Muscle fiber typology substantially influences time to recover from high-intensity exercise

Anaerobic speed reserve and acute responses to a short-format high-intensity interval session in runners

OBJECTIVES

This study aimed to assess relationships of acute responses to short-format high-intensity interval training (HIIT) with the anaerobic speed reserve (ASR) of adolescent runners.

DESIGN

Pre-post intervention design.

METHODS

Eighteen highly-trained youth runners (15.83 ± 0.86 years) underwent maximal sprinting speed (MSS) and maximal aerobic speed (MAS) assessments to determine ASR (MSS minus MAS) and a standardized HIIT protocol (2 × (20 × 15 s/15 s @110 % MAS)) was administered. Pre/post-HIIT assessments included biochemical (i.e., creatine kinase (CK)), neuromuscular (countermovement jump, CMJ; reactive strength index, RSI), cardiac (i.e., heart rate recovery (HRR)), and athlete-reported outcome measures (e.g., single item for fatigue). Pearson's r was calculated to assess relationships between acute responses and ASR, MSS, MAS, and relative intensity of the HIIT (%ASR).

RESULTS

Athletes' ASR and %ASR were significantly associated with the pre/post difference of CK (r = -0.75; p < 0.001; r = 0.74; p < 0.001, respectively), CMJ height, and RSI (r ≥ 0.69; p ≤ 0.002; r ≤ -0.49; p ≤ 0.04, respectively). However, HRR did not correlate significantly with ASR or %ASR (r ≤ 0.37, p ≥ 0.131, r ≥ -0.31; p ≥ 0.22, respectively). The pre/post difference of RSI correlated with MAS (r = -0.54; p = 0.02), and the pre/post difference of CK (r = -0.50; p = 0.034) and of CMJ height (r = 0.76; p < 0.001) with MSS. Regarding athlete-reported measures, ASR and %ASR showed significant associations with most fatigue and recovery variables (r ≥ 0.57; p ≤ 0.014, r ≥ 0.57; p ≤ 0.013, respectively). The pre/post difference of the single item for fatigue showed a positive relationship with MSS (r = 0.49; p = 0.037).

CONCLUSIONS

Acute biochemical, neuromuscular, and athlete-reported responses to short-format HIIT showed strong relationships with ASR and MSS, indicating higher internal load in athletes with a lower ASR and MSS by using a higher %ASR, compared to athletes with a higher ASR and MSS. These findings can help to tailor training programs to individual needs and avoid possible overload.

Changes in Physical Performance Following Operational Military Training: A Meta-Analysis

BACKGROUND

To best simulate armed combat-related emergencies, military personnel undergo operational training that attempts to recreate multiple physical stressors. Understanding the specific aspects of physical performance degradation after operational training helps to better inform future training practices, as these declines translate to real-world operations. The purpose of this meta-analysis was to investigate the effects of operational trainings on physical performance metrics in military personnel.

METHODS

Six electronic databases were searched for studies that investigated physical performance metrics in active-duty military personnel before and after multiple-day operational training. Sample sizes, means and standard deviations were extracted from included studies, and random-effects meta-analyses were conducted using standardized mean differences (Hedge's g) with 95% confidence intervals (p ≤ 0.05).

RESULTS

Seventeen studies (N = 1592 participants) were included for final review and grouped by physical performance metric. Meta-analyses revealed a large pre-to-post decline in lower body jump power (n = 4, g = 0.87, 95% CI [0.28, 1.47]), and small declines in short-duration lower body power-jump distance (n = 5, g = 0.39, 95% CI [0.16, 0.63]), upper body endurance (n = 4, g = 0.40, 95% CI [0.09, 0.71]), and core endurance (n = 2, g = 0.46, 95% CI [0.10, 0.82]). Substantial heterogeneity was observed in the included studies (I2 = 0-91%).

CONCLUSIONS

Military operational training temporarily diminishes lower body performance to a greater extent than other constructs. The most consistent findings showed reductions in power production (large magnitude) and muscular endurance (small magnitude), which appear to have implications for military training and risk reduction. In real-world operational applications, reduced power- and endurance-generating capabilities may be detrimental to meeting tactical requirements, making these metrics valuable for military leadership to focus on during personnel training.

Effect of home-based hot bathing on exercise-induced adaptations associated with short-term resistance exercise training in young men

This study investigated whether home-based bathing intervention (HBBI) improve muscle strength gain and protect cardiovascular function by short-term resistance training (RT). Thirty-one healthy young men measured the maximum voluntary isometric contraction (MVC) of knee extensor, electrically evoked knee extension torque, and mean arterial pressure (MAP). Then, participants were divided into three groups with matching MVC: shower without bathing (control, n = 10), thermoneutral bathing (36°C-bathing, n = 10), and hot bathing (40°C-bathing, n = 11), and conducted 2 weeks of HBBI. Following familiarization for HBBI, participants completed 2 weeks of HBBI and acute RT (five sessions of three sets of 10 isometric knee extension at 60% MVC). Baseline neuromuscular and cardiovascular function was assessed again following completion of the 2 weeks of intervention. MVC was non-significantly increased after the RT period in 40°C-bathing with large effect size (partial η2 = 0.450). The electrically evoked knee extension torque (10/100-Hz ratio) was significantly increased after the RT period in control (p = 0.020). MAP did not alter due to bathing intervention and RT (all p > 0.05). HBBI improved muscle strength without RT-induced alteration of peripheral muscle condition. Shower without bathing reduced muscle strength gain but increased peripheral muscle condition. Short-term RT does not adversely affect the cardiovascular function, regardless of HBBI.

A proposal to analyze muscle fiber type composition in the soleus muscle of untrained subjects and sprinters using surface EMG signals

Muscle fiber type proportion is a key determinant of fatigue, force generation, and functions of different skeletal muscles. Analysis of muscle fiber type composition aids in the assessment of athletic abilities and individualization of training methods. This study attempts to non-invasively analyze the muscle fiber type composition in the soleus (SOL) of untrained subjects (UT) and sprinters (SP) using surface electromyography-based time-frequency analysis. Signals are recorded from both groups during an isometric calf raise test with loads until fatigue. Filtered signals are segmented into epochs of 1-s duration and processed using a reassigned Morlet scalogram. Four time-frequency features namely averaged frequency, squared frequency bandwidth, averaged time, and squared time duration are extracted from the reassigned distribution and are subjected to linear regression analysis. A fiber-type-specific reassigned profile is noticed for UT and SP reflecting their distinct muscle composition during their non-fatigue and fatigue states. The regression parameters namely slope, intercept, and Adjusted R-square values are higher for the signals of SP indicating their fast-fatigue characteristics. Greater variation of features during fatigue is noticed in the signals of UT compared to SP. Among the features, the squared time duration exhibits the highest significance of p = 8.75E-07 in differentiating the signals of UT and SP during the non-fatigue state. Thus, the proposed approach is found suitable for analyzing the fiber type differences in both subject groups. This work may be further extended in sports biomechanics for studying the fiber-type transformations in muscles due to different athletic training strategies.

Muscle-Fiber Typology Is Associated With Sprint-Cycling Characteristics in World-Class and Elite Track Cyclists

PURPOSE

Identifying the determinants of performance is fundamental to talent identification and individualizing training prescription. Consequently, the aim of this study was to determine whether estimated muscle typology is associated with the key mechanical characteristics of track sprint cycling.

METHODS

Sixteen world-class and elite track cyclists (n = 7 female) completed a laboratory session wherein torque-cadence and power-cadence profiles were constructed to determine maximal power output (Pmax), optimal cadence (Fopt), and maximal cadence (Fmax), and fatigue rate per pedal stroke was determined during a 15-second maximal sprint at Fopt. Muscle typology was estimated by measuring carnosine content via proton magnetic resonance spectroscopy in the gastrocnemius and soleus.

RESULTS

Using partial correlation analysis to account for sex, greater muscle carnosine content (ie, greater estimated proportion of type II fibers) was associated with a greater Pmax (r = .68, P = .007), Fmax (r = .77, P = .0014), Fopt (r = .61, P = .0196), and absolute fatigue rate (W·stroke-1; r = -.55, P = .0418) but not relative fatigue rate (%peak power·stroke-1; r = -.33, P = .246).

CONCLUSIONS

The findings from this study substantiate the mechanical differences in muscle-fiber types derived from single muscle-fiber studies and highlight the importance of estimated muscle typology for sprint cycling performance.

In vivo imaging of glycogen in human muscle

Probing regional glycogen metabolism in humans non-invasively has been challenging due to a lack of sensitive approaches. Here we studied human muscle glycogen dynamics post-exercise with a spatial resolution of millimeters and temporal resolution of minutes, using relayed nuclear Overhauser effect (glycoNOE) MRI. Data at 5T showed a homogeneous distribution of glycogen in resting muscle, with an average concentration of 99 ± 13 mM. After plantar flexion exercise following fasting with recovery under fasting conditions, the calf muscle showed spatially heterogeneous glycogen depletion and repletion kinetics that correlated with the severity of this depletion. Three types of regional glycogen kinetics were observed: (i) single exponential repletion (type a); (ii) biphasic recovery of rapid repletion followed by additional depletion (type b); (iii) biphasic recovery where continued depletion is followed by an exponential recovery (type c). The study of the complex patterns of glycogen kinetics suggests that glycogen breakdown may be quantitatively important during the initial recovery.

