Effect of time of day on force variation in a human muscle

Intrinsic skeletal muscle function and contraction-stimulated glucose uptake do not vary by time-of-day in mice

A growing body of data suggests that skeletal muscle contractile function and glucose metabolism vary by time-of-day, with chronobiological effects on intrinsic skeletal muscle properties being proposed as the underlying mediator. However, no studies have directly investigated intrinsic contractile function or glucose metabolism in skeletal muscle over a 24 h circadian cycle. To address this, we assessed intrinsic contractile function and endurance, as well as contraction-stimulated glucose uptake, in isolated extensor digitorum longus and soleus from female mice at four times-of-day (Zeitgeber Times 1, 7, 13, 19). Significantly, while both muscles demonstrated circadian-related changes in gene expression, intrinsic contractile function, endurance, and contraction-stimulated glucose uptake were not different between the four time points. Overall, these results demonstrate that time-of-day variation in exercise performance and the glycemia-reducing benefits of exercise are not due to chronobiological effects on intrinsic muscle function or contraction-stimulated glucose uptake.

Impact statement

Ex vivo testing demonstrates that there is no time-of-day variation in the intrinsic contractile properties of skeletal muscle (including no effect on force production or endurance) or contraction-stimulated glucose uptake.

Influence of acute dietary nitrate supplementation timing on nitrate metabolism, central and peripheral blood pressure and exercise tolerance in young men

PURPOSE

Dietary nitrate (NO3-) supplementation can lower systolic blood pressure (SBP) and improve exercise performance. Salivary flow rate (SFR) and pH are key determinants of oral NO3- reduction and purported to peak in the afternoon. We tested the hypotheses that NO3--rich beetroot juice (BR) would increase plasma [nitrite] ([NO2-]), lower SBP and improve exercise performance to a greater extent in the afternoon (AFT) compared to the morning (MORN) and evening (EVE).

METHOD

Twelve males completed six experimental visits in a repeated-measures, crossover design. NO3--depleted beetroot juice (PL) or BR (~ 13 mmol NO3-) were ingested in the MORN, AFT and EVE. SFR and pH, salivary and plasma [NO3-] and [NO2-], brachial SBP and central SBP were measured pre and post supplementation. A severe-intensity exercise tolerance test was completed to determine cycling time to exhaustion (TTE).

RESULTS

There were no between-condition differences in mean SFR or salivary pH. The elevation in plasma [NO2-] after BR ingestion was not different between BR-MORN, BR-AFT and BR-EVE. Brachial SBP was unchanged following BR supplementation in all conditions. Central SBP was reduced in BR-MORN (- 3 ± 4 mmHg), BR-AFT (- 4 ± 3 mmHg), and BR-EVE (- 2 ± 3 mmHg), with no differences between timepoints. TTE was not different between BR and PL at any timepoint.

CONCLUSION

Acute BR supplementation was ineffective at improving TTE and brachial SBP and similarly effective at increasing plasma [NO2-] and lowering central SBP across the day, which may have implications for informing NO3- supplementation strategies.

Diurnal variation in variables related to cognitive performance: a systematic review

PURPOSE

The aim of this review was to assess current evidence regarding changes in cognitive function according to time-of-day (TOD) and assess the key components of research design related to manuscripts of chronobiological nature.

METHODS

An English-language literature search revealed 523 articles through primary database searches, and 1868 via organization searches/citation searching. The inclusion criteria were met by eleven articles which were included in the review. The inclusion criteria set were healthy adult males, a minimum of two timepoints including morning and evening, cognitive measures of performance, and peer-reviewed academic paper.

RESULTS

It was established that cognitive performance varies with TOD and the degree of difference is highly dependent on the type of cognitive task with differences ranging from 9.0 to 34.2% for reaction time, 7.3% for alertness, and 7.8 to 40.3% for attention. The type of cognitive function was a determining factor as to whether the performance was better in the morning, evening, or afternoon.

CONCLUSION

Although some studies did not establish TOD differences, reaction time and levels of accuracy were highest in the evening. This implies that cognitive processes are complex, and existing research is contradictory. Some studies or cognitive variables did not show any measurable TOD effects, which may be due to differences in methodology, subjects involved, testing protocols, and confounding factors. No studies met all requirements related to chronobiological research, highlighting the issues around methodology. Therefore, future research must use a rigorous, approach, minimizing confounding factors that are specific to examinations of TOD.

Variability of corticospinal and spinal reflex excitability for the ankle dorsiflexor tibialis anterior across repeated measurements in people with and without incomplete spinal cord injury

To adequately evaluate the corticospinal and spinal plasticity in health and disease, it is essential to understand whether and to what extent the corticospinal and spinal responses fluctuate systematically across multiple measurements. Thus, in this study, we examined the session-to-session variability of corticospinal excitability for the ankle dorsiflexor tibialis anterior (TA) in people with and without incomplete spinal cord injury (SCI). In neurologically normal participants, the following measures were obtained across 4 days at the same time of day (N = 13) or 4 sessions over a 12-h period (N = 9, at 8:00, 12:00, 16:00, and 20:00): maximum voluntary contraction (MVC), maximum M-wave and H-reflex (Mmax and Hmax), motor evoked potential (MEP) amplitude, and silent period (SP) after MEP. In participants with chronic incomplete SCI (N = 17), the same measures were obtained across 4 days. We found no clear diurnal variation in the spinal and corticospinal excitability of the TA in individuals with no known neurological conditions, and no systematic changes in any experimental measures of spinal and corticospinal excitability across four measurement days in individuals with or without SCI. Overall, mean deviations across four sessions remained in a range of 5-13% for all measures in participants with or without SCI. The study shows the limited extent of non-systematic session-to-session variability in the TA corticospinal excitability in individuals with and without chronic incomplete SCI, supporting the utility of corticospinal and spinal excitability measures in mechanistic investigation of neuromodulation interventions. The information provided through this study may serve as the reference in evaluating corticospinal plasticity across multiple experimental sessions.

Effects of Repeated Jump Testing and Diurnal Changes on Subsequent Countermovement Jump and Squat Jump Output and Force-Time Characteristics

ABSTRACT

Harrison, PW, James, LP, Jenkins, DG, Holmberg, PM, and Kelly, VG. Effects of repeated jump testing and diurnal changes on subsequent countermovement jump and squat jump output and force-time characteristics. J Strength Cond Res 38(1): 174-179, 2024-The aim of this brief study was to investigate the effects of repeated jump testing on performance over 2 consecutive days while considering the possibility of diurnal changes. Fourteen male subjects and 14 recreationally active female subjects completed countermovement jump (CMJ) and squat jump (SJ) testing on 5 occasions (baseline [0,800], 5 minutes [0,820], 8 hours [1,600], 24 hours [0,800], and 32 hours [1,600]) over 32 hours. An additional rested baseline test was conducted on a separate day in the afternoon (1,600) to compare jump performance between morning and afternoon baseline values. Excluding small decreases in CMJ height at 24 hours (p = 0.292, Cliff's delta = -0.225) in male subjects and similar decreases in CMJ height at 5 minutes (p = 0.034, Cliff's delta = -0.245) in addition to SJ height:contraction time at 32 hours (p = 0.126, Cliff's delta = 0.153) in female subjects, findings generally showed no changes in jump performance over multiple assessments. Squat jump metrics may have showed small improvements between morning and afternoon baseline values in male subjects (SJ height:contraction time [p = 0.030, Cliff's delta = 0.225]) and female subjects (SJ height [p = 0.013, Cliff's delta = 0.173] and SJ height:contraction time [p = 0.091, Cliff's delta = 0.163)]. As jump performance was largely unaffected by repeated jump testing, the present findings support the use of monitoring practices and research designs that require multiple jump assessments within acute periods (∼32 hours).

