Power-duration relationship: Physiology, fatigue, and the limits of human performance

A single bout of downhill running attenuates subsequent level running-induced fatigue

Fatigue can be defined as exercise-induced strength loss. During running, fatigue can be partially explained by repetitive low-intensity eccentric contractions-induced muscle damage (EIMD). Previous studies showed that a bout of downhill running (DR) attenuated subsequent EIMD. Thus, we tested if a 30-min DR bout would attenuate fatigue induced by subsequent 60-min level running (LR). Twenty-seven male college students were randomly allocated to an experimental (EXP) or a control (CON) group. All participants performed LR on a treadmill at 70% of the velocity (vVO₂peak) corresponding to peak oxygen uptake (VO₂peak). Only EXP performed a 30-min DR (− 15%) on a treadmill at 70% vVO₂peak fourteen days before LR. Indirect EIMD markers and neuromuscular function were assessed before, immediately and 48 h after DR and LR. Knee extension isometric peak torque (IPT) decreased (− 36.3 ± 26%, p < 0.05) immediately following DR with full recovery reached 48 h post-DR. Muscle soreness developed (p < 0.05) immediately (37 ± 25 mm) and 48 h (45 ± 26 mm) post-DR. IPT and rate of torque development (RTD) at late phases (> 150 ms) from the onset of muscle contraction decreased significantly (− 10.7 ± 6.1% and from − 15.4 to − 18.7%, respectively) immediately after LR for the CON group and remained below baseline values (− 5.6 ± 8.5% and from − 13.8 to − 14.9%, respectively) 48 h post-LR. However, IPT and late RTD were not significantly affected by LR for the EXP group, showing a group x time interaction effect. We concluded that a single DR bout can be used to attenuate fatigue induced by a LR performed fourteen days after.

High-intensity decreasing interval training (HIDIT) increases time above 90% V ˙ O2peak

PURPOSE

Training nearV ˙ O2max is considered to be the most effective way to enhanceV ˙ O2max. High-intensity interval training (HIIT) is a well-known time-efficient training method for improving cardiorespiratory and metabolic function andV ˙ O2max. While long HIIT bouts allowV ˙ O2max to be achieved quickly, short HIIT bouts improve time to exhaustion (Tlim). The aim of this study was to evaluate the time spent above 90%V ˙ O2peak (T > 90%V ˙ O2peak) during three different HIIT protocols.

METHODS

Twelve cyclists performed three HIIT sessions. Each protocol had the same work and recovery power and ratio of work·recovery-1. The protocols consisted of long-interval HIIT (LIHIIT, 3 min work-2 min recovery), short-interval HIIT (SIHIIT, 30 s work-20 s recovery), and high-intensity decreasing interval training (HIDIT, work from 3 min to 30 s and recovery from 2 min to 20 s). T > 90%V ˙ O2peak, Tlim, blood lactate [La], and rate of perceived exertion (RPE) were measured at Tlim.

RESULTS

T > 90%V ˙ O2peak was greater in HIDIT (312 ± 207 s) than in SIHIIT (182 ± 225 s; P = 0.036) or LIHIIT (179 ± 145 s; P = 0.027). Tlim was not significantly different (P > 0.05) between HIDIT (798 ± 185 s), SIHIIT (714 ± 265 s), and LIHIIT (664 ± 282). At Tlim, no differences in [La] and RPE were found between protocols (P > 0.05).

CONCLUSION

HIDIT showed the highest T > 90%V ˙ O2peak, suggesting that it may be a good strategy to increase time close toV ˙ O2peak, despite similar Tlim, [La], and RPE at Tlim.

Physiological sex differences affect the integrative response to exercise: acute and chronic implications

NEW FINDINGS

What is the topic of this review? We review sex differences within physiological systems implicated in exercise performance; specifically, how they integrate to determine metabolic thresholds and fatigability. Thereafter, we discuss the implications that these sex differences might have for long-term adaptation to exercise. What advances does it highlight? The review collates evidence from recent physiological studies that have investigated sex as a biological variable, demonstrating that the physiological response to equivalent 'dosages' of exercise is not the same in males and females; thus, highlighting the need to research diversity in physiological responses to interventions.

ABSTRACT

The anatomical and physiological differences between males and females are thought to determine differences in the limits of human performance. The notion of studying sex as a biological variable has recently been emphasized in the biosciences as a vital step in enhancing human health. In this review, we contend that the effects of biological sex on acute and chronic responses must be studied and accounted for when prescribing aerobic exercise, much like any intervention targeting the optimization of physiological function. Emerging evidence suggests that the response of physiological systems to exercise differs between males and females, potentially mediating the beneficial effects in healthy and clinical populations. We highlight evidence that integrative metabolic thresholds during exercise are influenced by phenotypical sex differences throughout many physiological systems. Furthermore, we discuss evidence that female skeletal muscle is more resistant to fatigue elicited by equivalent dosages of high-intensity exercise. How the different acute responses affect the long-term trainability of males and females is considered, with discussion about tailoring exercise to the characteristics of the individual presented within the context of biological sex. Finally, we highlight the influence of endogenous and exogenous sex hormones on physiological responses to exercise in females. Sex is one of many mediating influences on the outcomes of exercise, and with careful experimental designs, physiologists can advance the collective understanding of diversity in physiology and optimize outcomes for both sexes.

Sex differences in fatigability following exercise normalised to the power-duration relationship

KEY POINTS

Knee-extensors demonstrate greater fatigue resistance in females compared to males during single-limb and whole-body exercise. For single-limb exercise, the intensity-duration relationship is different between sexes, with females sustaining a greater relative intensity of exercise. This study established the power-duration relationship during cycling, then assessed fatigability during critical power-matched exercise within the heavy and severe intensity domains. When critical power and the curvature constant were expressed relative to maximal ramp test power, no sex difference was observed. No sex difference in time to task failure was observed in either trial. During heavy and severe intensity cycling, females experienced lesser muscle de-oxygenation. Following both trials, females experienced lesser reductions in knee-extensor contractile function, and following heavy intensity exercise, females experienced less reduction in voluntary activation. These data demonstrate that whilst the relative power-duration relationship is not different between males and females, the mechanisms of fatigability during critical power-matched exercise are mediated by sex.

ABSTRACT

Due to morphological differences, females demonstrate greater fatigue resistance of locomotor muscle during single-limb and whole-body exercise modalities. Whilst females sustain a greater relative intensity of single-limb, isometric exercise than males, limited investigation has been performed during whole-body exercise. Accordingly, this study established the power-duration relationship during cycling in 18 trained participants (eight females). Subsequently, constant-load exercise was performed at critical power (CP)-matched intensities within the heavy and severe domains, with the mechanisms of fatigability assessed via non-invasive neurostimulation, near-infrared spectroscopy and pulmonary gas exchange during and following exercise. Relative CP (72 ± 5 vs. 74 ± 2% P_max , P = 0.210) and curvature constant (51 ± 11 vs. 52 ± 10 J P_max ^-1 , P = 0.733) of the power-duration relationship were similar between males and females. Subsequent heavy (P = 0.758) and severe intensity (P = 0.645) exercise time to task failures were not different between sexes. However, females experienced lesser reductions in contractile function at task failure (P ≤ 0.020), and greater vastus lateralis oxygenation (P ≤ 0.039) during both trials. Reductions in voluntary activation occurred following both trials (P < 0.001), but were less in females following the heavy trial (P = 0.036). Furthermore, during the heavy intensity trial only, corticospinal excitability was reduced at the cortical (P = 0.020) and spinal (P = 0.036) levels, but these reductions were not sex-dependent. Other than a lower respiratory exchange ratio in the heavy trial for females (P = 0.039), no gas exchange variables differed between sexes (P ≥ 0.052). Collectively, these data demonstrate that whilst the relative power-duration relationship is not different between males and females, the mechanisms of fatigability during CP-matched exercise above and below CP are mediated by sex.