Changes in torque complexity and maximal torque after a fatiguing exercise protocol

Torque outputs exhibit non-random fluctuations in their temporal structure, i.e., complexity. Fatigue has been shown to alter this structure. The torque outputs typically become more regular, resulting in decreased adaptability. Importantly, torque complexity was shown a different recovery pattern after fatigue compared to maximal torque. However, it remains to be understood if these uncoupled patterns of recovery are muscle dependent. In addition, it also remains to be investigated if changes in maximal torque and complexity are correlated. This study investigated (i) the effects of a fatiguing protocol on the complexity and maximal torque from plantar flexors and (ii) the relationship between changes in these two outputs. Ten participants visited the laboratory, and measures were taken at baseline, immediately after, 1 h after and 24 h after the fatiguing protocol. Maximum voluntary contraction, isometric contractions at 30% of maximum and pain pressure threshold were collected. Both legs were assessed, but only one was given the fatiguing protocol. Two-way ANOVAs and correlations were conducted. The fatiguing protocol decreased torque complexity (~35%) and maximal torque (~20%), and they exhibited uncoupled patterns of recovery. Moreover, the correlation analysis showed no correlation between changes in these parameters. These findings support that these parameters are independent of each other.

Zebrafish Models for Skeletal and Extraskeletal Osteogenesis Imperfecta Features: Unveiling Pathophysiology and Paving the Way for Drug Discovery

In the last decades, the easy genetic manipulation, the external fertilization, the high percentage of homology with human genes and the reduced husbandry costs compared to rodents, made zebrafish a valid model for studying human diseases and for developing new therapeutical strategies. Since zebrafish shares with mammals the same bone cells and ossification types, it became widely used to dissect mechanisms and possible new therapeutic approaches in the field of common and rare bone diseases, such as osteoporosis and osteogenesis imperfecta (OI), respectively. OI is a heritable skeletal disorder caused by defects in gene encoding collagen I or proteins/enzymes necessary for collagen I synthesis and secretion. Nevertheless, OI patients can be also characterized by extraskeletal manifestations such as dentinogenesis imperfecta, muscle weakness, cardiac valve and pulmonary abnormalities and skin laxity. In this review, we provide an overview of the available zebrafish models for both dominant and recessive forms of OI. An updated description of all the main similarities and differences between zebrafish and mammal skeleton, muscle, heart and skin, will be also discussed. Finally, a list of high- and low-throughput techniques available to exploit both larvae and adult OI zebrafish models as unique tools for the discovery of new therapeutic approaches will be presented.

Effects of intensified training with insufficient recovery on joint level and single muscle fibre mechanical function: the role of myofibrillar Ca2+ sensitivity

Intense exercise training with insufficient recovery time is associated with reductions in neuromuscular performance. However, it is unclear how single muscle fibre mechanical function and myofibrillar Ca2+ sensitivity contribute to these impairments. We investigated the effects of overload training on joint-level neuromuscular performance and cellular-level mechanical function. Fourteen athletes (4 female and 10 male) underwent a 3-week intensified training protocol consisting of up to 150% of their regular training hours with three additional high-intensity training sessions per week. Neuromuscular performance of the knee extensors was assessed via maximal voluntary contraction (MVC) force, electrically evoked twitch contractions, and a force-frequency relationship. Muscle biopsies were taken from the vastus lateralis to assess single fibre mechanical function. Neither MVC force nor twitch parameters were altered following training (all p > 0.05), but a rightward shift in the force-frequency curve was observed with average reduction in force of 6%-27% across frequencies 5-20 Hz (all p < 0.05). In single fibres, maximal force output was not reduced following training, but there was a rightward shift in the force-pCa curve driven by a 6% reduction in Ca2+ sensitivity (p < 0.05). These data indicate intensified training leads to impaired Ca2+ sensitivity at the single fibre level, which in part explains impaired neuromuscular function at the joint level during lower frequencies of activation. This is an important consideration for athletes, as performance is often assessed at maximal levels of activation, and these underlying impairments in force generation may be less obvious.

Long-Term Physiological Adaptations Induced by Short-Interval High-Intensity Exercises: An RCT Comparing Active and Passive Recovery

Background: High-intensity interval training (HIIT) is one of the most debated methods involving several parameters that could be modulated, but the long-term adaptations it induces are still unclear. This investigation aimed to evaluate the efficacy of running and whole-body exercises with high-intensity (>80% heart rate) short intervals (30 s) in body composition and physical performance and compare the effects between groups with active (AR) or passive recovery (PR), both in males and females. Methods: Eighteen trained young adults (55.56% ♀) were randomly allocated to the PR (n = 9, 23.09 ± 2.56 years, 163.69 ± 9.88 cm, 68.96 ± 14.62 kg) or AR (n = 9, 22.05 ± 1.54 years, 170.61 ± 11.5 cm, 68.78 ± 12.45 kg) group. Both groups performed eight weeks of HIIT, with an equal progression, training, and volume load (TL: F = 1.55, p = 0.214; VL: F = 0.81, p = 0.505). Body fat (BF), fat-free mass (FFM), upper and lower limb fat (UFI, LFI) and muscle areas (UMA, LMA), handgrip strength (HGS), power (countermovement jump, CMJ), agility (5-0-5), and maximal oxygen consumption (V˙O2p) were tested before and after treatments. Results: The proposed HIIT reduced BF by 9.57% and increased FFM by 2.09%. Females reported better adaptations in LMA (8.34 times higher than males), while both sexes' upper limb mass distribution was better affected by PR (♀: UFI g = 1.851, 95% CI: 0.51, 3.14; ♂: UFI g = 2.456, 95% CI: 0.336, 4.487). Concerning conditioning, the protocol increased V˙O2p by 6.47%. Females showed better adaptations in CMJ (RR = 1.8), while males showed better adaptations in agility (RR = 3.76). The interaction effects were significant for PR females (right = +6.28%; left = +9.28%) and for AR males (right = +19.21%; left = +19.04%) in HGS. Conclusions: Short-interval HIIT with different exercise recovery types may be a practical solution in training where several physiological improvements are needed. Coaches and trainers can take advantage of the versatile nature of HIIT, relying on desired movement patterns and long-term responses in both male and female individuals.

Estimating Muscle Fiber-Type Composition in Elite Athletes: A Survey on Current Practices and Perceived Merit

PURPOSE

To gather information on practices and perceptions of high-performance experts regarding their athletes' muscle fiber-type composition (MFTC) and its estimation.

METHODS

A questionnaire on the noninvasive versus invasive estimation of MFTC was completed by 446 experts including coaches and sport-science/sports-medicine staff. Moreover, the perceived importance of MFTC for training and performance optimization was assessed. Differences between sport types (individual and team sports) were analyzed using chi-square tests.

RESULTS

Forty percent of the experts implemented MFTC assessment in pursuit of performance optimization, while 50% did not know their athletes' MFTC but expressed a desire to implement it if they would be able to assess MFTC. Ten percent did not perceive value in MFTC assessment. Only 18% of experts believed that their athletes would undergo a muscle biopsy, leading to the adoption of alternative noninvasive techniques. Experts primarily relied on their experience to estimate MFTC (65%), with experts working in individual sports using their experience more frequently than those working in team sports (68% vs 51%; P = .009). Jump tests emerged as the second-most commonly employed method for estimating MFTC (56%). When only considering experts who are currently using MFTC, 87% use MFTC to individualize training volume and 84% to individualize training intensity.

CONCLUSIONS

Experts value MFTC assessment primarily to individualize training but mainly rely on noninvasive methods to estimate MFTC. Some of these methods lack scientific validity, suggesting a continuing need for education and further research in this area.

Transcriptomic signatures of human single skeletal muscle fibers in response to high-intensity interval exercise

The heterogeneous fiber type composition of skeletal muscle makes it challenging to decipher the molecular signaling events driving the health- and performance benefits of exercise. We developed an optimized workflow for transcriptional profiling of individual human muscle fibers before, immediately after, and after 3 h of recovery from high-intensity interval cycling exercise. From a transcriptional point-of-view, we observe that there is no dichotomy in fiber activation, which could refer to a fiber being recruited or nonrecruited. Rather, the activation pattern displays a continuum with a more uniform response within fast versus slow fibers during the recovery from exercise. The transcriptome-wide response immediately after exercise is characterized by some distinct signatures for slow versus fast fibers, although the most exercise-responsive genes are common between the two fiber types. The temporal transcriptional waves further converge the gene signatures of both fiber types toward a more similar profile during the recovery from exercise. Furthermore, a large heterogeneity among all resting and exercised fibers was observed, with the principal drivers being independent of a slow/fast typology. This profound heterogeneity extends to distinct exercise responses of fibers beyond a classification based on myosin heavy chains. Collectively, our single-fiber methodological approach points to a substantial between-fiber diversity in muscle fiber responses to high-intensity interval exercise.NEW & NOTEWORTHY By development of a single-fiber transcriptomics technology, we assessed the transcriptional events in individual human skeletal muscle fibers upon high-intensity exercise. We demonstrate a large variability in transcriptional activation of fibers, with shared and distinct gene signatures for slow and fast fibers. The heterogeneous fiber-specific exercise response extends beyond this traditional slow/fast categorization. These findings expand on our understanding of exercise responses and uncover a profound between-fiber diversity in muscle fiber activation and transcriptional perturbations.