Time-of-Day Effects of Exercise on Cardiorespiratory Responses and Endurance Performance-A Systematic Review and Meta-Analysis

ABSTRACT

Kang, J, Ratamess, NA, Faigenbaum, AD, Bush, JA, Finnerty, C, DiFiore, M, Garcia, A, and Beller, N. Time-of-day effects of exercise on cardiorespiratory responses and endurance performance-A systematic review and meta-analysis. J Strength Cond Res 37(10): 2080-2090, 2023-The time-of-day effect of exercise on human function remains largely equivocal. Hence, this study aimed to further analyze the existing evidence concerning diurnal variations in cardiorespiratory responses and endurance performance using a meta-analytic approach. Literature search was conducted through databases, including PubMed, CINAHL, and Google Scholar. Article selection was made based on inclusion criteria concerning subjects' characteristics, exercise protocols, times of testing, and targeted dependent variables. Results on oxygen uptake (V̇ o2 ), heart rate (HR), respiratory exchange ratio, and endurance performance in the morning (AM) and late afternoon or evening (PM) were extracted from the chosen studies. Meta-analysis was conducted with the random-effects model. Thirty-one original research studies that met the inclusion criteria were selected. Meta-analysis revealed higher resting V̇ o2 (Hedges' g = -0.574; p = 0.040) and resting HR (Hedges' g = -1.058; p = 0.002) in PM than in AM. During exercise, although V̇ o2 remained indifferent between AM and PM, HR was higher in PM at submaximal (Hedges' g = -0.199; p = 0.046) and maximal (Hedges' g = -0.298; p = 0.001) levels. Endurance performance as measured by time-to-exhaustion or the total work accomplished was higher in PM than in AM (Hedges' g = -0.654; p = 0.001). Diurnal variations in V̇ o2 appear less detectable during aerobic exercise. The finding that exercising HR and endurance performance were greater in PM than in AM emphasizes the need to consider the effect of circadian rhythm when evaluating athletic performance or using HR as a criterion to assess fitness or monitor training.

Mood Status Response to Physical Activity and Its Influence on Performance: Are Chronotype and Exercise Timing Affect?

PURPOSE

It is well known that there is an obvious 24 h diurnal variation in the individual's mood state and physiological activity, and training at different times of the day may lead to different exercise performance and metabolic outcomes; however, the time-dependent effect of emotional state on physical activity and the influence of its circadian rhythm on exercise performance are still not comprehensively understood. Based on this, this study summarizes the rhythmic experimental research in the field of sport psychology, and it aims to provide the basis for coaches to optimize sports training scientifically and to improve the mental health of the related crowd to the greatest extent.

METHODS

The systematic review was performed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. We searched the PubMed, Web of Science, Medline, and CNKI databases for relevant literature; the search scope was research before September 2022.

RESULTS

13 studies comprising 382 subjects examined the effects of exercise timing on mood responses to exercise or the effects of circadian rhythms of mood on exercise performance, which included 3 RCTs and 10 Non-RCTs. The subjects included athletes (both training or retired), college students, and healthy adults. Two studies were designed for long-term exercise intervention (aerobic training and RISE) and the rest for acute intervention (CrossFit training, HIIT, aerobic combined with muscle conditioning training, constant power exhaustion training, and cycling) or physical function tests (RSA + BTV tests, 30 s Wingate test, muscle strength + CMJ + swimming performance test, RSSJA, shooting accuracy tests + 10 × 20 m dribbling sprint, 200 m time trials). All trials reported specific exercise timing; of these, 10 studies reported subjects' chronotypes, most commonly using the MEQ scale, while 1 recorded with the CSM. Mood responses were assessed with the POMS scale in 10 studies, while 3 other studies used the UMACL, PANAS, and GAS scales, respectively.

CONCLUSION

There was much inconsistency between the results, with subjects likely to be exposed to more sunlight (the main timing factor of the circadian rhythm) during early morning exercise, resulting in feeling more positive emotions; however, following a night's rest, delayed responses and poor functioning of the various organ systems of the human body may also lead to higher feelings of fatigue and negative emotions indirectly. Conversely, for athletes, their physical function tests are also more susceptible to the circadian rhythm of emotions, suggesting the importance of synchronizing them. In addition, night owls' emotional state during physical activity seems to be more susceptible to exercise timing than that of early birds. In order to achieve the best emotional state, it is suggested that night owls arrange courses in the afternoon or evening in future training.

Daily variation in time-trial sporting performance: A systematic review

Few functional measures related to time-trial display diurnal variation. The diversity of tests/protocols used to assess time-trial performance on diurnal effects and the lack of a standardised approach hinder agreement in the literature. Therefore, the aims of the present study were to investigate and systematically review the evidence relating to time-of-day differences in time-trial measures and to examine the main aspects related to research design important for studies of a chronobiological nature. The entire content of Manipal Academy of Higher Education electronic library and Qatar National Library, and electronic databases: PubMed (MEDLINE), Scopus and Web of Science were searched. Research studies published in peer-reviewed journals and non-peer reviewed studies, conducted in male adult participants aged ≥18 y before November 2021 were screened/included. Studies assessing tests related to time-trials in any modality between a minimum of 2 time-points during the day (morning [06:30-10:30 h] vs evening [14:30-20:00 h]) were deemed eligible. The Grading of Recommendations Assessment, Development, and Evaluation (GRADE) process was used to assess the evidence to inform recommendations. The primary search revealed that a total of 10 from 40 articles were considered eligible and subsequently included. Six were conducted using cycling, two using running and two using swimming as the mode of exercise. Distances ranged from 1 to 16.1-km in distance or 15 to 20-min time in the cycling and running time-trials, and 50 to 200-m in the swimming time-trials. Only four studies found one or several of their performance variables to display daily variations, with significantly better values in the evening; while six studies found no time-of-day significance in any variables assessed. The magnitude of difference ranged from 2.9% to 7.1% for performance time to complete a cycling time-trial, while running and swimming did not find any differences for performance time. Power output during a 16.1-km time trial in cycling also found evening performance to be significantly better by 10%. The only other observed differences were stroke rate and stroke length during a swimming time-trial and stroke rate (cycles.min^-1) during a cycling time-trial. The magnitude of difference is dependent on exercise modality, individual chronotype, the training status of the individual and sample size differences. The lack of diurnal variation present in the majority of studies can in-part be explained with some of the methodological limitations and issues present related to quality and control. It is paramount that research assessing diurnal variation in performance uses appropriate session timings around the core body temperature minimum (~05:00 h; morning) and maximum (~17:00 h; evening). Although, differences in motivation/arousal, habitual training times, chronotypes and genotypes could provide an explanation as to why some studies/variables did not display time-of-day variation, more work is needed to provide an accurate conclusion. There is a clear demand for a rigorous, standardised approach to be adopted by future investigations which control factors that specifically relate to investigations of time-of-day, such as appropriate familiarisation, counterbalancing the order of administration of tests, providing sufficient recovery time between sessions and testing within a controlled environment.

Sex and limb comparisons of neuromuscular function in the morning versus the evening

The time-of-day influence on neuromuscular function is well-documented, but important details remain elusive. It is currently unknown whether males and females differ in their diurnal variation for optimal neuromuscular performance. The purpose of this study is to identify the time-of-day influence on neuromuscular function between sexes and determine whether these responses differ for the upper versus lower limbs. A group of males (n = 12) and females (n = 15) completed neuromuscular performance testing in the morning (07:00-09:00) and evening (17:00-19:00) on separate days in a randomized order. Maximal force, the normalized rate of force development, EMG, normalized EMG rise, and submaximal force steadiness were compared between morning and evening hours. The main findings show that maximal force was greater in the evening for the knee extensors (d = 0.570, p < 0.01) but not the elbow flexors (d = 0.212, p = 0.281), whereas maximal muscle excitation was greater in the evening for the biceps brachii (d = 0.348, p < 0.01) but not the vastus lateralis (d = 0.075, p = 0.526) with no influence of sex. However, force steadiness during knee extension was superior in the evening versus the morning for males (d = 0.734, p = 0.025) and compared to evening values for females (g = 1.19, p = 0.032). Overall, these findings show that time-of-day affects the knee extensors more than the elbow flexors and that diurnal variability between sexes appears to be task-dependent.

