Intermittent Isometric Contractions Research Articles

Sex differences in knee extensor torque control.

There is currently equivocal evidence regarding sex-related differences in measures of muscle force and torque control. To that end, we investigated sex differences in knee extensor muscle torque control, using both magnitude- and complexity-based measures, across contraction intensities typical of activities of daily living. 50 participants (25 male, median age [and interquartile range] 23.0 [20.0-33.0]; 25 female, median age [and interquartile range] 21.0 [20.0-40.5]) performed a series of intermittent isometric knee extensor contractions at 10, 20 and 40% maximal voluntary contraction (MVC). Torque was measured in N·m and torque control was quantified according to the magnitude (standard deviation [SD], coefficient of variation [CV]) and complexity (approximate entropy [ApEn], detrended fluctuation analysis [DFA] α) of torque fluctuations. Males exhibited a significantly greater absolute magnitude (i.e., SD) of knee extensor torque fluctuations during contractions at 10% (P=0.011), 20% (P=0.002) and 40% MVC (P=0.003), though no sex differences were evident when fluctuations were normalised to mean torque output (i.e., CV). Males exhibited significantly lower ApEn during contractions at 10% (P=0.002) and 20% MVC (P=0.024) and significantly greater DFA α during contractions at 10% (P=0.003) and 20% MVC (P=0.001). These data suggest sex differences in muscle torque control strategies and highlight the need to consider both the magnitude and complexity of torque fluctuations when examining sex differences in muscle force control.

In Vivo Monitoring of Acute and Intermittent Fatigue in Sport Climbing Using Near-Infrared Spectroscopy Wearable Biosensors

The objectification of acute fatigue (during isometric muscle contraction) and cumulative fatigue (due to multiple intermittent isometric muscle contractions) plays an important role in sport climbing. The data of 42 participants were used in the study. Climbing performance was operationalized using maximal climbing-specific holding time (CSHT) by performing dead hangs. The test started with an initial measurement of handgrip strength (HGS) followed by three intermittent measurements of CSHT and HGS. During the test, finger flexor muscle oxygen saturation (SmO2) was measured using a near-infrared spectroscopy wearable biosensor. Significant reductions in CSHT and HGS could be found (p < 0.001), which indicates that the consecutive maximal isometric holding introduces cumulative fatigue. The reduction in CSHT did not correlate with a reduction in HGS over multiple consecutive maximal dead hangs (p > 0.35). Furthermore, there were no significant differences in initial SmO2 level, SmO2 level at termination, SmO2 recovery, and mean negative slope of the SmO2 saturation reduction between the different measurements (p > 0.24). Significant differences were found between pre-, termination-, and recovery- (10 s after termination) SmO2 levels (p < 0.001). Therefore, monitoring acute fatigue using athletes' termination SmO2 saturation seems promising. By contrast, the measurement of HGS and muscle oxygen metabolism seems inappropriate for monitoring cumulative fatigue during intermittent isometric climbing-specific muscle contractions.

Short-term cold-water immersion does not alter neuromuscular fatigue development during high-intensity intermittent exercise.

Pre-exercise cold-water immersion affects physical performance under ambient environment, however the mechanisms leading to this decrease remains to be elucidated. The purpose was to determine whether short-term lower-body immersion in cold water could induce acute changes in the development of neuromuscular fatigue after high-intensity exercise. Ten participants performed on two separate visits a fatigue task (60 intermittent isometric maximal voluntary contractions maintained over 3s and spaced by 2s of recovery) once after lower-body cold-water immersion (Pre-Cooling, 6min at 8.9°C ± 1.6°C) and another time without prior immersion (Control). Before and after the fatigue task, neuromuscular function was assessed during voluntary or evoked contractions (electrical stimulation performed on the femoral nerve) on contracted and relaxed on knee extensor muscles. No differences in neuromuscular fatigue were measured between Pre-Cooling and Control conditions, despite maximal voluntary contraction reductions (-49 and -48%, respectively, both p < 0.05), peripheral contractile capacities (both -28%, p < 0.05). Additionally, rate of perceived exhaustion increases over time for both conditions (both p < 0.05) with differences in the time course. Lower body immersion in extreme cold water for a short period of time was not a sufficient stimulus to induce a significant disruption of human body homeostasis: neuromuscular function was not significantly altered during a maximum intensity fatigue task.

Effect of a sex stereotype on cortical activity during a self-paced exercise: A motor-related cortical potential approach

Recent research has shown that inducing a negative stereotype toward women does not always decrease the subsequent motor performance of women, but can increase it, especially during endurance tasks. The mechanisms involved are nonetheless still poorly understood. The main aim of the present study was to investigate the effect of a negative stereotype toward women on men’s and women’s performance during an endurance task, and to analyze the neuropsychological mechanisms involved through motor-related cortical potentials and motivation toward men/women. Thirty-four participants were assigned to a negative stereotype toward women condition and a nullified-stereotype condition and performed 80 self-paced intermittent isometric elbow contractions at a moderate perceived intensity. Results showed that women performed better when assigned to the negative stereotype toward women condition, they were more motivated to outperform men, and their MRCP amplitudes were higher in this same condition over the prefrontal cortex (i.e., FP1 and FP2). Concerning men, they also performed better when the negative stereotype toward women was induced. However, no effect emerged on motivation toward women and MRCP amplitudes. This study showed that inducing a negative stereotype during an endurance task led to a performance increase in women, which is contrary to the stereotype threat theory, strengthening the idea of a task-dependency effect when inducing a negative stereotype. This performance improvement observed in women may be caused by increased motivation to outperform men and a planning of the upcoming movement. Concerning men, more research is needed to clarify the mechanisms involved in such performance improvement.