The Relationship Between the Anaerobic Speed Reserve and Acute Responses to High-Intensity Interval Training in Female Soccer Players

ABSTRACT

Aspin, GL, Graham, M, Franklin, J, Hicks, KM, and Taylor, JM. The relationship between the anaerobic speed reserve and acute responses to high-intensity interval training in female soccer players. J Strength Cond Res 38(11): 1911-1916, 2024-The anaerobic speed reserve (ASR) is a popular method of profiling soccer players, often used to individualize training prescription. This study explored the reliability of ASR profiling, and the relationship between the ASR and acute physiological responses to high-intensity interval training (HIIT). Acute physiological responses to different HIIT types were also compared. Thirteen subelite female soccer players aged 20.2 ± 4.6 years completed 6 exercise sessions. In sessions 1-2, players completed a 40-m sprint to assess maximal sprint speed (MSS) and 1600-m time-trial to estimate maximal aerobic speed (MAS), which were used to calculate ASR and assess test-retest reliability. In sessions 3-6, players completed 4 HIIT sessions (repeated-sprint training, sprint interval training, long intervals, and short intervals HIIT). Intensities for long and short intervals HIIT were individualized according to MAS. Ratings of perceived exertion (RPE), heart rate (HR), and postsession blood lactates were recorded throughout. Relationships between the ASR and acute responses to HIIT, and between HIIT session comparisons in outcome measures were assessed. Anaerobic speed reserve (coefficient of variation ± 95% confidence limits; 3.1 ± 1.5%), MAS (1.8 ± 1.3%), and MSS (0.8 ± 0.6%) indicated acceptable reliability. Moderate correlations between ASR and RPE ( r = 0.33), postsession blood lactate ( r = 0.34), and HR ( r = 0.37) were observed during long intervals HIIT. A strong correlation was observed between ASR and RPE during SIT ( r = 0.50). Sprint interval training elicited higher RPE's and postsession blood lactate's than other HIIT sessions. Anaerobic speed reserve has good reliability and may influence acute physiological responses to HIIT in female soccer players.

Sex Differences in the Energy System Contribution during Sprint Exercise in Speed-Power and Endurance Athletes

Background/Objectives: A high level of specific metabolic capacity is essential for maximal sprinting in both male and female athletes. Various factors dictate sex differences in maximal power production and energy utilization. This study aims to compare the contribution of energy systems between male and female athletes with similar sport-specific physiological adaptations during a 15-s sprint exercise. Methods: The endurance group consisted of 17 males (23 ± 7 y) and 17 females (20 ± 2 y). The speed-power group included 14 males (21.1 ± 2.6 y) and 14 females (20 ± 3 y). The contribution of phosphagen, glycolytic, and aerobic systems was determined using the three-component PCr-LA-O2 method. Results: Significant differences were observed in the energy expenditure for all systems and total energy expenditure between males and females in both groups (p = 0.001-0.013). The energy expenditure in kJ for individual systems (phosphagen-glycolytic-aerobic) was 35:25:7 vs. 20:16:5 in endurance males vs. female athletes, respectively. In the speed-power group, male athletes expended 33:37:6 kJ and female athletes expended 21:25:4 kJ, respectively. The percentage proportions did not differ between males and females in any system. The contribution of the phosphagen-glycolytic-aerobic systems was 52:37:11 vs. 48:39:13 in endurance male and female athletes, respectively. For speed-power males vs. female athletes, the proportions were 42:50:8 vs. 41:50:9, respectively. Conclusions: Despite the differences in body composition, mechanical output, and absolute energy expenditure, the energy system contribution appears to have a similar metabolic effect between male and female athletes engaged in sprint exercises with similar sport-related adaptations. The magnitude and profile of sex differences are related to sports discipline.

Extracellular C1qbp inhibits myogenesis by suppressing NFATc1

Aging and lack of exercise are the most important etiological factors for muscle loss. We hypothesized that new factors that contribute to muscle loss could be identified from ones commonly altered in expression in aged and exercise-limited skeletal muscles. Mouse gastrocnemius muscles were subjected to mass spectrometry-based proteomic analysis. The muscle proteomes of hindlimb-unloaded and aged mice were compared to those of exercised and young mice, respectively. C1qbp expression was significantly upregulated in the muscles of both hindlimb-unloaded and aged mice. In vitro myogenic differentiation was not affected by altering intracellular C1qbp expression but was significantly suppressed upon recombinant C1qbp treatment. Additionally, recombinant C1qbp repressed the protein level but not the mRNA level of NFATc1. NFATc1 recruited the transcriptional coactivator p300, leading to the upregulation of acetylated histone H3 levels. Furthermore, NFATc1 silencing inhibited p300 recruitment, downregulated acetylated histone H3 levels, and consequently suppressed myogenic differentiation. The expression of C1qbp was inversely correlated with that of NFATc1 in the gastrocnemius muscles of exercised or hindlimb-unloaded, and young or aged mice. These findings demonstrate a novel role of extracellular C1qbp in suppressing myogenesis by inhibiting the NFATc1/p300 complex. Thus, C1qbp can serve as a novel therapeutic target for muscle loss.

The Utility of High-Intensity, Intermittent Exercise Protocols to Induce Fatigue for Screening Purposes in Jump-Landing Sports

Short-term fatigue protocols simulating sports participation are scarce and not well-documented in jump-landing sports. Therefore, this study investigated physiological and physical responses following high-intensity, intermittent exercise protocols (HIIPs) with a standardized level of subjective exhaustion (Borg ≥18/20) and a modified fixed version of five circuits (HIIP-5) for future inclusion in biomechanical screening protocols. Twenty male volleyball and basketball players participated in this study to complete the HIIP and the HIIP-5. Physiological and physical variables were assessed before and up to 30 min after cessation of both protocols. Regarding physiological variables, heart rate values increased (+104 bpm, p < 0.001) and remained elevated up to 30 min (+34 bpm, p < 0.001), and blood lactate levels increased (+17 mmol/l, p < 0.001) compared to baseline. Regarding physical variables, decreased jump height (-4 cm, p = 0.001-0.009) and quadriceps muscle strength (p = 0.001-0.050) were observed up to 30 min compared to baseline. The type of the fatigue protocol did not have an effect on the investigated variables (p > 0.05). To conclude, both the HIIP and the HIIP-5 seem valuable tools to induce acute and long-lasting responses, providing a sufficiently large time window of 30 min within which biomechanical markers of injury can be assessed under fatigued conditions in future risk factor screenings. In practice, the fatigue protocol can be terminated after only five circuits if athletes had not yet been stopped at that point due to exhaustion (Borg ≥18/20).

Three-week sprint interval training (SIT) reduces cell-free DNA and low-frequency fatigue but does not induce VO2max improvement in older men

PURPOSE

This study aimed to investigate the impact of sprint interval training (SIT) on both the acute and 3-week modulations of cell-free DNA (cfDNA), as well as its association with neuromuscular fatigue and physical performance in healthy young and old men.

METHODS

Ten young (20-25 year old) and nine elderly (63-72 year old) healthy men performed nine SIT sessions consisting of 4-to-6-all-out cycling repetitions of 30 s interspaced with 4-min rest intervals. We compared the maximal voluntary contractions torque, central activation ratio, low-frequency fatigue (LFF), and cfDNA concentrations between the groups before, immediately after, 1 h after, and 24 h after the first and ninth SIT sessions.

RESULTS

The plasma cfDNA levels were increased post-exercise (from 1.4 ± 0.258 to 1.91 ± 0.278 ng/ml (P < 0.01) on a log10 scale), without significant differences between the groups. However, older individuals showed a slight decrease in the baseline cfDNA values, from 1.39 ± 0.176 to 1.29 ± 0.085 ng/ml on a log10 scale, after 3 weeks (P = 0.043). Importantly, the elevation of the post-exercise cfDNA values was correlated with an increase in LFF in both groups. Three weeks of SIT induced an improvement in the recovery of LFF (main session effect, P = 0.0029); however, only the young group showed an increase in aerobic capacity (VO2max) (from 40.8 ± 6.74 to 43.0 ± 5.80 ml/kg/min, P = 0.0039).

CONCLUSION

Three weeks of SIT diminished the baseline cfDNA values in the old group, together with an improvement in the recovery of LFF. However, VO2max was increased only in the young group.

Human lower leg muscles grow asynchronously

Muscle volume must increase substantially during childhood growth to generate the power required to propel the growing body. One unresolved but fundamental question about childhood muscle growth is whether muscles grow at equal rates; that is, if muscles grow in synchrony with each other. In this study, we used magnetic resonance imaging (MRI) and advances in artificial intelligence methods (deep learning) for medical image segmentation to investigate whether human lower leg muscles grow in synchrony. Muscle volumes were measured in 10 lower leg muscles in 208 typically developing children (eight infants aged less than 3 months and 200 children aged 5 to 15 years). We tested the hypothesis that human lower leg muscles grow synchronously by investigating whether the volume of individual lower leg muscles, expressed as a proportion of total lower leg muscle volume, remains constant with age. There were substantial age-related changes in the relative volume of most muscles in both boys and girls (p < 0.001). This was most evident between birth and five years of age but was still evident after five years. The medial gastrocnemius and soleus muscles, the largest muscles in infancy, grew faster than other muscles in the first five years. The findings demonstrate that muscles in the human lower leg grow asynchronously. This finding may assist early detection of atypical growth and allow targeted muscle-specific interventions to improve the quality of life, particularly for children with neuromotor conditions such as cerebral palsy.

Baselining physiological parameters in three muscles across three equine breeds. What can we learn from the horse?