Daily variation in performance measures related to anaerobic power and capacity: A systematic review

Numerous functional measures related to anaerobic performance display daily variation. The diversity of tests and protocols used to assess anaerobic performance related to diurnal effects and the lack of a standardized approach have hindered agreement in the literature. Therefore, the aim of the present study was to investigate and systematically review the evidence relating to time-of-day differences in anaerobic performance measures. The entire content of PubMed (MEDLINE), Scopus, SPORTDiscus® (via EBSCOhost) and Web of Science and multiple electronic libraries were searched. Only experimental research studies conducted in male adult participants aged ≥ 18 yrs before May 2021 were included. Studies assessing tests related to anaerobic capacity or anaerobic power between a minimum of two time-points during the day (morning vs evening) were deemed eligible. The primary search revealed that a total of 55 out of 145 articles were considered eligible and subsequently included. Thirty-nine studies assessed anaerobic power and twenty-five anaerobic capacity using different modes of exercise and test protocols. Forty-eight studies found several of their performance variables to display time-of-day effects, with higher values in the evening than the morning, while seven studies did not find any time-of-day significance in any variables which were assessed. The magnitude of difference is dependent on the modality and the exercise protocol used. Performance measures for anaerobic power found jump tests displayed 2.7 to 12.3% differences, force velocity tests ~8% differences, sprint tests 2.7 to 11.3% differences and 5-m multiple shuttle run tests 3.7 to 13.1% differences in favour of the evening. Performance measures for anaerobic capacity found Wingate test to display 1.8 to 11.7% differences and repeated sprint tests to display 3.4 to 10.2% differences. The only test not to display time-of-day differences was the running based anaerobic sprint test (RAST). Time-of-day variations in anaerobic performance has previously been partially explained by higher core-body and/or muscle temperature and better muscle contractile properties in the afternoon, although recent findings suggest that differences in methodology, motivation/arousal, habitual training times and chronotypes could provide additional explanations. There is a clear demand for a rigorous, standardised approach to be adopted by future investigations which control factors that specifically relate to investigations of time-of-day.

Factors Contributing to Diurnal Variation in Athletic Performance and Methods to Reduce Within-Day Performance Variation: A Systematic Review

ABSTRACT

Kusumoto, H, Ta, C, Brown, SM, and Mulcahey, MK. Factors contributing to diurnal variation in athletic performance and methods to reduce within-day performance variation: A systematic review. J Strength Cond Res 35(12S): S119-S135, 2021-For many individuals, athletic performance (e.g., cycle ergometer output) differs based on the time of day (TOD). This study identified factors contributing to diurnal variation in athletic performance and methods to reduce TOD performance variation. Comprehensive searches of PubMed, Ovid, EMBASE, Web of Science, and Cochrane Libraries were conducted in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Peer-reviewed publications reporting quantitative, significant diurnal variation (p ≤ 0.05) of athletic performance with explanations for the differences were included. Studies providing effective methods to reduce diurnal variation were also included. Literature reviews, studies involving nonhuman or nonadult subjects, studies that intentionally manipulated sleep duration or quality, and studies deemed to be of poor methodological quality using NIH Quality Assessment Tools were excluded. Forty-nine studies met the inclusion criteria. Body temperature differences (n = 13), electromyographic parameters (n = 10), serum biomarker fluctuations (n = 5), athlete chronotypes (n = 4), and differential oxygen kinetics (n = 3) were investigated as significant determinants of diurnal variation in sports performance. Successful techniques for reducing diurnal athletic performance variability included active or passive warm-up (n = 9), caffeine ingestion (n = 2), and training-testing TOD synchrony (n = 3). Body temperature was the most important contributor to diurnal variation in athletic performance. In addition, extended morning warm-up was the most effective way to reduce performance variation. Recognizing contributors to diurnal variation in athletic performance may facilitate the development of more effective training regimens that allow athletes to achieve consistent performances regardless of TOD.

Chronic fatigue syndrome: Abnormally fast muscle fiber conduction in the membranes of motor units at low static force load

OBJECTIVE

Chronic fatigue syndrome (CFS) and fibromyalgia (FM) are disorders of unknown etiology and unclear pathophysiology, with overlapping symptoms of - especially muscular -fatigue and pain. Studies have shown increased muscle fiber conduction velocity (CV) in the non-painful muscles of FM patients. We investigated whether CFS patients also show CV abnormalities.

METHODS

Females with CFS (n = 25), with FM (n = 22), and healthy controls (n = 21) underwent surface electromyography of the biceps brachii, loaded up to 20% of maximum strength, during short static contractions. The mean CV and motor unit potential (MUP) velocities with their statistical distribution were measured.

RESULTS

The CV changes with force differed between CFS-group and both FM-group and controls (P = 0.01). The CV of the CFS-group increased excessively with force (P < 0.001), whereas that of the controls increased only slightly and non-significantly, and that of the FM-group did not increase at all. In the CFS-group, the number of MUPs conveying very high conduction velocities increased abundantly with force and the MUPs narrowed.

CONCLUSION

Our results suggest disturbed muscle membrane function in CFS patients, in their motor units involved in low force generation. Central neural deregulation may contribute to this disturbance.

SIGNIFICANCE

These findings help to detangle the underlying mechanisms of CFS.

Temporal organisation of the brain's intrinsic motor network: The relationship with circadian phenotype and motor performance

Background

Functional connectivity (FC) of the motor network (MN) is often used to investigate how intrinsic properties of the brain are associated with motor abilities and performance. In addition, the MN is a key feature in clinical work to map the recovery after stroke and aid the understanding of neurodegenerative disorders. Time of day variation and individual differences in circadian timing, however, have not yet been considered collectively when looking at FC.

Methods

A total of 33 healthy, right handed individuals (13 male, 23.1 ± 4.2 years) took part in the study. Actigraphy, sleep diaries and circadian phase markers (dim light melatonin onset and cortisol awakening response) were used to determine early (ECP, n = 13) and late (LCP, n = 20) circadian phenotype groups. Resting state functional MRI testing sessions were conducted at 14:00 h, 20:00 h and 08:00 h and preceded by a maximum voluntary contraction test for isometric grip strength to measure motor performance.

Results

Significant differences in FC of the MN between ECPs and LCPs were found, as well as significant variations between different times of day. A higher amplitude in diurnal variation of FC and performance was observed in LCPs compared to ECPs, with the morning being most significantly affected. Overall, lower FC was significantly associated with poorer motor performance.

Discussion

Our findings uncover intrinsic differences between times of day and circadian phenotype groups. This suggests that central mechanisms contribute to diurnal variation in motor performance and the functional integrity of the MN at rest influences the ability to perform in a motor task.