Blackcurrant extract does not affect the speed-duration relationship during high-intensity running

Anthocyanin-rich blackcurrant extract (BC) has been shown to ergogenically aid high-intensity exercise. Capacity for such exercise is evaluated by the hyperbolic speed-tolerable duration (S-D_tol) relationship. Therefore, in double-blinded and cross-over randomised controlled trials, 15 males underwent treadmill running incremental exercise testing and were assessed for S-D_tol, quantified by critical speed (CS) and D' (distance), and assessments of time to exhaustion performance to empirically test the limits of the S-D_tol relationship, after daily supplementation of 300 mg/d BC (105 mg/d anthocyanin) or placebo. Supplementation with BC did not change CS (placebo 12.1 ± 1.0 km/h vs BC 11.9 ± 1.0 km/h, p > .05) or D' (placebo 918.6 ± 223.2 m vs BC 965.2 ± 231.2 m, p > .05), although further analysis indicated D' increased in 60% of subject (p = .08), indicating a trend toward cohorts potentially benefiting from BC supplementation. BC supplementation did not change time to exhaustion at or above CS, maximal oxygen uptake (VO_2max), lactate threshold (LT), submaximal running economy (C_R), or substrate utilisation during exercise (all p > .05). In conclusion, we could not detect any beneficial effect of BC supplementation during high-intensity running exercise, including the determining factors S-D_tol relationship, VO_2max, LT or C_R. Hence, no ergogenic effect was observed.

Potential Biomarkers of Peripheral and Central Fatigue in High-Intensity Trained Athletes at High-Temperature: A Pilot Study with Momordica charantia (Bitter Melon)

Among potent dietary supplements, Momordica charantia, commonly called bitter melon, has various biological effects, such as antioxidant and anti-inflammatory effects, and improves energy metabolism and fatigue recovery. However, it is unknown whether Momordica charantia extract (MCE) induces antifatigue effects during exercise training in high-temperature environments. This study aimed at investigating the efficacy of MCE by examining 10 male tennis players consuming 100 mL MCE/dose (6 times a day over 4 weeks) during the summer training season. Peripheral (ammonia and uric acid) and central (serotonin, dopamine, and prolactin) fatigue parameters were measured before and after MCE consumption; before, during, and after exercise; and the next morning. After consuming MCE supplements, ammonia levels were higher during and after exercise and recovered the next morning, whereas uric acid levels did not change at any time point. Serotonin levels were lower during exercise. Dopamine levels were higher, especially during exercise. Prolactin levels were lower at all time points, especially during and after exercise. Although high-intensity training in a hot environment causes accumulation of fatigue-related metabolites, our results indicate that 4 weeks of MCE intake positively influenced fatigue parameters, suggesting that MCE can efficiently combat fatigue.

Is V̇O2peak a Valid Estimation of V̇O2max in Swimmers with Physical Impairments?

Purpose: Peak and maximal oxygen uptake ([Formula: see text] and [Formula: see text], respectively) are used in assessing aerobic power. For swimmers with physical impairments, it is unclear whether the physiological variables obtained in 200-m and Nx200-m tests are similar. The objective of this study is to assess the validity of [Formula: see text] as an estimator of [Formula: see text] and complementary physiological variables, in particular, carbon dioxide production ([Formula: see text]), respiratory exchange ratio (RER), minute-ventilation ([Formula: see text] and absolute (HR) and relative (%HRmax) heart rates-which were obtained in a time trial test (200-m) and an incremental intermittent test (Nx200-m) performed by swimmers with physical impairments. Methods: Eleven well-trained swimmers with physical impairments performed 200-m all-out and Nx200-m from low to all-out (controlled by a visual pacer), both with a respiratory valve system and a portable gas analyzer. Results: A paired Student's t-test showed no statistical difference (p > .05) for all comparisons. The intraclass correlation coefficient (ICC) was 0.97 and 0.98 for [Formula: see text] in l/min and ml/kg/min, respectively; ICC = 0.75 to 0.9 for [Formula: see text] (l/min and ml/kg/min),[Formula: see text] (in l/min) and HR (beats/min); ICC = 0.5 and 0.75 for %HRmax; and ICC < 0.5 for RER. Passing-Bablok regression showed that the dispersions were acceptable, considering the proportionality, except for HR and %HRmax. Bland-Altman method showed a high level of agreement for all variables. Conclusions: The [Formula: see text] and [Formula: see text], as well as the physiological variables [Formula: see text] and HR obtained, respectively, by 200-m and Nx200-m tests in swimmers with physical impairment were not different.

Fatigue Induced Changes in Muscle Strength and Gait Following Two Different Intensity, Energy Expenditure Matched Runs

Purpose

To investigate changes in hip and knee strength, kinematics, and running variability following two energy expenditure matched training runs; a medium intensity continuous run (MICR) and a high intensity interval training session (HIIT).

Methods

Twenty (10 Females, 10 Males) healthy master class runners were recruited. Each participant completed the HIIT consisting of six repetitions of 800 m with a 1:1 work: rest ratio. The MICR duration was set to match energy expenditure of the HIIT session. Hip and knee muscular strength were examined pre and post both HIIT and MICR. Kinematics and running variability for hip and knee, along with spatiotemporal parameters were assessed at start and end of each run-type. Changes in variables were examined using both 2 × 2 ANOVAs with repeated measures and on an individual level when the change in a variable exceeded the minimum detectable change (MDC).

Results

All strength measures exhibited significant reductions at the hip and knee (P < 0.05) with time for both run-types; 12% following HIIT, 10.6% post MICR. Hip frontal plane kinematics increased post run for both maximum angle (P < 0.001) and range of motion (P = 0.003). Runners exhibited increased running variability for nearly all variables, with the HIIT having a greater effect. Individual assessment revealed that not all runners were effected post run and that following HIIT more runners had reduced muscular strength, altered kinematics and increased running variability.

Conclusion

Runners exhibited fatigue induced changes following typical training runs, which could potentially present risk of injury development. Group and individual assessment revealed different findings where the use of MDC is recommended over that of P-values.

Effects of Prior Upper Body Exercise on the 3-min All-Out Cycling Test in Men

INTRODUCTION

Prior upper body exercise reduces the curvature constant (W') of the hyperbolic power-duration relationship without affecting critical power. This study tested the hypothesis that prior upper body exercise reduces the work done over the end-test power (WEP; analog of W') during a 3-min all-out cycling test (3MT) without affecting the end-test power (EP; analog of critical power).