Mapping-out baseline physiological muscle parameters with their metabolic blueprint across multiple archetype equine breeds, will contribute to better understanding their functionality, even across species. Aims: 1) to map out and compare the baseline fiber type composition, fiber type and mean fiber cross-sectional area (fCSA, mfCSA) and metabolic blueprint of three muscles in 3 different breeds 2) to study possible associations between differences in histomorphological parameters and baseline metabolism. Methods: Muscle biopsies [m. pectoralis (PM), m. vastus lateralis (VL) and m. semitendinosus (ST)] were harvested of 7 untrained Friesians, 12 Standardbred and 4 Warmblood mares. Untargeted metabolomics was performed on the VL and PM of Friesian and Warmblood horses and the VL of Standardbreds using UHPLC/MS/MS and GC/MS. Breed effect on fiber type percentage and fCSA and mfCSA was tested with Kruskal-Wallis. Breeds were compared with Wilcoxon rank-sum test, with Bonferroni correction. Spearman correlation explored the association between the metabolic blueprint and morphometric parameters. Results: The ST was least and the VL most discriminative across breeds. In Standardbreds, a significantly higher proportion of type IIA fibers was represented in PM and VL. Friesians showed a significantly higher representation of type IIX fibers in the PM. No significant differences in fCSA were present across breeds. A significantly larger mfCSA was seen in the VL of Standardbreds. Lipid and nucleotide super pathways were significantly more upregulated in Friesians, with increased activity of short and medium-chain acylcarnitines together with increased abundance of long chain and polyunsaturated fatty acids. Standardbreds showed highly active xenobiotic pathways and high activity of long and very long chain acylcarnitines. Amino acid metabolism was similar across breeds, with branched and aromatic amino acid sub-pathways being highly active in Friesians. Carbohydrate, amino acid and nucleotide super pathways and carnitine metabolism showed higher activity in Warmbloods compared to Standardbreds. Conclusion: Results show important metabolic differences between equine breeds for lipid, amino acid, nucleotide and carbohydrate metabolism and in that order. Mapping the metabolic profile together with morphometric parameters provides trainers, owners and researchers with crucial information to develop future strategies with respect to customized training and dietary regimens to reach full potential in optimal welfare.

Electromyography-driven musculoskeletal models with time-varying fatigue dynamics improve lumbosacral joint moments during lifting

Muscle fatigue is prevalent across different aspects of daily life. Tracking muscle fatigue is useful to understand muscle overuse and possible risk of injury leading to musculoskeletal disorders. Current fatigue models are not suitable for real-world settings as they are either validated using simulations or non-functional tasks. Moreover, models that capture the changes to muscle activity due to fatigue either assume a linear relationship between muscle activity and muscle force or utilize a simple muscle model. Personalised electromygraphy (EMG)-driven musculoskeletal models (pEMS) offer person-specific approaches to model muscle and joint kinetics during a wide repertoire of daily life tasks. These models utilize EMG, thus capturing central fatigue-dependent changes in multi-muscle bio-electrical activity. However, the peripheral muscle force decay is missing in these models. Thus, we studied the influence of fatigue on a large scale pEMS of the trunk. Eleven healthy participants performed functional asymmetric lifting task. Average peak body-weight normalized lumbosacral moments (BW-LM) were estimated to be 2.55 ± 0.26 Nm/kg by reference inverse dynamics. After complete exhaustion of the lower back, the pEMS overestimated the peak BW-LM by 0.64 ± 0.37 Nm/kg. Then, we developed a time-varying muscle force decay model resulting in a time-varying pEMS (t-pEMS). This reduced the difference between BW-LM estimated by the t-pEMS and reference to 0.49 ± 0.14 Nm/kg. We also showed that five fatiguing contractions are sufficient to calibrate the t-pEMS. Thus, this study presents a person and muscle specific model to track fatigue during functional tasks.

In professional football the decline in high-intensity running activities from first to second half is more pronounced in players with a fast muscle typology

Muscle typology is heterogeneous among national level football (soccer) players, but positional differences remain unclear. Furthermore, fast typology (FT) individuals fatigue more than slow typology (ST) individuals in lab conditions. Therefore, we investigated if muscle typology is different between playing positions and if the decay in high-intensity activities from the first to the second half is larger in FT football players than in ST players. We estimated muscle typology in 147 male professional football players by measuring soleus and gastrocnemius muscle carnosine via proton magnetic resonance spectroscopy. Players were classified as ST, intermediate typology (IT) or FT and categorized as goalkeeper, center back, full back, midfielder, winger or forward. Across four seasons in-game distances covered in multiple running speed, acceleration and deceleration zones were collected during the first and second half. We found no differences in muscle typology between positions (p = 0.412). FT players covered 10.9% more high acceleration distance (>3 m.s-2 ) in the first half than ST players (p = 0.021) and high acceleration distance decay was larger for FT players (-12.4%) than ST (-7.7%; p = 0.006) and IT players (-7.3%; p = 0.010). Moreover, the decline in distance covered in several high-intensity zones tended to be larger in FT players (-11.2% high-intensity >15 km.h-1 ; -12.7% high deceleration <-3 m.s-2 ; -11.5% medium acceleration 2-3 m.s-2 ) than in ST players (-7.1% high-intensity; -8.1% high deceleration; -8.1% medium acceleration; 0.05 < p < 0.1). In conclusion, possessing a particular muscle typology is not required to play any football position at the national level. However, there are indications that FT players might fatigue more toward the end of the game compared to ST players.

Psychophysiological Responses in Soldiers during Close Combat: Implications for Occupational Health and Fitness in Tactical Populations

This study explores the psychophysiological responses and subjective exertion experiences of soldiers in simulated hand-to-hand combat, aligning these findings with established physiological benchmarks. Active military personnel were monitored for heart rate, blood lactate levels, subjective exertion, cortical arousal, and muscle strength during combat scenarios. The results showed significant increases in heart rate and blood lactate, indicating intense cardiovascular demands and a reliance on anaerobic energy systems. Contrary to these physiological changes, soldiers reported lower levels of exertion, suggesting a possible underestimation of physical effort or individual differences in perception and mental resilience to stress. Notably, a decrease in cortical arousal post-combat was observed, potentially signaling cognitive function deficits in decision-making and information processing in high-stress environments. This decline was more pronounced than typically seen in other high-stress situations, highlighting the unique cognitive demands of hand-to-hand combat. Additionally, an increase in muscle strength was noted, underscoring the physiological adaptations arising from intensive combat training. These findings provide valuable insights into the psychophysiological effects of hand-to-hand combat, emphasizing the complex interplay between physical exertion, cognitive function, and stress response in military contexts. The study underscores the need for comprehensive training strategies that address both physical and psychological aspects to enhance combat readiness and decision-making under stress.

Optimizing mechanical performance in the bench press: The combined influence of inter-set rest periods and proximity to failure

This study examined the influence of inter-set rest periods of 1 (R1), 3 (R3) and 5 (R5) minutes on the number of repetitions completed before exceeding the minimum velocity thresholds of 0.45 m⋅s-1 (MVT0.45) and 0.35 m⋅s-1 (MVT0.35) during the bench press exercise. Twenty-three physically active individuals, 15 men and eight women, randomly completed six testing sessions consisting of four sets of the bench press exercise performed with maximal intent against 75% of the one-repetition maximum. Testing sessions differed in the length of inter-set rest periods (R1, R3, and R5) and MVT applied (MVT0.45 and MVT0.35). The number of repetitions was lower using shorter inter-set rest periods (R1 < R3 < R5), but R3 was more similar to R1 and R5 using MVT0.45 and MVT0.35, respectively. The fastest velocity of the set was reduced with the increment in the number of sets for the three protocols using MVT0.35 (greater reduction for shorter rest periods), but it was only reduced for R1 when using MVT0.45. The results suggest that, to maintain bench press mechanical performance, 5-min inter-set rest periods are necessary when sets are terminated close to failure (MVT0.35), while 3 min may suffice when sets are terminated farther from failure (MVT0.45).

A dynamic model of the bi-exponential reconstitution and expenditure of W' in trained cyclists

ABSTRACTThe aim of this study was to investigate the effects of different recovery power outputs on the reconstitution of W' and to develop a dynamic bi-exponential model of W' during depletion and reconstitution. Ten trained cyclists (mass 71.7 ± 8.4 kg; V̇O2max 60.0 ± 6.3 ml·kg-1·min-1) completed three incremental ramps (20 W·min-1) to the limit of tolerance on each of six occasions with recovery durations of 30 and 240 s. Recovery power outputs varied between 50 W (LOW); 60% of critical power (CP) (MOD) and 85% of CP (HVY). W' reconstitution was measured following each recovery and fitted to a bi-exponential model. Amplitude and time constant (τ) parameters were then determined via regression analysis accounting for relative intensity and duration to produce a dynamic model of W'. W' reconstitution slowed disproportionately as recovery power output increased (p < 0.001) and increased with recovery duration (p < 0.001). The amplitudes of each recovery component were strongly correlated to W' reconstitution after 240 s at HVY (r = 0.95), whilst τ parameters were found to be related to the fractional difference between recovery power and CP. The predictive capacity of the resultant model was assessed against experimental data with no differences found between predicted and experimental values of W' reconstitution (p > 0.05). The dynamic bi-exponential model of W' accounting for varying recovery intensities closely described W' kinetics in trained cyclists facilitating real-time decisions about pacing and tactics during competition. The model can be customised for individuals from known CP and W' and a single additional test session.HighlightsA dynamic bi-exponential model of W' accounting for both varying power output and duration.Individual customisation of the model can be achieved with a single specific test session.W' reconstitution slows disproportionally with increasing intensity after repeated bouts.

Match Running Performance in Australian Football Is Related to Muscle Fiber Typology

PURPOSE

To examine the association between muscle fiber typology and match running performance in professional Australian football (AF) athletes.