Acute Beetroot Juice Supplementation Attenuates Morning-Associated Decrements in Supramaximal Exercise Performance in Trained Sprinters

Diurnal fluctuations in power output have been well established with power loss typically occurring in morning (AM) times. Beetroot juice (BRJ) is a source of dietary nitrate that possess ergogenic properties, but it is unknown if ingestion can mitigate performance decrements in the morning. The purpose of this study was to examine the effects of acute BRJ supplementation on diurnal fluctuations in anaerobic performance in trained sprinters. Male Division 1 National Collegiate Athletic Association (NCAA) sprinters (n = 10) participated. In a double-blinded crossover study design, participants completed three counterbalanced exercise trials under different conditions: Morning–placebo (8:00 HR, AM-PL), Morning–BRJ (8:00 HR, AM-BRJ), and Afternoon–no supplement (15:00 HR, PM). For each trial, participants completed 3 × 15 s Wingate anaerobic tests separated by 2 min of rest. Each trial was separated by a 72 h washout period. Mean power output (p = 0.043), anaerobic capacity (p = 0.023), and total work (p = 0.026) were significantly lower with the AM-PL condition compared to PM. However, BRJ supplementation prevented AM losses of mean power output (p = 0.994), anaerobic capacity (p = 0.941), and total work (p = 0.933) in the AM-BRJ compared to the PM condition. Rate of perceived exertion was not significantly different between any conditions (p = 0.516). Heart rate was significantly lower during the AM-BRJ condition compared to AM-PL (p = 0.030) and PM (p < 0.001). Findings suggest anaerobic capacity suffers during AM versus PM times in trained sprinters, but BRJ ingestion abolishes AM-associated decrements in performance.

Reliability and time-of-day effect on measures of change of direction deficit in young healthy physical education students

This study aimed to examine the reliability and the time-of-day effect of the 505 change of direction (CoD), 10-m sprint, and change of direction deficit test (CoDD). At two different time of days, 39 young diurnally active physical education male students performed different physical tests: 505 CoD, and sprint tests. Measurements were taken at two separate testing sessions, i.e. in the morning (07:00-08:30 h) and early evening (17:00-18:30 h) in a randomized and counter-balanced setting on nonconsecutive days in 21 of them (21.5 ± 1.5 y of age). The results showed that the 505 CoD test, 10-m sprint, and CoDD performances were a reliable test, and performances were better in the evening the 505 CoD, 10-m sprint, and CoDD testing provided reliable and sensitive scores. In addition, phase 2 showed that CoD, speed, and CoDD are affected by the time of day.

Does Performance-Related Information Augment the Maximal Isometric Force in the Elbow Flexors?

The effects of different feedback conditions on the elbow flexors maximal voluntary isometric (MVIC) force were examined. In a single visit, twenty participants (10 males) were exposed to four conditions including, Visual only; (i.e., knowledge of results [VI]); Verbal encouragement (i.e., verbally stating pull, pull [VE]); VI and VE (VIVE); and no VI or VE (NOVIVE). Three, 5-s MVIC trials were completed for each condition. Separate 2 × 4 (sex × condition) repeated measures analyses of variance (ANOVA) were used to examine MVIC force, rate of force development (RFD₂₀₀), and rate of electromyography (EMG) rise (RER₂₀₀) for the 0-200-ms window. There was a significant main condition effect for MVIC force (p < 0.001). Significantly greater MVIC force was shown for VIVE compared to VE (p < 0.001) and NOVIVE (p = 0.002) conditions, and VI compared to VE (p = 0.011) and NOVIVE (p = 0.009) conditions. A significant sex × condition interaction (p = 0.036) was observed for RFD₂₀₀, with significantly (p = 0.003) greater RFD₂₀₀ in males compared to females, in the VI condition only. There were no significant differences in RFD₂₀₀ among condition for males (p = 0.033) or females (p = 0.194). There was a significant main effect for sex (p = 0.006) for RER₂₀₀, with significantly greater (p = 0.003) RER₂₀₀ (conditions merged) in males (mean ± standard deviation (SD): 422.99 ± 124.67%), compared to females (220.68 ± 134.01%). Here we suggest the importance of providing VI feedback alone, or in conjunction with VE, for the measurement of elbow flexion MVIC. Additionally, these results provide a foundational framework for future studies aiming to improve feedback modalities during training or rehabilitation.

A Time to Eat and a Time to Exercise

Analogous to exercise training, time-restricted eating may rescue some of the deleterious effects on metabolic health induced by our modern-day lifestyle.

This Perspective for Progress provides a synopsis for the potential of time-restricted eating (TRE) to rescue some of the deleterious effects on circadian biology induced by our modern-day lifestyle. We provide novel insights into the comparative and potential complementary effects of TRE and exercise training on metabolic health.

Diurnal Differences in Human Muscle Isometric Force In Vivo Are Associated with Differential Phosphorylation of Sarcomeric M-Band Proteins

We investigated whether diurnal differences in muscle force output are associated with the post-translational state of muscle proteins. Ten physically active men (mean ± SD; age 26.7 ± 3.7 y) performed experimental sessions in the morning (08:00 h) and evening (17:00 h), which were counterbalanced in order of administration and separated by at least 72 h. Knee extensor maximal voluntary isometric contraction (MVIC) force and peak rate of force development (RFD) were measured, and samples of vastus lateralis were collected immediately after exercise. MVIC force was greater in the evening (mean difference of 67 N, 10.2%; p < 0.05). Two-dimensional (2D) gel analysis encompassed 122 proteoforms and discovered 6 significant (p < 0.05; false discovery rate [FDR] = 10%) diurnal differences. Phosphopeptide analysis identified 1693 phosphopeptides and detected 140 phosphopeptides from 104 proteins that were more (p < 0.05, FDR = 22%) phosphorylated in the morning. Myomesin 2, muscle creatine kinase, and the C-terminus of titin exhibited the most robust (FDR < 10%) diurnal differences. Exercise in the morning, compared to the evening, coincided with a greater phosphorylation of M-band-associated proteins in human muscle. These protein modifications may alter the M-band structure and disrupt force transmission, thus potentially explaining the lower force output in the morning.

Time-of-Day Effects on Short-Duration Maximal Exercise Performance

Time-of-day dependent fluctuations in exercise performance have been documented across different sports and seem to affect both endurance and resistance modes of exercise. Most of the studies published to date have shown that the performance in short-duration maximal exercises (i.e. less than 1 min - e.g. sprints, jumps, isometric contractions) exhibits diurnal fluctuations, peaking between 16:00 and 20:00 h. However, the time-of-day effects on short duration exercise performance may be minimized by the following factors: (1) short exposures to moderately warm and humid environments; (2) active warm-up protocols; (3) intermittent fasting conditions; (4) warming-up while listening to music; or (5) prolonged periods of training at a specific time of day. This suggests that short-duration maximal exercise performance throughout the day is controlled not only by body temperature, hormone levels, motivation and mood state but also by a versatile circadian system within skeletal muscle. The time of day at which short-duration maximal exercise is conducted represents an important variable for training prescription. However, the literature available to date lacks a specific review on this subject. Therefore, the present review aims to (1) elucidate time-of-day specific effects on short-duration maximal exercise performance and (2) discuss strategies to promote better performance in short-duration maximal exercises at different times of the day.

Circadian rhythms and exercise - re-setting the clock in metabolic disease

Perturbed diurnal rhythms are becoming increasingly evident as deleterious events in the pathology of metabolic diseases. Exercise is well characterized as a crucial intervention in the prevention and treatment of individuals with metabolic diseases. Little is known, however, regarding optimizing the timing of exercise bouts in order to maximize their health benefits. Furthermore, exercise is a potent modulator of skeletal muscle metabolism, and it is clear that skeletal muscle has a strong circadian profile. In humans, mitochondrial function peaks in the late afternoon, and the circadian clock might be inherently impaired in myotubes from patients with metabolic disease. Timing exercise bouts to coordinate with an individual's circadian rhythms might be an efficacious strategy to optimize the health benefits of exercise. The role of exercise as a Zeitgeber can also be used as a tool in combating metabolic disease. Shift work is known to induce acute insulin resistance, and appropriately timed exercise might improve health markers in shift workers who are at risk of metabolic disease. In this Review, we discuss the literature regarding diurnal skeletal muscle metabolism and the interaction with exercise bouts at different times of the day to combat metabolic disease.