METHODS

Ten endurance-trained men (V˙O2max = 62 ± 5 mL·kg·min) performed a 3MT without (CYC) and with (ARM-CYC) prior severe-intensity, intermittent upper body exercise. EP was calculated as the mean power output over the last 30 s of the 3MT, whereas WEP was calculated as the power-time integral above EP.

RESULTS

At the start of the 3MT, plasma [La] (1.8 ± 0.4 vs 14.1 ± 3.4 mmol·L) and [H] (42.8 ± 3.1 vs 58.6 ± 5.5 nmol·L) were higher, whereas the strong ion difference (41.4 ± 2.2 vs 30.9 ± 4.6 mmol·L) and [HCO3] (27.0 ± 1.9 vs 16.9 ± 3.2 mmol·L) were lower during ARM-CYC than CYC (P < 0.010). EP was 12% lower during the 3MT of ARM-CYC (298 ± 52 W) than CYC (338 ± 60 W; P < 0.001), whereas WEP was not different (CYC: 12.8 ± 3.3 kJ vs ARM-CYC: 13.5 ± 4.1 kJ, P = 0.312). EP in CYC was positively correlated with the peak [H] (r = 0.78, P = 0008) and negatively correlated with the lowest [HCO3] (r = -0.74, P = 0.015).

CONCLUSIONS

These results suggest that EP during a 3MT in endurance-trained men is sensitive to fatigue-related ionic perturbation.

Sex differences in fatigability and recovery relative to the intensity-duration relationship

KEY POINTS

Females demonstrate greater fatigue resistance than males during contractions at intensities relative to maximum force. However, previous studies have not accounted for the influence of metabolic thresholds on fatigability. This study is the first to test whether sex differences in fatigability exist when exercise intensity is normalised relative to a metabolic threshold: the critical intensity derived from assessment of the intensity-duration relationship during intermittent, isometric knee extensor contractions. We show that critical intensity in females occurred at a higher percentage of maximum force compared to males. Furthermore, females demonstrated greater fatigue resistance at exercise intensities above and below this metabolic threshold. Our data suggest that the sex difference was mediated by lesser deoxygenation of the knee extensors during exercise. These data highlight the importance of accounting for metabolic thresholds when comparing fatigability between sexes, whilst emphasising the notion that male data are not generalisable to female populations.

ABSTRACT

Females are less fatigable than males during isometric exercise at intensities relative to maximal voluntary contraction (MVC); however, whether a sex difference in fatigability exists when exercise is prescribed relative to a critical intensity is unknown. This study established the intensity-duration relationship, and compared fatigability and recovery between sexes following intermittent isometric contractions normalised to critical intensity. Twenty participants (10 females) completed four intermittent isometric knee extension trials to task failure to determine critical intensity and the curvature constant (W'), followed by fatiguing tasks at +10% and -10% relative to critical intensity. Neuromuscular assessments were completed at baseline and for 45 min post-exercise. Non-invasive neurostimulation, near-infrared spectroscopy, and non-invasive haemodynamic monitoring were used to elucidate the physiological mechanisms responsible for sex differences. Females demonstrated a greater critical intensity relative to MVC than males (25 ± 3 vs. 21 ± 2% MVC, P = 0.003), with no sex difference for W' (18,206 ± 6331 vs. 18,756 ± 5762 N s, P = 0.850). Time to task failure was greater for females (62.37 ± 17.25 vs. 30.43 ± 12.75 min, P < 0.001) during the +10% trial, and contractile function recovered faster post-exercise (P = 0.034). During the -10% trial females experienced less contractile dysfunction (P = 0.011). Throughout the +10% trial, females demonstrated lesser decreases in deoxyhaemoglobin (P = 0.007) and an attenuated exercise pressor reflex. These data show that a sex difference in fatigability exists even when exercise is matched for critical intensity. We propose that greater oxygen availability during exercise permits females to sustain a higher relative intensity than males, and is an explanatory factor for the sex difference in fatigability during intermittent, isometric contractions.

The effect of low dose marine protein hydrolysates on short-term recovery after high intensity performance cycling: a double-blinded crossover study

Background

Knowledge of the effect of marine protein hydrolysate (MPH) supplementation to promote recovery after high intensity performance training is scarce. The aim of this study was to examine the effect of MPH supplementation to whey protein (WP) and carbohydrate (CHO): (CHO-WP-MPH), on short-term recovery following high intensity performance, compared to an isoenergetic and isonitrogenous supplement of WP and CHO: (CHO-WP), in male cyclists.

Methods

This was a double-blinded crossover study divided into three phases. Fourteen healthy men participated. In phase I, an incremental bicycle exercise test was performed for establishment of intensities used in phase II and III. In phase II (9–16 days after phase 1), the participants performed first one high intensity performance cycling session, followed by nutrition supplementation (CHO-WP-MPH or CHO-WP) and 4 hours of recovery, before a subsequent high intensity performance cycling session. Phase III (1 week after phase II), was similar to phase II except for the nutrition supplementation, where the participants received the opposite supplementation compared to phase II. Primary outcome was difference in time to exhaustion between the cycling sessions, after nutrition supplementations containing MPH or without MPH. Secondary outcomes were differences in heart rate (HR), respiratory exchange ratio (RER), blood lactate concentration and glucose.

Results

The mean age of the participants was 45.6 years (range 40–58). The maximal oxygen uptake (mean ± SD) measured at baseline was 54.7 ± 4.1 ml∙min^− 1∙kg^− 1. There were no significant differences between the two nutrition supplementations measured by time to exhaustion at the cycling sessions (mean_diff = 0.85 min, p = 0.156, 95% confidence interval (CI), − 0.37, 2.06), HR (mean_diff = 0.8 beats pr.min, p = 0.331, 95% CI, − 0.9, 2.5), RER (mean_diff = − 0.05, p = 0.361, 95% CI -0.07 – 0.17), blood lactate concentration (mean_diff = − 0.24, p = 0.511, 95% CI, − 1.00, 0.53) and glucose (mean_diff = 0.23, p = 0.094, 95% CI, − 0.05, 0.51).

Conclusions

A protein supplement with MPH showed no effects on short-term recovery in middle-aged healthy male cyclists compared to a protein supplement without MPH.

Trial registration

The study was registered 02.05.2017 at ClinicalTrials.gov (Protein Supplements to Cyclists, NCT03136133, https://clinicaltrials.gov/ct2/show/NCT03136133?cond=marine+peptides&rank=1.