METHODS

An observational time-motion analysis was performed on 23 professional AF athletes during 224 games throughout the 2020 competitive season. Athletes were categorized by position as hybrid, small, or tall. Athlete running performance was measured using Global Navigation Satellite System devices. Mean total match running performance and maximal mean intensity values were calculated for moving mean durations between 1 and 10 minutes for speed (in meters per minute), high-speed-running distance (HSR, >4.17 m·s-1), and acceleration (in meters per second squared), while intercept and slopes were calculated using power law. Carnosine content was quantified by proton magnetic resonance spectroscopy in the gastrocnemius and soleus and expressed as a carnosine aggregate z score (CAZ score) to estimate muscle fiber typology. Mixed linear models were used to determine the association between CAZ score and running performance.

RESULTS

The mean (range) CAZ score was -0.60 (-1.89 to 1.25), indicating that most athletes possessed a greater estimated proportion of type I muscle fibers. A greater estimated proportion of type I fibers (ie, lower CAZ score) was associated with a larger accumulation of HSR (>4.17 m·s-1) and an increased ability to maintain HSR as the peak period duration increased.

CONCLUSION

AF athletes with a greater estimated proportion of type I muscle fibers were associated with a greater capacity to accumulate distance running at high speeds, as well as a greater capacity to maintain higher output of HSR running during peak periods as duration increases.

Muscle typology influences the number of repetitions to failure during resistance training

This study examined whether muscle typology (muscle fibre type composition) is related to maximal strength and whether it can explain the high inter-individual variability in number of repetitions to failure during resistance training. Ninety-five resistance training novices (57 males) were assessed for their maximal isometric knee extension strength and muscle typology. Muscle typology was estimated by measuring carnosine in the soleus, gastrocnemius and/or vastus lateralis using proton magnetic resonance spectroscopy. Forty-four subjects (22 males) performed dynamic strength tests (1RM) and 3 sets of leg extensions and curls to failure (60%1RM) to determine the association between muscle typology and (total) number of repetitions. Twenty-one subjects performed additional biceps curls and triceps extensions (60%1RM) to assess influence of exercise, 23 subjects performed additional leg extensions and curls at 80% and 40%1RM to evaluate influence of training load. There was a weak but significant relationship between muscle typology and maximal isometric strength (r = 0.22, p = 0.03) favouring the fast typology individuals. Slow and fast typology individuals did not differ in upper arm and upper leg 1RM. Total number of repetitions was related to muscle typology at 80% (r = -0.42; p = 0.04) and 60% (p = -0.44; p = 0.003) but not at 40%1RM. Slow typology individuals performed more repetitions to failure at 60%1RM in the leg extension (p = 0.03), leg curl (p = 0.01) and biceps curl (p = 0.02). In conclusion, muscle typology has a small contribution to maximal isometric strength but not dynamic strength and partly determines the number of repetitions to failure during resistance training. This insight can help individualizing resistance training prescriptions.

Neuromuscular and autonomic function is fully recovered within 24 h following a sprint interval training session

BACKGROUND

Recovery is a key factor to promote adaptations and enhance performance. Sprint Interval Training (SIT) is known to be an effective approach to improve overall physical function and health. Although a 2-day rest period is given between SIT sessions, the time-course of recovery after SIT is unknown.

PURPOSE

The aim of this study was to determine whether the neuromuscular and autonomic nervous systems would be impaired 24 and 48 h after an SIT session.

METHODS

Twenty-five healthy subjects performed an 8 × 15 s all-out session on a braked cycle ergometer with 2 min of rest between repetitions. Isometric maximal voluntary contraction (iMVC) and evoked forces to electrical nerve stimulation during iMVC and at rest were used to assess muscle contractile properties and voluntary activation before (Pre), 1 (Post24h), and 2 (Post48h) days after the session. Two maximal 7 s sprints with two different loads were performed at those same time-points to evaluate the maximal theoretical force (F0), velocity (V0) and maximal power (Pmax) production during a dynamic exercise. Additionally, nocturnal heart rate variability (HRV) was assessed the previous and the three subsequent nights to the exercise bout.

RESULTS

No significant impairments were observed for the iMVC or for the force evoked by electrical stimulation 1 day after the session. Similarly, F0, V0, and Pmax were unchanged at Post24h and Post48h. Furthermore, HRV did not reveal any temporal or frequential significant difference the nights following SIT compared to Pre.

CONCLUSION

The results of this study show a full recovery of neuromuscular and autonomic functions a day after an all-out SIT session.

Anaerobic Speed Reserve and Performance Relationships Between International and World-Class Short-Track Speed Skating

PURPOSE

Short-track speed skating race distances of 500, 1000, and 1500 m that last ∼40 seconds to ∼2.5 minutes and require a maximal intensity at speeds beyond maximal oxygen uptake (VO2max). Recently, the anaerobic speed reserve (ASR) has been applied by scientists and coaches in middle-distance sports to deepen understanding of 1- to 5-minute event performance where different physiological profiles (speed, hybrid, and endurance) can have success.

METHODS

World-class (women, n = 2; men, n = 3) and international-level (women, n = 4; men, n = 5) short-track speed skaters completed maximal aerobic speed and maximal skating speed tests. ASR characteristics were compared between profiles and associated with on-ice performance.

RESULTS

World-class athletes raced at a lower %ASR in the 1000- (3.1%; large; almost certainly) and 1500-m (1.8%; large; possibly) events than international athletes. Men's and women's speed profiles operated at a higher %ASR in the 500-m than hybrid and endurance profiles, whereas in the 1500-m, endurance profiles worked at a substantially lower %ASR than hybrid and speed profiles. Women's 500-m performance is very largely associated with maximal skating speed, while women's maximal aerobic speed appears to be a key determining factor in the 1000- and 1500-m performance.

CONCLUSION

World-class short-track speed skaters can be developed in speed, hybrid, and endurance profiles but achieve their performance differently by leveraging their strongest characteristics. These results show nuanced differences between men's and women's 500-, 1000- and 1500-m event performance across ASR profile that unlock new insights for individualizing athlete performance in these disciplines.

Non-invasive Analysis of Fiber Type Composition in Lower Limb Skeletal Muscles Using Reduced Interference Rihaczek Distribution

Fiber composition is an important factor influencing force generation and endurance of different skeletal muscles. The analysis of the heterogeneous composition of muscles has gained importance in the field of sports biomechanics and biomedicine. In this work, an attempt is made to analyze the fiber composition of Rectus femoris (type II dominant) and Soleus (type I dominant) muscles using surface electromyography. Isometric signals are acquired from the muscles of 15 participants using a well-defined protocol and are further processed using reduced interference Rihaczek distribution. Instantaneous median frequency (IMDF) is extracted from the non-fatigue (NF) and fatigue (F) segments of the signals and is analyzed. From the distributions, it is found that the spectral power increases in the lower frequencies of the signal recorded from the rectus femoris and in the higher frequencies of signals recorded from the soleus during fatigue. The soleus is showing higher IMDF values than the rectus femoris in both segments. A reduction of 14% and an increase of 10% is observed in the IMDF during fatigue for rectus femoris and soleus, respectively. Thus, the extracted feature is found to be sensitive and statistically significant (p<0.05) to differentiate fiber types as well as the NF and F states of the two muscles.Clinical Relevance- This study may be extended to non-invasively analyze the fiber type shifts in muscles due to athletic training and pathological conditions.

Can muscle typology explain the inter-individual variability in resistance training adaptations?

Considerable inter-individual heterogeneity exists in the muscular adaptations to resistance training. It has been proposed that fast-twitch fibres are more sensitive to hypertrophic stimuli and thus that variation in muscle fibre type composition is a contributing factor to the magnitude of training response. This study investigated if the inter-individual variability in resistance training adaptations is determined by muscle typology and if the most appropriate weekly training frequency depends on muscle typology. In strength-training novices, 11 slow (ST) and 10 fast typology (FT) individuals were selected by measuring muscle carnosine with proton magnetic resonance spectroscopy. Participants trained both upper arm and leg muscles to failure at 60% of one-repetition maximum (1RM) for 10 weeks, whereby one arm and leg trained 3×/week and the contralateral arm and leg 2×/week. Muscle volume (MRI-based 3D segmentation), maximal dynamic strength (1RM) and fibre type-specific cross-sectional area (vastus lateralis biopsies) were evaluated. The training response for total muscle volume (+3 to +14%), fibre size (-19 to +22%) and strength (+17 to +47%) showed considerable inter-individual variability, but these could not be attributed to differences in muscle typology. However, ST individuals performed a significantly higher training volume to gain these similar adaptations than FT individuals. The limb that trained 3×/week had generally more pronounced hypertrophy than the limb that trained 2×/week, and there was no interaction with muscle typology. In conclusion, muscle typology cannot explain the high variability in resistance training adaptations when training is performed to failure at 60% of 1RM. KEY POINTS: This study investigated the influence of muscle typology (muscle fibre type composition) on the variability in resistance training adaptations and on its role in the individualization of resistance training frequency. We demonstrate that an individual's muscle typology cannot explain the inter-individual variability in resistance training-induced increases in muscle volume, maximal dynamic strength and fibre cross-sectional area when repetitions are performed to failure. Importantly, slow typology individuals performed a significantly higher training volume to obtain similar adaptations compared to fast typology individuals. Muscle typology does not determine the most appropriate resistance training frequency. However, regardless of muscle typology, an additional weekly training (3×/week vs. 2×/week) increases muscle hypertrophy but not maximal dynamic strength. These findings expand on our understanding of the underlying mechanisms for the large inter-individual variability in resistance training adaptations.

Time Decay in the Performance Benefits From Repeated-Sprint Training in Hypoxia in World-Class Short-Track Speed Skaters

PURPOSE

In short-track speed skating, athletes need to possess specific physiological capabilities to perform explosive starts and to finish races with faster lap times than their opponents. Repeated-sprint training in hypoxia (RSH) can enhance repeated-sprint ability and high-intensity performance. This study aimed to evaluate the relationship between on- and off-ice performance indicators for training and testing purposes and how these are optimized with RSH in world-class short-track speed skaters including world and Olympic champions.