Circadian and Circannual Regulation in the Horse: Internal Timing in an Elite Athlete

Biological rhythms evolved to provide temporal coordination across all tissues and organs and allow synchronization of physiology with predictable environmental cycles. Most important of these are circadian and circannual rhythms, primarily regulated via photoperiod signals from the retina. Understanding the nature of physiological rhythms in horses is crucially important for equine management. Predominantly, they have been removed from exposure to their natural environmental stimuli; the seasonally changing photoperiod, continuous foraging and feeding activity, social herd interactions, and the continuous low-intensity exercise of a grassland dweller. These have been replaced in many cases with confined indoor housing, regimental feeding and exercise times, social isolation, and exposure to lighting that is often erratic and does not come close to mimicking the spectral composition of sunlight. Man has further altered seasonal timing cues through the use of artificial lighting programs that impact reproductive behavior, breeding efficiency, and the development of youngstock. Understanding how these new environmental cues (some stronger and some weaker) impact the internal physiology of the horse in the context of the natural endogenous rhythms that evolved over millennia is key to helping to improve equine health, welfare, and performance, now and into the future. This review provides an overview of the field, highlights the recent discoveries related to biological timing in horses, and discusses the implications that these findings may have for the production and management of the elite equine athlete.

The effects of time of day-specific resistance training on adaptations in skeletal muscle hypertrophy and muscle strength: A systematic review and meta-analysis

The present paper endeavored to elucidate the topic on the effects of morning versus evening resistance training on muscle strength and hypertrophy by conducting a systematic review and a meta-analysis of studies that examined time of day-specific resistance training. This systematic review was performed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines with searches conducted through PubMed/MEDLINE, Scopus, and SPORTDiscus databases. The Downs and Black checklist was used for the assessment of the methodological quality of the included studies. Studies that examined the effects of time of day-specific resistance training (while equating all other training variables, such as training frequency and volume, between the groups) on muscle strength and/or muscle size were included in the present review. The random effects model was used for the meta-analysis. Meta-analyses explored (1) the differences in strength expression between morning and evening hours at baseline; (2) the differences in strength within the groups training in the morning and evening by using their post-intervention strength data from the morning and evening strength assessments; (3) the overall differences between the effects of morning and evening resistance training (with subgroup analyses conducted for studies that assessed strength in the morning hours and for the studies that assessed strength in the evening hours). Finally, a meta-analysis was also conducted for studies that assessed muscle hypertrophy. Eleven studies of moderate and good methodological quality were included in the present review. The primary findings of the review are as follows: (1) at baseline, a significant difference in strength between morning and evening is evident, with greater strength observed in the evening hours; (2) resistance training in the morning hours may increase strength assessed in the morning to similar levels as strength assessed in the evening; (3) training in the evening hours, however, maintains the general difference in strength across the day, with greater strength observed in the evening hours; (4) when comparing the effects between the groups training in the morning versus in the evening hours, increases in strength are similar in both groups, regardless of the time of day at which strength assessment is conducted; and (5) increases in muscle size are similar irrespective of the time of day at which the training is performed.

Effects of an active warm-up on variation in bench press and back squat (upper and lower body measures)

The present study investigated the magnitude of diurnal variation in back squat and bench press using the MuscleLab linear encoder over three different loads and assessed the benefit of an active warm-up to establish whether diurnal variation could be negated. Ten resistance-trained males underwent (mean ± SD: age 21.0 ± 1.3 years, height 1.77 ± 0.06 m, and body mass 82.8 ± 14.9 kg) three sessions. These included control morning (M, 07:30 h) and evening (E, 17:30 h) sessions (5-min standardized warm-up at 150 W, on a cycle ergometer), and one further session consisting of an extended active warm-up morning trial (M_E, 07:30 h) until rectal temperature (T_rec) reached previously recorded resting evening levels (at 150 W, on a cycle ergometer). All sessions included handgrip, followed by a defined program of bench press (at 20, 40, and 60 kg) and back squat (at 30, 50, and 70 kg) exercises. A linear encoder was attached to an Olympic bar used for the exercises and average force (AF), peak velocity (PV), and time to peak velocity (tPV) were measured (MuscleLab software; MuscleLab Technology, Langesund, Norway) during the concentric phase of the movements. Values for T_rec were higher in the E session compared to values in the M session (Δ0.53 °C, P < 0.0005). Following the extended active warm-up in the morning (M_E), T_rec and T_m values were no different to the E values (P < 0.05). Values for T_m were lower in the M compared to the E condition throughout (P < 0.05). There were time-of-day effects for hand grip with higher values of 6.49% for left (P = 0.05) and 4.61% for right hand (P = 0.002) in the E compared to the M. Daily variations were apparent for both bench press and back squat performance for AF (3.28% and 2.63%), PV (13.64% and 11.50%), and tPV (-16.97% and -14.12%, where a negative number indicates a decrease in the variable from morning to evening). There was a main effect for load (P < 0.0005) such that AF and PV values were larger at higher masses on the bar than lower ones and tPV was smaller at lower masses on the bar than at higher masses for both bench press and back squat. We established that increasing T_rec in the M-E values did not result in an increase of any measures related to bench press and back squat performance (P > 0.05) to increase from M to E levels. Therefore, MuscleLab linear encoder could detect meaningful differences between the morning and evening for all variables. However, the diurnal variation in bench press and back squat (measures of lower and upper body force and power output) is not explained by time-of-day oscillations in T_rec.

Effect of sleep deprivation on diurnal variation of vertical perception and postural control

The aim of the study was to test the effect of total sleep deprivation on performance and time-of-day pattern of subjective visual vertical (SVV) and postural control. Nineteen healthy, young participants (4 women and 15 men 21.9 ± 1.2 yr) were engaged in two counterbalanced experimental sessions with or without total sleep deprivation. Oral temperature, Karolinska Sleepiness Scale, and visual analogic scale for fatigue, postural control, and SVV were randomly measured every 4 h, from 0600 to 2200. A linear mixed model was used to capture the effect of time of day and sleep condition as factors. A classical adjusted COSINOR function was then used to modelize this daily variation. After the control night of sleep, SVV as well as oral temperature, sleepiness, and fatigue showed significant time-of-day variation, contrasting with measures of postural control which remained stable across the day. After sleep deprivation, SVV showed no diurnal variation, but its mean deviation value increased by 29%. Postural control capability also decreased after sleep deprivation, with a higher center of pressure surface (+70.4%) and total length (+7.37%) but remained stable throughout the day. These results further confirm the negative effect of sleep loss on postural control capability. Even if a direct relationship cannot be confirmed, the disruption of SVV capacity after sleep deprivation could strongly play a role in postural control capacity changes. Sleep deprivation should be considered as a potent factor involved in balance loss and subsequent fall. NEW & NOTEWORTHY The topic of sleep deprivation and postural control is not understood, with discrepancy among results. This study described that postural control displays a stable level throughout the day and that sleep deprivation, even if it increases postural sway, does not affect this stable diurnal pattern. The modification of the perception of the vertical level after sleep deprivation could strongly play a role in the observed changes in postural control capacity.