Characterization of performance fatigability during a self-paced exercise

Pacing during a high-intensity cycling time trial (TT) appears to prevent premature task failure, but the performance fatigability during a self-paced exercise is currently unknown. Therefore, the current study characterized the time course of performance fatigability during a 4-km TT. Eleven male cyclists performed three separated TTs in a crossover, counterbalanced design. The TTs lasted until the end of the fast-start (FS; 600 ± 205 m), even-pace (EP; 3,600 ± 190 m), and end-spurt (ES; 4,000 m) phases. Performance fatigability was characterized by using isometric maximal voluntary contractions (IMVCs), whereas the muscle activation [i.e., voluntary activation (VA)] and contractile function of knee extensors [e.g., peak torque of potentiated twitches (TwPt)] were evaluated using electrically evoked contractions performed before and 1 min after each specific part of the trial. Gas exchange, power output (PO), and electromyographic activity (EMG) were also recorded. EMG/PO showed an abrupt increase followed by a continuous decrease toward the end of FS, resulting in a drop in IMVC (−12%), VA (−8%), and TwPt (−23%). EMG/PO was stable during EP, with no additional drop on IMVC, VA, or TwPt (−12%, −6%, and −22%, respectively). EMG/PO increased abruptly during the ES, but there was no change in IMVCs, VA, or TwPt (−13%, −8%, and −26%, respectively). These findings demonstrate that the performance fatigability during a self-paced exercise is characterized by a large drop in contractile function and muscle activation at the beginning of the trial (i.e., FS), without additional change during the middle and end phases (i.e., EP and ES).

NEW & NOTEWORTHY The time course of performance fatigability throughout a self-paced exercise is currently unknown. The results showed that a large amount of muscle activation and contractile function impairments are attained early on a self-paced exercise (first ∼15% of the total time trial distance) and maintained throughout the test. This novel finding characterizes the performance fatigability from a contractile function and muscle activation perspective, which brings new insights for future studies focused on real-world exercise training and competition.

Fatigue, Sleep, and Autoimmune and Related Disorders

Profound and debilitating fatigue is the most common complaint reported among individuals with autoimmune disease, such as systemic lupus erythematosus, multiple sclerosis, type 1 diabetes, celiac disease, chronic fatigue syndrome, and rheumatoid arthritis. Fatigue is multi-faceted and broadly defined, which makes understanding the cause of its manifestations especially difficult in conditions with diverse pathology including autoimmune diseases. In general, fatigue is defined by debilitating periods of exhaustion that interfere with normal activities. The severity and duration of fatigue episodes vary, but fatigue can cause difficulty for even simple tasks like climbing stairs or crossing the room. The exact mechanisms of fatigue are not well-understood, perhaps due to its broad definition. Nevertheless, physiological processes known to play a role in fatigue include oxygen/nutrient supply, metabolism, mood, motivation, and sleepiness-all which are affected by inflammation. Additionally, an important contributing element to fatigue is the central nervous system-a region impacted either directly or indirectly in numerous autoimmune and related disorders. This review describes how inflammation and the central nervous system contribute to fatigue and suggests potential mechanisms involved in fatigue that are likely exhibited in autoimmune and related diseases.

Dynamics of the power-duration relationship during prolonged endurance exercise and influence of carbohydrate ingestion

We tested the hypotheses that the parameters of the power-duration relationship, estimated as the end-test power (EP) and work done above EP (WEP) during a 3-min all-out exercise test (3MT), would be reduced progressively after 40 min, 80 min, and 2 h of heavy-intensity cycling and that carbohydrate (CHO) ingestion would attenuate the reduction in EP and WEP. Sixteen participants completed a 3MT without prior exercise (control), immediately after 40 min, 80 min, and 2 h of heavy-intensity exercise while consuming a placebo beverage, and also after 2 h of heavy-intensity exercise while consuming a CHO supplement (60 g/h CHO). There was no difference in EP measured without prior exercise (260 ± 37 W) compared with EP after 40 min (268 ± 39 W) or 80 min (260 ± 40 W) of heavy-intensity exercise; however, after 2 h EP was 9% lower compared with control (236 ± 47 W; P < 0.05). There was no difference in WEP measured without prior exercise (17.9 ± 3.3 kJ) compared with after 40 min of heavy-intensity exercise (16.1 ± 3.3 kJ), but WEP was lower (P < 0.05) than control after 80 min (14.7 ± 2.9 kJ) and 2 h (13.8 ± 2.7 kJ). Compared with placebo, CHO ingestion negated the reduction of EP following 2 h of heavy-intensity exercise (254 ± 49 W) but had no effect on WEP (13.5 ± 3.4 kJ). These results reveal a different time course for the deterioration of EP and WEP during prolonged endurance exercise and indicate that EP is sensitive to CHO availability.NEW & NOTEWORTHY The parameters of the power-duration relationship [critical power (CP) and the curvature constant (W')] have typically been considered to be static. Here we report the time course for reductions in CP and W', as estimated with the 3-min all-out cycle test, during 2 h of heavy-intensity exercise. We also show that carbohydrate ingestion during exercise preserves CP, but not W', without altering muscle glycogen depletion. These results provide new mechanistic and practical insight into the power-duration curve and its relationship to exercise-related fatigue development.

Pompe disease: what are we missing?

Pompe disease is a multisystemic metabolic disorder caused by a deficiency of lysosomal acid alpha-glucosidase (GAA) leading to progressive accumulation of lysosomal glycogen, lysosomal swelling and rupture in all tissues of the human body. Furthermore, autophagic buildup, organelle abnormalities, and energy deficit are regularly observed. Enzyme replacement therapy has been available for patients living with Pompe disease for more than 15 years. Although our disease knowledge has grown enormously, we still have multiple challenges to overcome. Here, I will discuss unmet clinical needs, neglected or overlooked aspects of the pathophysiology, and issues related to future therapies.

Bioenergetic basis of skeletal muscle fatigue

Energetic demand from high-intensity exercise can easily exceed ATP synthesis rates of mitochondria leading to a reliance on anaerobic metabolism. The reliance on anaerobic metabolism results in the accumulation of intracellular metabolites, namely inorganic phosphate (P_i) and hydrogen (H⁺), that are closely associated with exercise-induced reductions in power. Cellular and molecular studies have revealed several steps where these metabolites impair contractile function demonstrating a causal role in fatigue. Elevated P_i or H⁺ directly inhibits force and power of the cross-bridge and decreases myofibrillar Ca²⁺ sensitivity, whereas P_i also inhibits Ca²⁺ release from the sarcoplasmic reticulum (SR). When both metabolites are elevated, they act synergistically to cause marked reductions in power, indicating that fatigue during high-intensity exercise has a bioenergetic basis.

The maximal metabolic steady state: redefining the 'gold standard'

The maximal lactate steady state (MLSS) and the critical power (CP) are two widely used indices of the highest oxidative metabolic rate that can be sustained during continuous exercise and are often considered to be synonymous. However, while perhaps having similarities in principle, methodological differences in the assessment of these parameters typically result in MLSS occurring at a somewhat lower power output or running speed and exercise at CP being sustainable for no more than approximately 20-30 min. This has led to the view that CP overestimates the 'actual' maximal metabolic steady state and that MLSS should be considered the 'gold standard' metric for the evaluation of endurance exercise capacity. In this article we will present evidence consistent with the contrary conclusion: i.e., that (1) as presently defined, MLSS naturally underestimates the actual maximal metabolic steady state; and (2) CP alone represents the boundary between discrete exercise intensity domains within which the dynamic cardiorespiratory and muscle metabolic responses to exercise differ profoundly. While both MLSS and CP may have relevance for athletic training and performance, we urge that the distinction between the two concepts/metrics be better appreciated and that comparisons between MLSS and CP, undertaken in the mistaken belief that they are theoretically synonymous, is discontinued. CP represents the genuine boundary separating exercise in which physiological homeostasis can be maintained from exercise in which it cannot, and should be considered the gold standard when the goal is to determine the maximal metabolic steady state.