METHODS

RSH training was administered for 3 consecutive weeks, 3 times per week, at 3500 m of simulated altitude. Testing sessions (on-ice: 3-lap, 27-lap; off-ice: cycling incremental test, 7-s and 30-s Wingate) were performed immediately before and 2 and 4 weeks after RSH to determine the time course of decay.

RESULTS

On-ice top speed showed a small and possibly beneficial change of ∼0.9% for the women and large and almost certain ∼0.7% improvement for the men 2 weeks post-RSH. Cycling peak power showed a moderate and probable ∼5.4% improvement for the men 2 weeks after RSH. These adaptations reverted to baseline 4 weeks post-RSH. Wingate average power showed a small and possibly beneficial gain (∼3.4%) in performance 4 weeks post-RSH.

CONCLUSION

Although scientific controls could not be added due to the extremely high caliber of these athletes and low sample size of the national team, this study suggests that cycling RSH can be added immediately after on-ice training and can transfer into meaningful improvements on the ice in both male and female skaters.

Electrical stimulation to regain lower extremity muscle perfusion and endurance in patients with post-acute sequelae of SARS CoV-2: A randomized controlled trial

Muscle deconditioning and impaired vascular function in the lower extremities (LE) are among the long-term symptoms experienced by COVID-19 patients with a history of severe illness. These symptoms are part of the post-acute sequelae of Sars-CoV-2 (PASC) and currently lack evidence-based treatment. To investigate the efficacy of lower extremity electrical stimulation (E-Stim) in addressing PASC-related muscle deconditioning, we conducted a double-blinded randomized controlled trial. Eighteen (n = 18) patients with LE muscle deconditioning were randomly assigned to either the intervention (IG) or the control (CG) group, resulting in 36 LE being assessed. Both groups received daily 1 h E-Stim on both gastrocnemius muscles for 4 weeks, with the device functional in the IG and nonfunctional in the CG. Changes in plantar oxyhemoglobin (OxyHb) and gastrocnemius muscle endurance (GNMe) in response to 4 weeks of daily 1 h E-Stim were assessed. At each study visit, outcomes were measured at onset (t₀ ), 60 min (t₆₀ ), and 10 min after E-Stim therapy (t₇₀ ) by recording ΔOxyHb with near-infrared spectroscopy. ΔGNMe was measured with surface electromyography at two time intervals: 0-5 min (Intv₁ ) and: 55-60 min (Intv₂ ). Baseline OxyHb decreased in both groups at t₆₀ (IG: p = 0.046; CG: p = 0.026) and t₇₀ (IG = p = 0.021; CG: p = 0.060) from t₀ . At 4 weeks, the IG's OxyHb increased from t₆₀ to t₇₀ (p < 0.001), while the CG's decreased (p = 0.003). The IG had higher ΔOxyHb values than the CG at t₇₀ (p = 0.004). Baseline GNMe did not increase in either group from Intv₁ to Intv₂ . At 4 weeks, the IG's GNMe increased (p = 0.031), whereas the CG did not change. There was a significant association between ΔOxyHb and ΔGNMe (r = 0.628, p = 0.003) at 4 weeks in the IG. In conclusion, E-Stim can improve muscle perfusion and muscle endurance in individuals with PASC experiencing LE muscle deconditioning.

The Relevance of Muscle Fiber Type to Physical Characteristics and Performance in Team-Sport Athletes

PURPOSE

The aim of this systematic review was to (1) determine the muscle fiber-type composition (or muscle fiber typology [MFT]) of team-sport athletes and (2) examine associations between MFT and the physical characteristics and performance tasks in team-sport athletes.

METHODS

Searches were conducted across numerous databases-PubMed, SPORTDiscus, MEDLINE, and Google Scholar-using consistent search terms. Studies were included if they examined the MFT of team-sport athletes. Included studies underwent critical appraisal using the McMasters University critical appraisal tool for quantitative research.

RESULTS

A total of 10 studies were included in the present review, wherein the MFT of athletes was measured from 5 different team sports (soccer, rugby union, rugby league, handball, and volleyball). There was large variability in the MFT of team-sport athletes both within (up to 27.5%) and between sports (24.0% relative difference). Male football players with a higher proportion of type II fibers had faster 10- and 30-m sprint times, achieved a greater total distance sprinting (distance at >6.67 m·s-1), and a greater peak 1-minute sprint distance.

CONCLUSIONS

MFT varies considerably between athletes both within and between different team sports. The results from some studies suggest that variation in MFT is associated with high-intensity running performance in a football match, as well as 10- and 30-m sprint times. Further experimental studies should focus on how determination of the MFT of team-sport athletes could be utilized to influence talent identification, team selection, and the individualization of training.

Repeated measurements of Adaptive Force: Maximal holding capacity differs from other maximal strength parameters and preliminary characteristics for non-professional strength vs. endurance athletes

The Adaptive Force (AF) reflects the neuromuscular capacity to adapt to external loads during holding muscle actions and is similar to motions in real life and sports. The maximal isometric AF (AFisomax) was considered to be the most relevant parameter and was assumed to have major importance regarding injury mechanisms and the development of musculoskeletal pain. The aim of this study was to investigate the behavior of different torque parameters over the course of 30 repeated maximal AF trials. In addition, maximal holding vs. maximal pushing isometric muscle actions were compared. A side consideration was the behavior of torques in the course of repeated AF actions when comparing strength and endurance athletes. The elbow flexors of n = 12 males (six strength/six endurance athletes, non-professionals) were measured 30 times (120 s rest) using a pneumatic device. Maximal voluntary isometric contraction (MVIC) was measured pre and post. MVIC, AFisomax, and AFmax (maximal torque of one AF measurement) were evaluated regarding different considerations and statistical tests. AFmax and AFisomax declined in the course of 30 trials [slope regression (mean ± standard deviation): AFmax = -0.323 ± 0.263; AFisomax = -0.45 ± 0.45]. The decline from start to end amounted to -12.8% ± 8.3% (p < 0.001) for AFmax and -25.41% ± 26.40% (p < 0.001) for AFisomax. AF parameters declined more in strength vs. endurance athletes. Thereby, strength athletes showed a rather stable decline for AFmax and a plateau formation for AFisomax after 15 trials. In contrast, endurance athletes reduced their AFmax, especially after the first five trials, and remained on a rather similar level for AFisomax. The maximum of AFisomax of all 30 trials amounted 67.67% ± 13.60% of MVIC (p < 0.001, n = 12), supporting the hypothesis of two types of isometric muscle action (holding vs. pushing). The findings provided the first data on the behavior of torque parameters after repeated isometric-eccentric actions and revealed further insights into neuromuscular control strategies. Additionally, they highlight the importance of investigating AF parameters in athletes based on the different behaviors compared to MVIC. This is assumed to be especially relevant regarding injury mechanisms.

Evaluating the Usefulness of the Modified Swimming Anaerobic Sprint Test (SAST) Based on the Relationship with the 100- and 200-m Freestyle

The construction of specialized physiological tests for swimmers is often based on those previously used in versions on land. So far, no attempt has been made to adapt the RAST test to conditions of an aquatic environment. The aim of the study was to evaluate the relationship between the results from the 6 × 25 m front crawl test (Swimming Anaerobic Sprint Test—SAST) and 100- and 200-m freestyle performance. In a group of 22 swimmers (9 girls, 13 boys, mean age 17.2 years, body mass 67.6 kg, body height 1.76 m), the SAST was performed (10-s interval between repetition). To determine speed (v), stroke rate (SR), stroke length (SL), and stroke index (SI), the test and races were recorded with the use of cameras. The analysis included times from the competition and fatigue indices (the decrease in SI during the SAST). Analysis of variance was performed, and a correlation matrix was created considering variables from the SAST and competition. The indices describing the 200-m event differed from those recorded for the SAST and 100 m freestyle race. The SAST results (v, SL, SI) were strongly correlated with the 100- and 200-m performances. Fatigue indices have not been shown to correlate with competition performance. The SAST may be useful in the context of predicting results for the 100- and, to a limited extent, 200-m freestyle races.

High-throughput muscle fiber typing from RNA sequencing data

Background

Skeletal muscle fiber type distribution has implications for human health, muscle function, and performance. This knowledge has been gathered using labor-intensive and costly methodology that limited these studies. Here, we present a method based on muscle tissue RNA sequencing data (totRNAseq) to estimate the distribution of skeletal muscle fiber types from frozen human samples, allowing for a larger number of individuals to be tested.

Methods

By using single-nuclei RNA sequencing (snRNAseq) data as a reference, cluster expression signatures were produced by averaging gene expression of cluster gene markers and then applying these to totRNAseq data and inferring muscle fiber nuclei type via linear matrix decomposition. This estimate was then compared with fiber type distribution measured by ATPase staining or myosin heavy chain protein isoform distribution of 62 muscle samples in two independent cohorts (n = 39 and 22).

Results

The correlation between the sequencing-based method and the other two were r_ATPas = 0.44 [0.13–0.67], [95% CI], and r_myosin = 0.83 [0.61–0.93], with p = 5.70 × 10^–3 and 2.00 × 10^–6, respectively. The deconvolution inference of fiber type composition was accurate even for very low totRNAseq sequencing depths, i.e., down to an average of ~ 10,000 paired-end reads.