Separate and combined effects of time of day and verbal instruction on knee extensor neuromuscular adjustments

We examined the effects of time of day and verbal instruction, separately and combined, on knee extensor neuromuscular adjustments, with special reference to rapid muscle force production capacity. Ten healthy male participants performed 4 experimental trials in counterbalanced order: morning “hard-and-fast” instruction, evening hard-and-fast instruction, morning “fast” instruction, and evening fast instruction. During each experimental trial, neuromuscular performance was assessed from the completion of 6 maximal isometric voluntary contractions (rest = 2 min) of the knee extensors with concomitant quadriceps surface electromyography recordings. For each contraction, we determined maximal voluntary force (Fmax), maximal rate of force development (RFDmax) and associated maximal electromechanical delay (EMDmax), and maximal rate of muscle activation (RMAmax). Globally, oral temperature (+2.2%), Fmax (+4.9%) and accompanying median frequency (+6.6%)/mean power frequency (+6.0%) as well as RFDmax (+13.5%) and RMAmax (+15.5%) were significantly higher in the evening than morning (p < 0.05). Conversely, evening in reference to morning values were lower for EMDmax (–4.3%, p < 0.05). Compared with a hard-and-fast instruction, RFDmax (+30.6%) and corresponding root mean square activity (+18.6%) were globally higher using a fast instruction (p < 0.05), irrespectively of the time of day. There was no significant interaction effect of time of day and verbal instruction on any parameter, except for EMDmax (p = 0.028). Despite diurnal variation in maximal or explosive force production of knee extensors and associated neuromuscular parameters, these adjustments occurred essentially independently of the verbal instruction provided.

Neural Correlates to the Increase in Maximal Force after Dexamethasone Administration

PURPOSE

This study investigated the effects of short-term glucocorticoid administration on voluntary activation and intracortical inhibitory and facilitatory circuits.

METHODS

Seventeen healthy men participated in a pseudorandomized double-blind study to receive either dexamethasone (8 mg·d, n = 9 subjects) or placebo (n = 8 subjects) for 7 d. The ankle dorsiflexion torque, corresponding EMG of the tibialis anterior, and voluntary activation assessed by the interpolated twitch method using transcranial magnetic stimulation (TMS) were measured during a maximal voluntary contraction (MVC). Short-latency intracortical inhibition (SICI) and intracortical facilitation (ICF) were assessed at rest and during submaximal contraction (50% MVC torque) by paired-pulse TMS with the conditioning stimulus set at 0.8× of motor threshold and delivered 2 ms (SICI) and 13 ms (ICF) before the test stimulus (1.2× motor threshold).

RESULTS

The MVC torque (+14%), tibialis anterior EMG (+31%), and voluntary activation (+3%) increased after glucocorticoid treatment (P < 0.05). The increase in voluntary activation was associated with the gain in MVC torque (r = 0.56; P = 0.032). The level of SICI and the duration of the EMG silent period that followed the test TMS decreased (-18.6% and -13.5%, respectively) during the 50% MVC after treatment (P < 0.05), whereas no significant change was observed for ICF. Neither SICI nor ICF changed after treatment when assessed at rest.

CONCLUSIONS

Short-term dexamethasone treatment induced specific decrease in the excitability of intracortical inhibitory circuits that likely contributed to the increase in the voluntary activation and associated MVC torque.

Comparison of maximum voluntary isometric contraction of the biceps on various posture and respiration conditions for normalization of electromyography data

[Purpose] Maximum voluntary isometric contraction can increase the reliability of electromyography data by controlling respiration; however, many studies that use normalization of electromyography data fail to account for this. This study aims to check changes in maximum voluntary isometric contraction based on changes in posture and respiration conditions. [Subjects and Methods] Twenty-two healthy volunteers were included in this study. Using 22 healthy subjects, MVIC of the biceps brachii muscle was measured in three respiration conditions: (1) Maximum voluntary isometric contraction during inspiration after maximal expiration, (2) Maximum voluntary isometric contraction during expiration after maximal inspiration and (3) Maximum voluntary isometric contraction during the Valsalva maneuver. The subjects were in tested in standing and supine postures under all three respiration conditions. [Results] A significant difference was observed in the standing and supine postures based on the respiration condition. A significant difference was observed in the maximum voluntary isometric contraction during inspiration after maximal expiration and maximum voluntary isometric contraction during the Valsalva maneuver conditions when the subjects were in the supine posture. [Conclusion] It is necessary to apply the same respiration condition and the same posture to each subject when measuring Maximum voluntary isometric contraction for the normalization of electromyography data.

Diurnal Rhythm of Muscular Strength Depends on Temporal Specificity of Self-Resistance Training

The study investigated the effect of the time-of-day at which maximal isometric voluntary co-contraction (MIVCC) training is conducted on the adaptation and diurnal variation of maximal and explosive force production. Twenty active men underwent a 6-week (3 times per week) MIVCC training of the right elbow joint. The participants were randomly assigned to a morning training group (MTG, 07:00-08:00 hours) and evening training group (ETG, 17:00-18:00 hours). The maximal voluntary force (MVF) and maximal rate of force development (MRFD) during isometric elbow flexion (MVFF and MRFD(F)) and extension (MVF(E) and MRFD(E)) were recorded before (T0) and after (T1) training in the morning and evening. At T0, MVF and MRFD were higher in the evening compared with those in the morning for the MTG and ETG (p ≤ 0.05). At T1, MVFF and MVFE increased in the morning and evening for both groups (p < 0.001). The MRFD(F) and MRFD(E) increased only if training and test session were scheduled at the same time. The relative increase of MVF was greater at the specific time of training for the MTG (12 and 17.6% in MVF(F) and MVF(E), respectively) and ETG (9.8 and 13.4% in MVF(F) and MVF(E), respectively). The diurnal variations in MVF and MRF(D) during flexion and extension disappeared in the MTG and persisted in the ETG. Maximal isometric voluntary co-contraction training enhanced muscle strength whatever the time-of-day at which the training was scheduled without alteration of explosive force. In contrast, to optimize the muscle strength, our results suggested that morning training may be accompanied by the greatest muscle strength gain and blunted muscle strength variation observed between the morning and evening.

Effect of kinesthetic illusion induced by visual stimulation on muscular output function after short-term immobilization

Kinesthetic illusions by visual stimulation (KiNVIS) enhances corticomotor excitability and activates motor association areas. The purpose of this study was to investigate the effect of KiNVIS induction on muscular output function after short-term immobilization. Thirty subjects were assigned to 3 groups: an immobilization group, with the left hand immobilized for 12h (immobilization period); an illusion group, with the left hand immobilized and additionally subjected to KiNVIS of the immobilized part during the immobilization period; and a control group with no manipulation. The maximum voluntary contraction (MVC), fluctuation of force (force fluctuation) during a force modulation task, and twitch force were measured both before (pre-test) and after (post-test) the immobilization period. Data were analyzed by performing two-way (TIME×GROUP) repeated measures ANOVA. The MVC decreased in the immobilization group only (pre-test; 37.8±6.1N, post-test; 32.8±6.9N, p<0.0005) after the immobilization period. The force fluctuation increased only in the immobilization group (pre-test; 2.19±0.54%, post-test; 2.78±0.87%, p=0.007) after the immobilization period. These results demonstrate that induction of KiNVIS prevents negative effect on MVC and force fluctuation after 12h of immobilization.

Circadian regulation of metabolic homeostasis: causes and consequences

Robust circadian rhythms in metabolic processes have been described in both humans and animal models, at the whole body, individual organ, and even cellular level. Classically, these time-of-day-dependent rhythms have been considered secondary to fluctuations in energy/nutrient supply/demand associated with feeding/fasting and wake/sleep cycles. Renewed interest in this field has been fueled by studies revealing that these rhythms are driven, at least in part, by intrinsic mechanisms and that disruption of metabolic synchrony invariably increases the risk of cardiometabolic disease. The objectives of this paper are to provide a comprehensive review regarding rhythms in glucose, lipid, and protein/amino acid metabolism, the relative influence of extrinsic (eg, neurohumoral factors) versus intrinsic (eg, cell autonomous circadian clocks) mediators, the physiologic roles of these rhythms in terms of daily fluctuations in nutrient availability and activity status, as well as the pathologic consequences of dyssynchrony.

Ethanol does not delay muscle recovery but decreases testosterone/cortisol ratio

PURPOSE

This study investigated the effects of ethanol consumption on recovery from traditional resistance exercise in recreationally trained individuals.