Dietary Nitrate Supplementation Improves Exercise Tolerance by Reducing Muscle Fatigue and Perceptual Responses

The present study was designed to provide further insight into the mechanistic basis for the improved exercise tolerance following dietary nitrate supplementation. In a randomized, double-blind, crossover design, twelve recreationally active males completed a dynamic time-to-exhaustion test of the knee extensors after 5 days of consuming both nitrate-rich (NITRATE) and nitrate-depleted beetroot juice (PLACEBO). Participants who improved their time-to-exhaustion following NITRATE performed a time-matched trial corresponding to the PLACEBO exercise duration with another 5 days of dietary nitrate supplementation. This procedure was performed to obtain time-matched exercise trials with (NITRATE_tm) and without dietary nitrate supplementation (PLACEBO). Neuromuscular tests were performed before and after each time-matched condition. Muscle fatigue was quantified as percentage change in maximal voluntary torque from pre- to post-exercise (ΔMVT). Changes in voluntary activation (ΔVA) and quadriceps twitch torque (ΔPS100) were used to quantify central and peripheral factors of muscle fatigue, respectively. Muscle oxygen saturation, quadriceps muscle activity as well as perceptual data (i.e., perception of effort and leg muscle pain) were recorded during exercise. Time-to-exhaustion was improved with NITRATE (12:41 ± 07:18 min) compared to PLACEBO (09:03 ± 04:18 min; P = 0.010). NITRATE_tm resulted in both lower ΔMVT and ΔPS100 compared to PLACEBO (P = 0.002; P = 0.001, respectively). ΔVA was not different between conditions (P = 0.308). NITRATE_tm resulted in reduced perception of effort and leg muscle pain. Our findings extend the mechanistic basis for the improved exercise tolerance by showing that dietary nitrate supplementation (i) attenuated the development of muscle fatigue by reducing the exercise-induced impairments in contractile muscle function; and (ii) lowered the perception of both effort and leg muscle pain during exercise.

Critical Speed as a Measure of Aerobic Fitness for Male Rugby Union Players

PURPOSE

To compare critical speed (CS) derived from all-out testing (AOT) for linear and shuttle running with metrics from a graded exercise test, the Yo-Yo Intermittent Recovery Test  Level 1 (YYIR1), and estimation of an 800-m-shuttle time trial.

METHODS

Twelve male rugby players completed a graded exercise test, the YYIR1, a linear AOT, shuttle AOTs of 25 and 50 m, and an 800-m-shuttle time trial consisting of 32 × 25-m shuttles.

RESULTS

Strong linear correlations were observed between maximum oxygen uptake ( V˙O2max ) and CS (m·s^-1) derived from the linear AOT (3.68 [0.62], r = .90, P < .01) and 50-m-shuttle AOT (3.19 [0.26], r = .83, P < .01). Conversely, V˙O2max showed lower correlations with speeds evoking CS from 25-m AOT (2.86 [0.18], r = .42, P = .18) and YYIR1 (4.36 [0.11], r = .55, P = .07). The 800-m time trial (213.58 [15.84] s) was best predicted using parameters from the 25-m AOT (r = .93, SEE = 6.60 s, P < .001).

CONCLUSIONS

The AOT is a valuable method of assessing performance-specific fitness, with CS from linear and 50-m-shuttle AOTs being strong predictors of V˙O2max , rivaling metrics from the graded exercise test. The YYIR1 offered limited utility compared with the AOT method.

Dynamics of pleasure-displeasure at the limit of exercise tolerance: conceptualizing the sense of exertional physical fatigue as an affective response

The search for variables involved in the regulation and termination of exercise performance has led to integrative models that attribute a central role to the brain and utilize an array of psychological terms (e.g. sensation, perception, discomfort, tolerance). We propose that theorizing about exercise regulation would benefit from establishing cross-disciplinary bridges to research fields, such as affective psychology and neuroscience, in which changes along the dimension of pleasure-displeasure are considered the main channel via which homeostatic perturbations enter consciousness and dictate corrective action (slowing down or stopping). We hypothesized that ratings of pleasure-displeasure would respond to the severity of homeostatic perturbation and would be related to time to exhaustion during exercise performed at an unsustainable intensity. In a within-subjects experiment (N=15, 13 men and 2 women, age 23.4±2.2 years; maximal oxygen uptake 46.0±8.0 ml kg^-1 min^-1), we compared the slope of ratings of pleasure-displeasure (acquired every 1 min) during cycling exercise at a power output 10% above critical power until volitional termination under glycogen-loaded and glycogen-depleted conditions. As hypothesized, ratings of pleasure-displeasure declined more steeply under glycogen depletion (P=0.009, d=0.70) and correlated closely with time to exhaustion under both glycogen-loaded (r=0.85; P<0.001) and glycogen-depleted conditions (r=0.83; P<0.001). We conclude that in exercise, as in other domains, changes in pleasure-displeasure may be the main channel via which homeostatic perturbations enter consciousness. This proposal may have important implications for conceptualizing and identifying the neurobiological mechanisms of the sense of exertional physical fatigue.

Prediction of power output at different running velocities through the two-point method with the Stryd™ power meter

BACKGROUND

The force- and power-velocity (F-V and P-V, respectively) relationships have been extensively studied in recent years. However, its use and application in endurance running events is limited.

RESEARCH QUESTION

This study aimed to determine if the P-V relationship in endurance runners fits a linear model when running at submaximal velocities, as well as to examine the feasibility of the "two-point method" for estimating power values at different running velocities.

METHODS

Eighteen endurance runners performed, on a motorized treadmill, an incremental running protocol to exhaustion. Power output was obtained at each stage with the Stryd™ power meter. The P-V relationship was determined from a multiple-point method (10, 12, 14, and 17 km·h^-1) as well as from three two-point methods based on proximal (10 and 12 km·h^-1), intermediate (10 and 14 km·h^-1) and distal (10 and 17 km·h^-1) velocities.

RESULTS

The P-V relationship was highly linear ( r = 0.999). The ANOVAs revealed significant, although generally trivial (effect size < 0.20), differences between measured and estimated power values at all the velocities tested. Very high correlations ( r = 0.92) were observed between measured and estimated power values from the 4 methods, while only the multiple-point method ( r² = 0.091) and two-point method distal ( r² = 0.092) did not show heteroscedasticity of the error.

SIGNIFICANCE

The two-point method based on distant velocities (i.e., 10 and 17 km·h^-1) is able to provide power output with the same accuracy than the multiple-point method. Therefore, since the two-point method is quicker and less prone to fatigue, we recommend the assessment of power output under only two distant velocities to obtain an accurate estimation of power under a wide range of submaximal running velocities.