Conclusions

This new method (https://github.com/OlaHanssonLab/PredictFiberType) consequently allows for measurement of fiber type distribution of a larger number of samples using totRNAseq in a cost and labor-efficient way. It is now feasible to study the association between fiber type distribution and e.g. health outcomes in large well-powered studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-022-00299-4.

“I Want to Create So Much Stimulus That Adaptation Goes Through the Roof”: High-Performance Strength Coaches' Perceptions of Planned Overreaching

Functional overreaching (FOR) occurs when athletes experience improved athletic capabilities in the days and weeks following short-term periods of increased training demand. However, prolonged high training demand with insufficient recovery may also lead to non-functional overreaching (NFOR) or the overtraining syndrome (OTS). The aim of this research was to explore strength coaches' perceptions and experiences of planned overreaching (POR); short-term periods of increased training demand designed to improve athletic performance. Fourteen high-performance strength coaches (weightlifting; n = 5, powerlifting; n = 4, sprinting; n = 2, throws; n = 2, jumps; n = 1) participated in semistructured interviews. Reflexive thematic analysis identified 3 themes: creating enough challenge, training prescription, and questioning the risk to reward. POR was implemented for a 7 to 14 day training cycle and facilitated through increased daily/weekly training volume and/or training intensity. Participants implemented POR in the weeks (~5–8 weeks) preceding competition to allow sufficient time for performance restoration and improvement to occur. Short-term decreased performance capacity, both during and in the days to weeks following training, was an anticipated by-product of POR, and at times used as a benchmark to confirm that training demand was sufficiently challenging. Some participants chose not to implement POR due to a lack of knowledge, confidence, and/or perceived increased risk of athlete training maladaptation. Additionally, this research highlights the potential dichotomy between POR protocols used by strength coaches to enhance athletic performance and those used for the purpose of inducing training maladaptation for diagnostic identification.

Knee extensor force production and discomfort during neuromuscular electrical stimulation of quadriceps with and without gluteal muscle co-stimulation

Purpose

To investigate whether Neuromuscular Electrical Stimulation (NMES) simultaneously applied on the quadriceps (Q) and gluteal (G) muscles, as compared to single Q-stimulation alters the knee extensor force production and discomfort.

Methods

A total of 11 healthy participants (6 females), with normal weight and age between 19 and 54 years were included. The unilateral, isometric maximal voluntary contraction (MVC) was assessed for each participant in an isokinetic dynamometer (Biodex, system 3). NMES was, in a randomized order, applied only on the Q-muscle and on the Q- and G-muscles (QG) simultaneously. NMES-intensity was increased stepwise until the maximal tolerable level was reached regarding discomfort, graded according to the visual analogue scale (VAS). VAS and the % of MVC produced by NMES, were registered for each level, expressed as median (inter-quartile range).

Results

The maximum tolerated NMES-intensity applied on Q compared to QG resulted in equally high discomfort, 8.0 (6.0–9.0) vs 8.0 (6.3–9.0), and in equivalent knee extensor force production, 36.7 (29.9–47.5) and 36.2 (28.9–49.3), respectively, in % of MVC. At 20% of MVC, NMES applied on Q compared to QG resulted in equal acceptable discomfort, 3.0 (2.0–4.5) vs 3.0 (3–5.5), and comparable intensity levels, 41.5 (38.0–45.8) vs 43.5 (37.0–48.8), respectively.

Conclusions

Simultaneous QG-NMES, as compared to single Q-NMES, does not seem to affect the knee extensor force production or discomfort. Q-NMES, without voluntary muscle contraction, can with an acceptable level of discomfort result in at least 20% of MVC.

The ergogenic effect of acute carnosine and anserine supplementation: dosing, timing, and underlying mechanism

Background

Recent studies suggest that acute-combined carnosine and anserine supplementation has the potential to improve the performance of certain cycling protocols. Yet, data on optimal dose, timing of ingestion, effective exercise range, and mode of action are lacking. Three studies were conducted to establish dosing and timing guidelines concerning carnosine and anserine intake and to unravel the mechanism underlying the ergogenic effects.

Methods

First, a dose response study A was conducted in which 11 men randomly received placebo, 10, 20, or 30 mg.kg⁻¹ of both carnosine and anserine. They performed 3x maximal voluntary isometric contractions (MVC), followed by a 5 x 6 s repeated cycling sprint ability test (RSA), once before the supplement and 30 and 60 minutes after. In a second study, 15 men performed 3x MVCs with femoral nerve electrical stimulation, followed by an RSA test, once before 30 mg.kg⁻¹ carnosine and anserine and 60 minutes after. Finally, in study C, eight men performed a high intensity cycling training after randomly ingesting 30 mg.kg⁻¹ of carnosine and anserine, a placebo or antihistamines (reduce post-exercise blood flow) to investigate effects on muscle perfusion.

Results

Study A showed a 3% peak power (p = 0.0005; 95% CI = 0.07 to 0.27; ES = 0.91) and 4.5% peak torque (p = 0.0006; 95% CI = 0.12 to 0.50; ES = 0.87) improvement on RSA and MVC, with 30 mg.kg⁻¹ carnosine + anserine ingestion 60 minutes before the performance yielding the best results. Study B found no performance improvement on group level; however, a negative correlation (r = −0.54; p = 0.0053; 95% CI = −0.77 to −0.19) was found between carnosinase enzyme activity (responsible for carnosine and anserine breakdown) and performance improvement. No effect of the supplement on neuromuscular function nor on muscle perfusion was found.

Conclusions

These studies reveal that acute ingestion of 30 mg.kg⁻¹ of both carnosine and anserine, 60 minutes before a high intensity exercise, can potentially improve performance, such as short cycling sprints or maximal muscle contractions. Subjects with lower carnosinase activity, and thus a slower breakdown of circulating dipeptides, appear to benefit more from this ergogenic effect. Finally, neither the involvement of a direct effect on neuromuscular function, nor an indirect effect on recovery through increased muscle perfusion could be confirmed as potential mechanism of action. The ergogenic mechanism therefore remains elusive.

The Effects of the Task Balance Training Program on the Glial Cell Line-Derived Neurotrophic Factor Levels, Cognitive Function, and Postural Balance in Old People

Exercise in the form of physical activity can provide neuroprotective benefits. The purpose of this study is to determine the effect of the task balance training program (TBT program) on the glial cell-derived neurotrophic factor levels, cognitive function, and postural balance in old people. The population of this study was the old people members of the Batara Hati Mulia Gowa Foundation who were willing to participate in the study (n = 66). The sample of this study was obtained through a random sampling technique to determine the treatment (n = 32) and control (n = 34) groups. Before and after implementing the TBT program, glial cell-derived neurotrophic factor (GDNF) level measurement and cognitive function and postural balance assessment were performed. Cognitive function was measured by using Montreal cognitive assessment (MoCA). Postural balance was measured in two ways by using the timed up and go (TUG) test and Tinetti performance-oriented mobility assessment (POMA). The treatment group showed significantly greater changes than the control group in GDNF levels (2.24 (±0.63) vs. 1.24 (±0.43), P = 0.001), cognitive function (24.66 (±3.42) vs. 19.18 (±2.67), P = 0.001), and postural balance (TUG [14.00 (±4.04) vs. 18.68 (±3.98)]; POMA [26.53 (±1.74) vs. 23.47 (±3.06)], P = 0.001) after training. The treatment group also showed a significant relationship between GDNF levels and cognitive function (r = 0.840, P = 0.001) and postural balance (TUG [r = 0.814, P = 0.001]; POMA [r = 0.630, P = 0.001]). The TBT program affects the levels of GDNF in old people. The TBT program involves cognitive function improvement and affects postural balance changes in old people.

A Systematic Review of the Effects of Different Resistance Training Volumes on Muscle Hypertrophy

The main goal of this study was to compare responses to moderate and high training volumes aimed at inducing muscle hypertrophy. A literature search on 3 databases (Pubmed, Scopus and Chocrane Library) was conducted in January 2021. After analyzing 2083 resultant articles, studies were included if they met the following inclusion criteria: a) studies were randomized controlled trials (with the number of sets explicitly reported), b) interventions lasted at least six weeks, c) participants had a minimum of one year of resistance training experience, d) participants’ age ranged from 18 to 35 years, e) studies reported direct measurements of muscle thickness and/or the cross-sectional area, and f) studies were published in peer-review journals. Seven studies met the inclusion criteria and were included in the qualitative analysis, whereas just six were included in the quantitative analysis. All participants were divided into three groups: “low” (<12 weekly sets), “moderate” (12-20 weekly sets) and “high” volume (>20 weekly sets). According to the results of this meta-analysis, there were no differences between moderate and high training volume responses for the quadriceps (p = 0.19) and the biceps brachii (p = 0.59). However, it appears that a high training volume is better to induce muscle mass gains in the triceps brachii (p = 0.01). According to the results of this review, a range of 12-20 weekly sets per muscle group may be an optimum standard recommendation for increasing muscle hypertrophy in young, trained men.

Performance and Recovery of Well-Trained Younger and Older Athletes during Different HIIT Protocols

Due to physiological and morphological differences, younger and older athletes may recover differently from training loads. High-intensity interval training (HIIT) protocols are useful for studying the progression of recovery. It was the objective of this study to determine age differences in performance and recovery following different HIIT protocols. Methods: 12 younger (24.5 ± 3.7 years) and 12 older (47.3 ± 8.6 years) well-trained cyclists and triathletes took part in this study. Between the age groups there were no significant differences in relative peak power to fat-free mass, maximal heart rate (HR), training volume, and VO_2max-percentiles (%). Participants performed different HIIT protocols consisting of 4 × 30 s Wingate tests with different active rest intervals (1, 3, or 10 min). Peak and average power, lactate, HR, respiratory exchange ratio (RER), subjective rating of perceived exertion (RPE), and recovery (Total Quality Recovery scale, TQR) were assessed. Results: During the different HIIT protocols, metabolic, cardiovascular, and subjective recovery were similar between the two groups. No significant differences were found in average lactate concentration, peak and average power, fatigue (%), %HR_max, RER, RPE, and TQR values between the groups (p > 0.05). Conclusion: The findings of this study indicate that recovery following HIIT does not differ between the two age groups. Furthermore, older and younger participants displayed similar lactate kinetics after the intermittent exercise protocols.