METHODS

Nine recreationally trained volunteers (eight males and one female, 26 ± 4 yr, 81 ± 4 kg) conducted four resistance exercise sessions and consumed a low (0.6 (females) and 0.7 (males) g · kg(-1) body mass) or a high dose (1.2 or 1.4 g · kg(-1) body mass) of ethanol 1-2.5 h after exercise on two occasions. The first session was for familiarization with the tests and exercises and was performed without ethanol consumption. As a control trial, alcohol-free drinks were consumed after the exercise session. The sequence of trials, with low and high ethanol doses and alcohol-free drinks (control), was randomized. Maximal voluntary contractions (MVC) (knee extension), electrically stimulated contractions (knee extension), squat jumps, and hand grip strength were assessed 10-15 min and 12 and 24 h after the ethanol/placebo drinks. In addition to a baseline sample, blood was collected 1, 12, and 24 h after the ethanol/placebo drinks. The exercise session comprised 4 × 8 repetition maximum of squats, leg presses, and knee extensions.

RESULTS

MVC were reduced by 13%-15% immediately after the exercise sessions (P < 0.01). MVC, electrically stimulated force, and squat jump performance were recovered 24 h after ethanol drinks. MVC was not fully recovered at 24 h in the control trial. Compared with those in the control, cortisol increased and the free testosterone/cortisol ratio were reduced after the high ethanol dose (P < 0.01).

CONCLUSIONS

Neither a low nor a high dose of ethanol adversely affected recovery of muscle function after resistance exercise in recreationally strength-trained individuals. However, the increased cortisol levels and reduced testosterone/cortisol ratio after the high ethanol dose could translate into long-term negative effects.

Effects of time-of-day on neuromuscular function in untrained men: Specific responses of high morning performers and high evening performers

It has been clearly established that maximal force varies during the day in human muscles but the exact mechanisms behind the diurnal rhythms are still not fully clarified. Therefore, the aim of this study was to examine the diurnal rhythms in maximal isometric force production in a large group of participants and also by separating the high morning performance types (n = 8) and the high evening performance types (n = 19) from the neutral types (n = 45) based on their actual maximal isometric force levels. Measurements were performed in the morning (7:26 h ± 63 min) and in the evening (17:57 h ± 74 min) for maximal bilateral isometric leg press force (MVCLP) together with myoelectric activity (EMGLP), maximal unilateral isometric knee extension force (MVCKE) and maximal voluntary activation level (VA%) during maximal unilateral isometric knee extension force (MVCVA) together with myoelectric activity (EMGVA). In addition, venous blood samples were drawn four times a day and serum testosterone and cortisol concentrations were analyzed. None of the participants belonged to the extreme morning or evening chronotype according to the Munich Chronotype Questionnaire. In the total group of participants, MVCLP and MVCKE were 4.4 ± 12.9% (p < 0.01) and 4.3 ± 10.6% (p < 0.01) higher in the evening compared to the morning. MVCVA and VA% did not show significant diurnal variation. The high morning performance types showed lower force values in the evening compared to the morning for MVCLP (10.8 ± 9.1%; p < 0.05) and MVCKE (5.7 ± 4.9%; p < 0.05). No significant diurnal variation was observed for MVCVA and VA%. The high evening performance types showed higher force values in the evening for MVCLP (16.1 ± 15.9%; p < 0.001), MVCKE (13.5 ± 11.3%; p < 0.001) and MVCVA (6.2 ± 9.9%; p < 0.05) with a concomitant higher VA% in the evening (p < 0.05). The neutral types showed significantly higher evening force values for the MVCLP (2.1 ± 6.7%; p < 0.05). All the other neuromuscular variables did not show significant diurnal variations. EMGLP and EMGVA did not show significant diurnal fluctuations in any group. Serum testosterone and cortisol concentrations showed normal daily rhythms with higher values observed in the morning in all of the groups (p < 0.001). Between-group differences were observed for MVCLP (p < 0.001) and MVCKE (p < 0.001) between all of the three groups. Diurnal changes in VA% differed between the high evening performance types and the neutral types (p < 0.05) and the testosterone/cortisol ratio (p < 0.05) as well as vastus lateralis EMGVA (p < 0.05) differed between the high morning and high evening performance types. In conclusion, we were able to identify the high morning performance types, the high evening performance types and the neutral types who showed significantly different diurnal rhythms in force production, irrespective of their actual chronotype. Therefore, the questionnaires designed to determine the chronotype may not always be sensitive enough to determine the "morningness" or "eveningness" in maximal neuromuscular performance. In general, central factors could partially explain the diurnal fluctuations in maximal strength performance, but peripheral mechanisms were also possibly involved.

The effect of nitrate supplementation on muscle contraction in healthy adults

This study examined the effect of dietary supplementation with inorganic nitrate ([Formula: see text] ) on markers of contractile function in human knee extensors. In a double-blinded, randomized cross-over design, 18 (12 M) healthy participants undertook four days of supplementation with either nitrate-rich beetroot juice (NITRATE; days 1-3: 525 mg [Formula: see text], day 4: 1050 mg [Formula: see text]) or nitrate-depleted beetroot juice (PLACEBO). On the fourth day, isometric knee extension force was assessed during a series of voluntary and electrically evoked (stimulation) tests. In addition, muscular fatigue was examined in two separate continuous-stimulation (0.8 s tetanus with a 1:1 work:rest ratio for 102.4 s) fatigue tests, one with and one without blood flow restriction. There were no differences for maximum voluntary contraction, peak twitch force, half-relaxation time and the force-frequency relationship for stimulations up to 100 Hz between the NITRATE and PLACEBO trials. No differences between trials were observed in the non-restricted fatigue test, however NITRATE was found to attenuate the decline in force during the restricted test, such that the force at the 80 s mark (PLACEBO: 66 ± 11 vs. NITRATE 74 ± 9% of initial force; P = .01) and 102 s mark (PLACEBO: 47 ± 8% vs. NITRATE 55 ± 8%; P < .01) were significantly higher. These results suggest that four days of [Formula: see text] supplementation elicits peripheral responses in muscle that attenuate muscular fatigue during exhaustive exercise under hypovolemic conditions. This ergogenic action is likely attributable to improved Ca(2+) handling in the muscle, or enhanced perfusion during ischemia.

Time of day affects the frequency and duration of breathing events and the critical closing pressure during NREM sleep in participants with sleep apnea

We investigated if the number and duration of breathing events coupled to upper airway collapsibility were affected by the time of day. Male participants with obstructive sleep apnea completed a constant routine protocol that consisted of sleep sessions in the evening (10 PM to 1 AM), morning (6 AM to 9 AM), and afternoon (2 PM to 5 PM). On one occasion the number and duration of breathing events was ascertained for each sleep session. On a second occasion the critical closing pressure that demarcated upper airway collapsibility was determined. The duration of breathing events was consistently greater in the morning compared with the evening and afternoon during N1 and N2, while an increase in event frequency was evident during N1. The critical closing pressure was increased in the morning (2.68 ± 0.98 cmH2O) compared with the evening (1.29 ± 0.91 cmH2O; P ≤ 0.02) and afternoon (1.25 ± 0.79; P ≤ 0.01). The increase in the critical closing pressure was correlated to the decrease in the baseline partial pressure of carbon dioxide in the morning compared with the afternoon and evening (r = -0.73, P ≤ 0.005). Our findings indicate that time of day affects the duration and frequency of events, coupled with alterations in upper airway collapsibility. We propose that increases in airway collapsibility in the morning may be linked to an endogenous modulation of baseline carbon dioxide levels and chemoreflex sensitivity (12), which are independent of the consequences of sleep apnea.

Diurnal and day-to-day variation of isometric muscle strength in myasthenia gravis

INTRODUCTION

In patients with myasthenia gravis (MG), muscle strength is expected to decrease gradually during the day due to physical activities.