VO₂FITTING: A Free and Open-Source Software for Modelling Oxygen Uptake Kinetics in Swimming and other Exercise Modalities

The assessment of oxygen uptake (VO₂) kinetics is a valuable non-invasive way to evaluate cardiorespiratory and metabolic response to exercise. The aim of the study was to develop, describe and evaluate an online VO₂ fitting tool (VO₂FITTING) for dynamically editing, processing, filtering and modelling VO₂ responses to exercise. VO₂FITTING was developed in Shiny, a web application framework for R language. Validation VO₂ datasets with both noisy and non-noisy data were developed and applied to widely-used models (n = 7) for describing different intensity transitions to verify concurrent validity. Subsequently, we then conducted an experiment with age-group swimmers as an example, illustrating how VO₂FITTING can be used to model VO₂ kinetics. Perfect fits were observed, and parameter estimates perfectly matched the known inputted values for all available models (standard error = 0; p < 0.001). The VO₂FITTING is a valid, free and open-source software for characterizing VO₂ kinetics in exercise, which was developed to help the research and performance analysis communities.

The magnitude of neuromuscular fatigue is not intensity dependent when cycling above critical power but relates to aerobic and anaerobic capacities

NEW FINDINGS

What is the central question of this study? Is the magnitude of neuromuscular fatigue dependent upon exercise intensity above critical power (CP) when W' (the curvature constant of the power-duration relationship) is depleted? What is the main finding and its importance? The magnitude of neuromuscular fatigue is the same after two bouts of supra-CP cycling (3 versus 12 min) when controlling for W' depletion but is larger for individuals of greater anaerobic capacity after the shorter bout and smaller for individuals of greater aerobic capacity after the longer exercise bout. These findings provide new insight into the mechanisms underpinning exercise above CP.

ABSTRACT

The aim of the present study was to test whether the development of neuromuscular fatigue within the severe-intensity domain could be linked to the depletion of the curvature constant (W') of the power-duration relationship. Twelve recreationally active men completed tests to determine peak oxygen consumption, critical power (CP) and W', followed by two randomly assigned constant-load supra-CP trials set to deplete W' fully in 3 (P-3) and 12 min (P-12). Pre- to postexercise changes in maximal voluntary contraction, potentiated quadriceps twitch force evoked by single (Q_pot ) and paired high- (PS100) and low-frequency (PS10) stimulations and voluntary activation were determined. Cycling above CP reduced maximal voluntary contraction (P-3, -20 ± 10% versus P-12, -15 ± 7%), measures associated with peripheral fatigue (Q_pot , -35 ± 13 versus -31 ± 14%; PS10, -38 ± 13 versus -37 ± 17%; PS100, -18 ± 9 versus -13 ± 8% for P-3 and P-12, respectively) and voluntary activation (P-3, -12 ± 3% versus P-12, -13 ± 3%; P < 0.05), with no significant difference between trials (P > 0.05). Changes in maximal voluntary contraction and evoked twitch forces were inversely correlated with CP and peak oxygen consumption after P-12, whereas W' was significantly correlated with changes in Q_pot and PS10 after P-3 (P < 0.05). Therefore, the magnitude of neuromuscular fatigue does not depend on exercise intensity when W' is fully exhausted during severe-intensity exercise; nonetheless, exploration of inter-individual variations suggests that mechanisms underpinning exercise tolerance within this domain differ between short- and long-duration exercise.

The relation between maximal voluntary force in m. palmaris longus and the temporal and spatial summation of muscle fiber recruitment in human subjects

This study aimed at looking at the frequency (T‐score) and the amplitude (S‐score) of fiber use during contraction of a forearm muscle, m. palmaris longus, as measured by acoustic myography (AMG). An additional aim was to relate the T‐ and S‐scores to the recorded force obtained from a hand dynamometer. The hypothesis being that temporal and spatial summation of muscle fiber contraction in a given muscle during a given movement, can together describe a given obtained force. Force measurements were carried out on 12 healthy human subjects aged 19–68 years (6 men & 6 women), while their m. palmaris longus contractile function was measured using an acoustic myography CURO device. Force production was varied from 90 to 10% of assessed maximal voluntary force (MVF), and also monitored over a 1 min period of 50% MVF. Linear regression analysis was applied to relate force to spatial and temporal summation. Muscle strength was sustained by changing the frequency and/or the number of active fibere at any given point in time. Force production, whilst stronger for men than women, was regulated in a similar fashion for both sexes and was closely correlated with the AMG T‐ and S‐scores. It is concluded that AMG is a noninvasive method which can be readily applied to accurately describe how a subject uses a given muscle during any given movement. These findings have relevance when considering training strategies in subjects with muscle trauma or disease, in the elderly, or for both amateur and top professional athletes.

A minimal power model for human running performance

Models for human running performances of various complexities and underlying principles have been proposed, often combining data from world record performances and bio-energetic facts of human physiology. The purpose of this work is to develop a novel, minimal and universal model for human running performance that employs a relative metabolic power scale. The main component is a self-consistency relation for the time dependent maximal power output. The analytic approach presented here is the first to derive the observed logarithmic scaling between world (and other) record running speeds and times from basic principles of metabolic power supply. Our hypothesis is that various female and male record performances (world, national) and also personal best performances of individual runners for distances from 800m to the marathon are excellently described by this model. Indeed, we confirm this hypothesis with mean errors of (often much) less than 1%. The model defines endurance in a way that demonstrates symmetry between long and short racing events that are separated by a characteristic time scale comparable to the time over which a runner can sustain maximal oxygen uptake. As an application of our model, we derive personalized characteristic race speeds for different durations and distances.

The cardinal exercise stopper: Muscle fatigue, muscle pain or perception of effort?

The capacity to sustain high-intensity aerobic exercise is essential for endurance performance. Therefore, it is important to understand what is the factor limiting time to exhaustion (TTE) in healthy and fit adults. In Study 1, maximal voluntary cycling power (MVCP) was measured in 11 volunteers before and immediately after a high-intensity TTE test on cycle ergometer. Cadence was 60 rpm in both the MVCP and TTE tests. Despite a 35% loss in MVCP, power produced during the final MVCP test (mean ± SD 469 ± 111 W) was significantly higher than the power required by the TTE test (269 ± 55 W) (P < 0.001). In Study 2, 12 participants performed a cold pressor test (CPT) to the limit of tolerance followed by a high-intensity TTE test on cycle ergometer. Ratings of pain unpleasantness (RPU) during the TTE test were anchored to the unpleasantness of pain experienced during the CPT. On average, the RPU was 9.7 ± 0.4 at completion of the CPT and 5.0 ± 0.9 at exhaustion during the TTE test. The difference between these two ratings of pain unpleasantness was statistically significant (P < 0.001). In both Studies 1 and 2, the slope of the rating of perceived exertion (RPE) during the TTE test correlated significantly with TTE (r = -0.75 and -0.83, P < 0.01). Results of this two-part investigation suggest that perception of effort, rather than severe locomotor muscle fatigue or intolerably unpleasant muscle pain, is the cardinal exercise stopper during high-intensity aerobic exercise.