Association of muscle fiber composition with health and exercise-related traits in athletes and untrained subjects

Skeletal muscle is a heterogenous and metabolically active tissue, the composition of which is associated with multiple traits. The aim of the study was to determine whether there are additional health and exercise-related traits associated with muscle fiber composition in athletes and non-athletes. This study recruited 164 Russian participants (51 endurance and 48 power athletes; 65 controls). Vastus lateralis muscle fiber composition was assessed by immunohistochemistry. Slow-twitch muscle fiber percentage (STMF%) was significantly greater in endurance than power athletes and non-athletes, and in non-athlete females than males. STMF% was positively associated with athletes' training frequency, non-athletes' and endurance athletes' age, endurance athletes' competition level and chest depth, and power athletes' training age. STMF% was negatively associated with diastolic blood pressure in power athletes and with systolic blood pressure and reaction time in non-athletes. In all participants, STMF% was positively associated with age, tolerance to long distance exercise, chest depth and fracture incidence, and negatively with systolic blood pressure and resting heart rate. Age, sex and training frequency explained 10.6% and 13.2% of the variance in STMF% in endurance and power athletes, respectively. This is one of the most comprehensive studies involving athletes and untrained subjects and provides novel information concerning associations of increased STMF percentage with lower resting heart rate, better tolerance to long distances, faster reaction time and larger chest depth. On the other hand, the increased percentage of fast-twitch muscle fibers was associated with rare fracture incidence.

Skeletal Muscle Phenotype in Patients Undergoing Long-Term Hemodialysis Awaiting Kidney Transplantation

BACKGROUND AND OBJECTIVES

Age and comorbidity-related sarcopenia represent a main cause of muscle dysfunction in patients on long-term hemodialysis. However, recent findings suggest muscle abnormalities that are not associated with sarcopenia. The aim of this study was to isolate functional and cellular muscle abnormalities independently of other major confounding factors, including malnutrition, age, comorbidity, or sedentary lifestyle, which are common in patients on maintenance hemodialysis. To overcome these confounding factors, alterations in skeletal muscle were analyzed in highly selected patients on long-term hemodialysis undergoing kidney transplantation.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In total, 22 patients on long-term hemodialysis scheduled for kidney transplantation with few comorbidities, but with a long-term uremic milieu exposure, and 22 age, sex, and physical activity level frequency-matched control participants were recruited. We compared biochemical, functional, and molecular characteristics of the skeletal muscle using maximal voluntary force and endurance of the quadriceps, 6-minute walking test, and muscle biopsy of vastus lateralis. For statistical analysis, mean comparison and multiple regression tests were used.

RESULTS

In patients on long-term hemodialysis, muscle endurance was lower, whereas maximal voluntary force was not significantly different. We observed a transition from type I (oxidative) to type II (glycolytic) muscle fibers, and an alteration of mitochondrial structure (swelling) without changes in DNA content, genome replication (peroxisome proliferator activator receptor γ coactivator-1α and mitochondrial transcription factor A), regulation of fusion (mitofusin and optic atrophy 1), or fission (dynamin-related protein 1). Notably, there were autophagosome structures containing glycogen along with mitochondrial debris, with a higher expression of light chain 3 (LC3) protein, indicating phagophore formation. This was associated with a greater conversion of LC3-I to LC3-II and the expression of Gabaralp1 and Bnip3l genes involved in mitophagy.

CONCLUSIONS

In this highly selected long-term hemodialysis population, a low oxidative phenotype could be defined by a poor endurance, a fiber-type switch, and an alteration of mitochondria structure, without evidence of sarcopenia. This phenotype could be related to uremia through the activation of autophagy/mitophagy.

CLINICAL TRIAL REGISTRATION NUMBERS

NCT02794142 and NCT02040363.

Muscle Fibre Typology as a Novel Risk Factor for Hamstring Strain Injuries in Professional Football (Soccer): A Prospective Cohort Study

BACKGROUND

Hamstring strain injuries (HSI) are prevalent in team sports and occur frequently in the later phase of matches. In the search for interindividual factors that determine muscle fatigue and possibly injury risk, muscle fibre typology is a likely candidate.

OBJECTIVE

The aim of the study was to determine whether muscle fibre typology is a risk factor for HSI.

METHODS

A prospective cohort study was conducted over three seasons in professional football players competing in the Belgian Jupiler Pro League (n = 118) and in the English Premier League (n = 47). A total of 27 HSI were sustained during this period. Muscle fibre typology was non-invasively estimated using proton magnetic resonance spectroscopy and was characterized as a fast, slow, or intermediate typology based on the carnosine concentration in the soleus. A multivariate Cox model was used to identify risk factors for HSI.

RESULTS

Football players exhibited a wide variety of muscle typologies (slow 44.9%, intermediate 39.8%, fast 15.3%). In the combined cohort, players with a fast typology displayed a 5.3-fold (95% confidence interval [CI] 1.92-14.8; P = 0.001) higher risk of sustaining an index HSI than slow typology players. This was also independently observed in both leagues separately as, respectively, a 6.7-fold (95% CI 1.3-34.1; P = 0.023) and a 5.1-fold (95% CI 1.2-20.4; P = 0.023) higher chance was found in fast typology players than in slow typology players of the Jupiler Pro League and the Premier League cohort.

CONCLUSION

We identified muscle fibre typology as a novel and potent risk factor for HSI in team sports.

W' Recovery Kinetics after Exhaustion: A Two-Phase Exponential Process Influenced by Aerobic Fitness

PURPOSE

The aims of this study were 1) to model the temporal profile of W' recovery after exhaustion, 2) to estimate the contribution of changing V˙O2 kinetics to this recovery, and 3) to examine associations with aerobic fitness and muscle fiber type (MFT) distribution.

METHODS

Twenty-one men (age = 25 ± 2 yr, V˙O2peak = 54.4 ± 5.3 mL·min-1·kg-1) performed several constant load tests to determine critical power and W' followed by eight trials to quantify W' recovery. Each test consisted of two identical exhaustive work bouts (WB1 and WB2), separated by a variable recovery interval of 30, 60, 120, 180, 240, 300, 600, or 900 s. Gas exchange was measured and muscle biopsies were collected to determine MFT distribution. W' recovery was quantified as observed W' recovery (W'OBS), model-predicted W' recovery (W'BAL), and W' recovery corrected for changing V˙O2 kinetics (W'ADJ). W'OBS and W'ADJ were modeled using mono- and biexponential fitting. Root-mean-square error (RMSE) and Akaike information criterion (∆AICC) were used to evaluate the models' accuracy.

RESULTS

The W'BAL model (τ = 524 ± 41 s) was associated with an RMSE of 18.6% in fitting W'OBS and underestimated W' recovery for all durations below 5 min (P < 0.002). Monoexponential modeling of W'OBS resulted in τ = 104 s with RMSE = 6.4%. Biexponential modeling of W'OBS resulted in τ1 = 11 s and τ2 = 256 s with RMSE = 1.7%. W'ADJ was 11% ± 1.5% lower than W'OBS (P < 0.001). ∆AICC scores favored the biexponential model for W'OBS, but not for W'ADJ. V˙O2peak (P = 0.009) but not MFT distribution (P = 0.303) was associated with W'OBS.

CONCLUSION

We showed that W' recovery from exhaustion follows a two-phase exponential time course that is dependent on aerobic fitness. The appearance of a fast initial recovery phase was attributed to an enhanced aerobic energy provision resulting from changes in V˙O2 kinetics.

Anaerobic Speed/Power Reserve and Sport Performance: Scientific Basis, Current Applications and Future Directions

Many individual and team sport events require extended periods of exercise above the speed or power associated with maximal oxygen uptake (i.e., maximal aerobic speed/power, MAS/MAP). In the absence of valid and reliable measures of anaerobic metabolism, the anaerobic speed/power reserve (ASR/APR) concept, defined as the difference between an athlete's MAS/MAP and their maximal sprinting speed (MSS)/peak power (MPP), advances our understanding of athlete tolerance to high speed/power efforts in this range. When exercising at speeds above MAS/MAP, what likely matters most, irrespective of athlete profile or locomotor mode, is the proportion of the ASR/APR used, rather than the more commonly used reference to percent MAS/MAP. The locomotor construct of ASR/APR offers numerous underexplored opportunities. In particular, how differences in underlying athlete profiles (e.g., fiber typology) impact the training response for different 'speed', 'endurance' or 'hybrid' profiles is now emerging. Such an individualized approach to athlete training may be necessary to avoid 'maladaptive' or 'non-responses'. As a starting point for coaches and practitioners, we recommend upfront locomotor profiling to guide training content at both the macro (understanding athlete profile variability and training model selection, e.g., annual periodization) and micro levels (weekly daily planning of individual workouts, e.g., short vs long intervals vs repeated sprint training and recovery time between workouts). More specifically, we argue that high-intensity interval training formats should be tailored to the locomotor profile accordingly. New focus and appreciation for the ASR/APR is required to individualize training appropriately so as to maximize athlete preparation for elite competition.