METHODS

Isometric muscle strength at the shoulder, knee, and ankle was determined in 10 MG patients (MGFA class II-IV) who were receiving usual medical treatment and in 10 control subjects. To determine diurnal and day-to-day variation, muscle strength was measured 4 times during day 1 and once at day 2.

RESULTS

Knee extension strength decreased during the day in both patients and controls. Neither diurnal nor day-to-day variation of muscle strength was higher in patients compared with controls.

CONCLUSIONS

Patients with mild to moderate MG did not have increased variation of isometric muscle strength during the day or from day-to-day compared with controls. This suggests that isometric muscle performance can be determined with high reproducibility in similar groups of MG patients without regard to time of day.

The effect of contralateral submaximal contraction on the development of biceps brachii muscle fatigue

OBJECTIVE

The aim of this study was to determine if a submaximal contraction in the contralateral limb affected the fatigability of the dominant limb.

BACKGROUND

Muscle fatigue is a known risk factor for musculoskeletal injury; however, it is unknown whether a submaximal contraction in the nondominant limb, such as for stabilizing a tool or load, affects the rate of development of fatigue, potentially increasing risk of injury. Current ergonomic assessments of injury risk do not involve consideration of submaximal contralateral demands. It was hypothesized that increased neuromuscular drive and active muscle mass during bilateral contractions would increase fatigability.

METHOD

Twelve males isometrically maintained a 30% unilateral contraction and a 30% dominant + 15% nondominant bilateral contraction until failure on two different collection days, separated by 7 days.

RESULTS

No statistically significant differences were found for time to task failure (p = .6204), decrease in maximal force (p = .1698), or alterations in electromyography amplitude (p = .7223) or frequency (p = .3292) between unilateral and bilateral conditions.

CONCLUSION

The hypothesis that the addition of a lesser submaximal isometric contraction would increase fatigability was rejected.

APPLICATION

These findings indicate that in ergonomic settings, muscle fatigability can be estimated by the more demanding task and do not need to be complicated by lesser submaximal contractions in the opposing limb.

Reliability of force per unit cross-sectional area measurements of the first dorsal interosseus muscle

The purpose of this study was to determine the reliability of maximum voluntary isometric force (MVIF), cross-sectional area (CSA) and force per unit CSA measures, of the first dorsal interosseus (FDI) muscle, using a custom-built dynamometer and ultrasonography. Twenty-seven participants completed MVIF and CSA measurements on two separate occasions under the same conditions. Reliability was determined using paired samples t-tests, systematic bias ratio and ratio limits of agreement (RLoA), intra-class correlation (ICC) and coefficient of variation (CV). MVIF of the FDI muscle (mean ± s; 31.8 ± 7.6 N and 31.6 ± 7.3 N) was not different between trials (P = 0.63); RLoA between trials were 1.00 ×/÷ 1.09, ICC = 0.990 and CV = 3.22%. CSA of the FDI muscle (22.6 ± 6.9 and 22.9 ± 6.9 mm²) was also not different between trials (P = 0.31); RLoA between trials were 0.98 ×/÷ 1.19, ICC = 0.979 and CV = 6.61%. Force per unit CSA was not different between trials (1.49 ± 0.43 and 1.46 ± 0.44 N·mm²; P = 0.18), RLoA were 1.02 ×/÷ 1.17, ICC = 0.985 and CV = 5.76%. The techniques used to determine MVIF and CSA of the FDI muscle were reliable and can be combined to calculate force per unit CSA.

Impact of range of motion during ecologically valid resistance training protocols on muscle size, subcutaneous fat, and strength

The impact of using different resistance training (RT) kinematics, which therefore alters RT mechanics, and their subsequent effect on adaptations remain largely unreported. The aim of this study was to identify the differences to training at a longer (LR) compared with a shorter (SR) range of motion (ROM) and the time course of any changes during detraining. Recreationally active participants in LR (aged 19 ± 2.6 years; n = 8) and SR (aged 19 ± 3.4 years; n = 8) groups undertook 8 weeks of RT and 4 weeks of detraining. Muscle size, architecture, subcutaneous fat, and strength were measured at weeks 0, 8, 10, and 12 (repeated measures). A control group (aged 23 ± 2.4 years; n = 10) was also monitored during this period. Significant (p > 0.05) posttraining differences existed in strength (on average 4 ± 2 vs. 18 ± 2%), distal anatomical cross-sectional area (59 ± 15 vs. 16 ± 10%), fascicle length (23 ± 5 vs. 10 ± 2%), and subcutaneous fat (22 ± 8 vs. 5 ± 2%), with LR exhibiting greater adaptations than SR. Detraining resulted in significant (p > 0.05) deteriorations in all muscle parameters measured in both groups, with the SR group experiencing a more rapid relative loss of postexercise increases in strength than that experienced by the LR group (p > 0.05). Greater morphological and architectural RT adaptations in the LR (owing to higher mechanical stress) result in a more significant increase in strength compared with that of the SR. The practical implications for this body of work follow that LR should be observed in RT where increased muscle strength and size are the objective, because we demonstrate here that ROM should not be compromised for greater external loading.

Changes in contractile and elastic properties of the triceps surae muscle induced by neuromuscular electrical stimulation training

PURPOSE

Neuromuscular electrical stimulation (NMES) training is known to induce improvement in force production capacities and fibre-type transition. The aim of this study was to determine whether NMES training also leads to changes in the mechanical properties of the human triceps surae (TS) muscle.

METHODS

Fifteen young male subjects performed a training protocol (4 weeks, 18 sessions, 4-5 sessions per week) based on a high-frequency isometric NMES programme of TS muscle. Quick-release test was used to evaluate Musculo-Tendinous (MT) stiffness index (SIMT) as the slope of the linear MT stiffness-torque relationships under submaximal contraction. Sinusoidal perturbations allowed the assessment of musculo-articular stiffness index (SIMA) as well as the calculation of the maximal angular velocity ([Formula: see text]) of TS muscle using an adaptation of Hill's equation.

RESULTS

After NMES training, Maximal Voluntary Contraction under isometric conditions and [Formula: see text] increased significantly by 17.5 and 20.6 %, respectively, while SIMT and SIMA decreased significantly (-12.7 and -9.3 %, respectively).

CONCLUSIONS

These changes in contractile and elastic properties may lead to functional changes of particular interest in sport-related activities as well as in the elderly.

The acute effect of upper-body complex training on power output of martial art athletes as measured by the bench press throw exercise

The purpose of this study was to examine the acute effect of upper body complex training on power output, as well as to determine the requisite preload intensity and intra-complex recovery interval needed to induce power output increases. Nine amateur-level combat/martial art athletes completed four distinct experimental protocols, which consisted of 5 bench press repetitions at either: 65% of one-repetition maximum (1RM) with a 4 min rest interval; 65% of 1RM with an 8 min rest; 85% of 1RM with a 4 min rest; or 85% of 1RM with an 8 min rest interval, performed on different days. Before (pre-conditioning) and after (post-conditioning) each experimental protocol, three bench press throws at 30% of 1RM were performed. Significant differences in power output pre-post conditioning were observed across all experimental protocols (F=26.489, partial eta2=0.768, p=0.001). Mean power output significantly increased when the preload stimulus of 65% 1RM was matched with 4 min of rest (p=0.001), and when the 85% 1RM preload stimulus was matched with 8 min of rest (p=0.001). Moreover, a statistically significant difference in power output was observed between the four conditioning protocols (F= 21.101, partial eta²=0.913, p=0.001). It was concluded that, in complex training, matching a heavy preload stimulus with a longer rest interval, and a lighter preload stimulus with a shorter rest interval is important for athletes wishing to increase their power production before training or competition.