Modelling of Running Performances: Comparisons of Power-Law, Hyperbolic, Logarithmic, and Exponential Models in Elite Endurance Runners

Many empirical and descriptive models have been proposed since the beginning of the 20th century. In the present study, the power-law (Kennelly) and logarithmic (Péronnet-Thibault) models were compared with asymptotic models such as 2-parameter hyperbolic models (Hill and Scherrer), 3-parameter hyperbolic model (Morton), and exponential model (Hopkins). These empirical models were compared from the performance of 6 elite endurance runners (P. Nurmi, E. Zatopek, J. Väätäinen, L. Virén, S. Aouita, and H. Gebrselassie) who were world-record holders and/or Olympic winners and/or world or European champions. These elite runners were chosen because they participated several times in international competitions over a large range of distances (1500, 3000, 5000, and 10000 m) and three also participated in a marathon. The parameters of these models were compared and correlated. The less accurate models were the asymptotic 2-parameter hyperbolic models but the most accurate model was the asymptotic 3-parameter hyperbolic model proposed by Morton. The predictions of long-distance performances (maximal running speeds for 30 and 60 min and marathon) by extrapolation of the logarithmic and power-law models were more accurate than the predictions by extrapolation in all the asymptotic models. The overestimations of these long-distance performances by Morton's model were less important than the overestimations by the other asymptotic models.

Mental fatigue does not alter performance or neuromuscular fatigue development during self-paced exercise in recreationally trained cyclists

PURPOSE

The purpose of this study was to investigate the effects of mental fatigue, characterized by a subjective feeling of tiredness, on the development of neuromuscular fatigue during a 4-km cycling time trial (4-km TT).

METHODS

Eight recreationally trained male cyclists performed a 4-km TT after either performing a prolonged cognitive task (mental fatigue) or after viewing emotionally neutral documentaries (control). The neuromuscular function of the knee extensors was assessed using electrical nerve stimulation at baseline, before (pre-TT), and after (post-TT) the 4-km TT. Rating of perceived exertion (RPE) and physiological variables were periodically measured during 4-km TT.

RESULTS

Subjective ratings of fatigue increased significantly only after a prolonged cognitive task (P = 0.022). Neuromuscular function at baseline was similar between conditions and remained unchanged at pre-TT. Time to complete the 4-km TT was similar between control (376 ± 27 s) and mental fatigue (376 ± 26 s). There was no significant difference between conditions for RPE, [Formula: see text], [Formula: see text], and HR throughout the exercise. The 4-km TT-induced similar decrease (from baseline to post-TT) in maximal voluntary contraction (mental fatigue - 11 ± 10%, control - 16 ± 12%), twitch force (mental fatigue - 26 ± 16%, control - 24 ± 17%), and voluntary activation (mental fatigue - 5 ± 7%, control - 3 ± 2%) for both conditions.

CONCLUSION

Mental fatigue induced by prolonged cognitive task does not impair performance nor alter the degree of central and peripheral fatigue development during self-paced exercise in recreationally trained cyclists.

Sex differences in diaphragmatic fatigue: the cardiovascular response to inspiratory resistance

KEY POINTS

Diaphragmatic fatigue (DF) elicits a sympathetically mediated metaboreflex resulting in increased heart rate, blood pressure and limb vascular resistance. Women may be more resistant to DF compared to men, and therefore it was hypothesised that women would experience an attenuated inspiratory muscle metaboreflex during inspiratory pressure-threshold loading (PTL) performed to task failure. At the time of PTL task failure, the severity of DF was not different between sexes; however, inspiratory muscle endurance time was significantly longer in women than in men. For a given cumulative diaphragmatic force output, the severity of DF was less in women than in men. Women exhibited a blunted cardiovascular response to inspiratory resistance (i.e. metaboreflex) that may have implications for exercise tolerance.

ABSTRACT

Diaphragmatic fatigue (DF) elicits reflexive increases in sympathetic vasomotor outflow (i.e. metaboreflex). There is some evidence suggesting women may be more resistant to DF compared to men, and therefore may experience an attenuated inspiratory muscle metaboreflex. To this end, we sought to examine the cardiovascular response to inspiratory resistance in healthy young men (n = 9, age = 24 ± 3 years) and women (n = 9, age = 24 ± 3 years). Subjects performed isocapnic inspiratory pressure-threshold loading (PTL, 60% maximal inspiratory mouth pressure) to task failure. Diaphragmatic fatigue was assessed by measuring transdiaphragmatic twitch pressure (P_di,tw ) using cervical magnetic stimulation. Heart rate (HR) and mean arterial pressure (MAP) were measured beat-by-beat throughout PTL via photoplethysmography, and low-frequency systolic pressure (LF_SBP ; a surrogate for sympathetic vasomotor tone) calculated from arterial waveforms using power spectrum analysis. At PTL task failure, the degree of DF was similar between sexes (∼23% reduction in P_di,tw ; P = 0.33). However, time to task failure was significantly longer in women than in men (27 ± 11 vs. 16 ± 11 min, respectively; P = 0.02). Women exhibited less of an increase in HR (13 ± 8 vs. 19 ± 12 bpm; P = 0.02) and MAP (10 ± 8 vs. 14 ± 9 mmHg; P = 0.01), and significantly lower LF_SBP (23 ± 11 vs. 34 ± 8 mmHg² ; P = 0.04) during PTL compared to men. An attenuation of the inspiratory muscle metaboreflex may influence limb and respiratory muscle haemodynamics with implications for exercise performance.

Using ramp-incremental V̇O2 responses for constant-intensity exercise selection

Despite compelling evidence to the contrary, the view that oxygen uptake (V̇O₂) increases linearly with exercise intensity (e.g., power output, speed) until reaching its maximum persists within the exercise physiology literature. This viewpoint implies that the V̇O₂ response at any constant intensity is predictable from a ramp-incremental exercise test. However, the V̇O₂ versus task-specific exercise intensity relationship constructed from ramp-incremental versus constant-intensity exercise are not equivalent preventing the use of V̇O₂ responses from 1 domain to predict those of the other. Still, this "linear" translational framework continues to be adopted as the guiding principle for aerobic exercise prescription and there remains in the sport science literature a lack of understanding of how to interpret V̇O₂ responses to ramp-incremental exercise and how to use those data to assign task-specific constant-intensity exercise. The objectives of this paper are to (i) review the factors that disassociate the V̇O₂ versus exercise intensity relationship between ramp-incremental and constant-intensity exercise paradigms; (ii) identify when it is appropriate (or not) to use ramp V̇O₂ responses to accurately assign constant-intensity exercise; and (iii) illustrate the technical and theoretical challenges with prescribing constant-intensity exercise solely on information acquired from ramp-incremental tests. Actual V̇O₂ data collected during cycling exercise and V̇O₂ kinetics modelling are presented to exemplify these concepts. Possible solutions to overcome these challenges are also presented to inform on appropriate intensity selection for individual-specific aerobic exercise prescription in both research and practical settings.

The effects of priming exercise on the V̇O2 slow component and the time-course of muscle fatigue during very-heavy-intensity exercise in humans

We hypothesized that prior exercise would attenuate the muscle fatigue accompanied by oxygen uptake slow-component (V̇O_2SC) behavior during a subsequent very-heavy (VH)-intensity cycling exercise. Thirteen healthy male subjects performed tests to determine the critical power (CP) and the fixed amount of work above CP ([Formula: see text]) and performed 6 square-wave bouts until 3 or 8 min, each at a work rate set to deplete 70% [Formula: see text] in 8 min, with a maximal isokinetic effort before and after the conditions without (VH_CON) and with prior exercise (VH_EXP), to measure the cycling peak torque decrement. The V̇O_2SC magnitude at 3 min (VH_CON = 0.280 ± 0.234, VH_EXP = 0.116 ± 0.109 L·min^-1; p = 0.04) and the V̇O_2SC trajectory were significantly lower for VH_EXP (VH_CON = 0.108 ± 0.042, VH_EXP = 0.063 ± 0.031 L·min^-2; p < 0.01), leading to a V̇O_2SC magnitude at the eighth minute that was significantly lower than VH_CON (VH_CON = 0.626 ± 0.296 L·min^-1, VH_EXP = 0.337 ± 0.179; p < 0.01). Conversely, peak torque progressively decreased from pre-exercise to 3 min (Δtorque = 21.5 ± 7.7 vs. 19.6 ± 9.2 Nm) and to 8 min (Δtorque = 29.4 ± 15.8 vs. 27.5 ± 12.0 Nm) at VH_CON and VH_EXP, respectively, without significant differences between conditions. Regardless of the condition, there was a significant relationship between Δtorque and the V̇O_2SC (R²: VH_CON = 0.23, VH_EXP = 0.25; p = 0.01). Considering that "priming" effects on the V̇O_2SC were not accompanied by the muscle force behavior, these findings do not support the hypothesis of a "causal" relationship between the time-course of muscle fatigue and V̇O_2SC.

Acute acetaminophen ingestion improves performance and muscle activation during maximal intermittent knee extensor exercise

Aim

Acetaminophen is a commonly used medicine for pain relief and emerging evidence suggests that it may improve endurance exercise performance. This study investigated some of the physiological mechanisms by which acute acetaminophen ingestion might blunt muscle fatigue development.

Methods

Thirteen active males completed 60 × 3 s maximum voluntary contractions (MVC) of the knee extensors with each contraction separated by a 2 s passive recovery period. This protocol was completed 60 min after ingesting 1 g of maltodextrin (placebo) or 1 g of acetaminophen on two separate visits. Peripheral nerve stimulation was administered every 6th contraction for assessment of neuromuscular fatigue development, with the critical torque (CT), which reflects the maximal sustainable rate of oxidative metabolism, taken as the mean torque over the last 12 contractions. Surface electromyography was recorded continuously as a measure of muscle activation.

Results

Mean torque (61 ± 11 vs. 58 ± 14% pre-exercise MVC) and CT (44 ± 13 vs. 40 ± 15% pre-exercise MVC) were greater in the acetaminophen trial compared to placebo (both P < 0.05). Voluntary activation and potentiated twitch declined at a similar rate in both conditions (P > 0.05). However, the decline in electromyography amplitude was attenuated in the acetaminophen trial, with electromyography amplitude being greater compared to placebo from 210 s onwards (P < 0.05).

Conclusion

These findings indicate that acute acetaminophen ingestion might be ergogenic by increasing CT and preserving muscle activation during high-intensity exercise.

Electronic supplementary material

The online version of this article (10.1007/s00421-017-3794-7) contains supplementary material, which is available to authorized users.

Intensity- and Duration-Based Options to Regulate Endurance Training

The regulation of endurance training is usually based on the prescription of exercise intensity. Exercise duration, another important variable of training load, is rarely prescribed by individual measures and mostly set from experience. As the specific exercise duration for any intensity plays a substantial role regarding the different kind of cellular stressors, degree, and kind of fatigue as well as training effects, concepts integrating the prescription of both intensity and duration within one model are needed. An according recent approach was the critical power concept which seems to have a physiological basis; however, the mathematical approach of this concept does not allow applying the three zones/two threshold model of metabolism and its different physiological consequences. Here we show the combination of exercise intensity and duration prescription on an individual basis applying the power/speed to distance/time relationship. The concept is based on both the differentiation of intensities by two lactate or gas exchange variables derived turn points, and on the relationship between power (or velocity) and duration (or distance). The turn points define three zones of intensities with distinct acute metabolic, hormonal, and cardio-respiratory responses for endurance exercise. A maximal duration exists for any single power or velocity such as described in the power-duration relationship. Using percentages of the maximal duration allows regulating fatigue, recovery time, and adaptation for any single endurance training session. Four domains of duration with respect to induced fatigue can be derived from maximal duration obtained by the power-duration curve. For any micro-cycle, target intensities and durations may be chosen on an individual basis. The model described here is the first conceptual framework of integrating physiologically defined intensities and fatigue related durations to optimize high-performance exercise training.

Determinants of curvature constant (W') of the power duration relationship under normoxia and hypoxia: the effect of pre-exercise alkalosis

Purpose

This study investigated the effect of induced alkalosis on the curvature constant (W’) of the power-duration relationship under normoxic and hypoxic conditions.

Methods

Eleven trained cyclists (mean ± SD) Age: 32 ± 7.2 years; body mass (bm): 77.0 ± 9.2 kg; VO_2peak: 59.2 ± 6.8 ml·kg⁻¹·min⁻¹ completed seven laboratory visits which involved the determination of individual time to peak alkalosis following sodium bicarbonate (NaHCO₃) ingestion, an environment specific ramp test (e.g. normoxia and hypoxia) and four x 3 min critical power (CP) tests under different experimental conditions. Participants completed four trials: alkalosis normoxia (ALN); placebo normoxia (PLN); alkalosis hypoxia (ALH); and placebo hypoxia (PLH). Pre-exercise administration of 0.3 g.kg⁻¹ BM of NaHCO₃ was used to induce alkalosis. Environmental conditions were set at either normobaric hypoxia (FiO₂: 14.5%) or normoxia (FiO₂: 20.93%).

Results

An increase in W’ was observed with pre-exercise alkalosis under both normoxic (PLN: 15.1 ± 6.2 kJ vs. ALN: 17.4 ± 5.1 kJ; P = 0.006) and hypoxic conditions (ALN: 15.2 ± 4.9 kJ vs. ALN: 17.9 ± 5.2 kJ; P < 0.001). Pre-exercise alkalosis resulted in a larger reduction in bicarbonate ion (HCO₃⁻) concentrations during exercise in both environmental conditions (p < 0.001) and a greater blood lactate accumulation under hypoxia (P = 0.012).

Conclusion

Pre-exercise alkalosis substantially increased W’ and, therefore, may determine tolerance to exercise above CP under normoxic and hypoxic conditions. This may be due to NaHCO₃ increasing HCO₃⁻ buffering capacity to delay exercise-induced acidosis, which may, therefore, enhance anaerobic energy contribution.