Sensitivity analysis of sex- and functional muscle group-specific parameters for a three-compartment-controller model of muscle fatigue

The three-compartment-controller with enhanced recovery (3CC-r) model of fatigue has been validated, in multiple stages and by different methods, for sustained (SIC) and intermittent isometric contractions (IIC). It has also been validated using a common methodology for both contraction types simultaneously to derive sex-specific representative model parameters for each functional muscle group, at the expense of reducing the sample size used to estimate each parameter set. In this study, a sensitivity analysis of the model to both variations in experimental measurements and to variations in the parameter values is carried out to estimate the robustness of the parameter sets. Torque decline prediction error is found to increase only slowly with increasing randomness injected into experimental data, with <1 % increases in error for 8–29 % variation in experimental endurance times. The results demonstrate that the obtained parameters from our previous study are reliable and can be used for fatigue prediction in multiple scenarios without significant loss of accuracy. For all sexes and functional muscle groups examined, the fatigue process dominates recovery in the experimental conditions examined. Finer estimates of the model’s recovery parameter will likely require changes to the experiment design in future studies.

EFFECT OF FATIGUE ON EXPLOSIVE STRENGTH AND MUSCLE ARCHITECTURE OF THE VASTUS LATERALIS

ABSTRACT Introduction: There has been little research on changes in rate of torque development (RTD) and muscle architecture. This study evaluated the effect of fatigue on RTD and muscle architecture of the vastus lateralis (VL). Methods: Seventeen volunteers (25.5 ± 6.2 years; 177.2 ± 12.9 cm; 76.4 ± 13.1 kg) underwent isokinetic knee extension assessment at 30°/s to obtain the peak torque (PT-ISK), before and after a set of intermittent maximal voluntary isometric contractions (MVIC) (15 reps – 3 s contraction, 3 s rest) used to promote muscle fatigue, monitored by the median frequency (MDF) of the electromyography from the VL, rectus femoris and vastus medialis muscles. Before and after the fatigue protocol, ultrasound images of the VL were obtained to measure muscle thickness (MT), fascicle length (FL), and fascicle angle (FA). The peak isometric torque (PT-ISM) and the RTDs in 50 ms windows were calculated for each MVIC. The RTDs were reported as absolute values and normalized by the PT-ISM. Results: Fatigue was confirmed due to significant reductions in MDF in all three muscles. After the fatigue protocol, the PT-ISK was reduced from 239.0±47.91 to 177.3±34.96 Nm, and the PT-MVIC was reduced from 269.5±45.63 to 220.49±46.94 Nm. All the RTD absolute values presented significant change after the fatigue protocol. However, the normalized RTD did not demonstrate any significant differences. No significant differences were found in the muscle architecture of the VL. Conclusions: The reduction in explosive strength occurred concomitantly with the reduction in maximum strength, as evidenced by the lack of changes in normalized TDT. Level of Evidence III.

Functional muscle group- and sex-specific parameters for a three-compartment controller muscle fatigue model applied to isometric contractions

The three-compartment controller with enhanced recovery (3CC-r) model of muscle fatigue has previously been validated separately for both sustained (SIC) and intermittent isometric contractions (IIC) using different objective functions, but its performance has not yet been tested against both contraction types simultaneously using a common objective function. Additionally, prior validation has been performed using common parameters at the joint level, whereas applications to many real-world tasks will require the model to be applied to agonistic and synergistic muscle groups. Lastly, parameters for the model have previously been derived for a mixed-sex cohort not considering the differece in fatigabilities between the sexes. In this work we validate the 3CC-r model using a comprehensive isometric contraction database drawn from 172 publications segregated by functional muscle group (FMG) and sex. We find that prediction errors are reduced by 19% on average when segregating the dataset by FMG alone, and by 34% when segregating by both sex and FMG. However, minimum prediction errors are found to be higher when validated against both SIC and IIC data together using torque decline as the outcome variable than when validated sequentially against hypothesized SIC intensity-endurance time curves with endurance time as the outcome variable and against raw IIC data with torque decline as the outcome variable.

Fatigue-induced changes in knee-extensor torque complexity and muscle metabolic rate are dependent on joint angle

PurposeJoint angle is a significant determinant of neuromuscular and metabolic function. We tested the hypothesis that previously reported correlations between knee-extensor torque complexity and metabolic rate ({text{m}dot{text{V}}text{O}}_{{2}}) would be conserved at reduced joint angles (i.e. shorter muscle lengths).MethodsEleven participants performed intermittent isometric knee-extensor contractions at 50% maximum voluntary torque for 30 min or until task failure (whichever occurred sooner) at joint angles of 30º, 60º and 90º of flexion (0º = extension). Torque and surface EMG were sampled continuously. Complexity and fractal scaling of torque were quantified using approximate entropy (ApEn) and detrended fluctuation analysis (DFA) α. {text{m}dot{text{V}}text{O}}_{{2}} was determined using near-infrared spectroscopy.ResultsTime to task failure/end increased as joint angle decreased (P < 0.001). Over time, complexity decreased at 90º and 60º (decreased ApEn, increased DFA α, both P < 0.001), but not 30º. {text{m}dot{text{V}}text{O}}_{{2}} increased at all joint angles (P < 0.001), though the magnitude of this increase was lower at 30º compared to 60º and 90º (both P < 0.01). There were significant correlations between torque complexity and {text{m}dot{text{V}}text{O}}_{{2}} at 90º (ApEn, r = − 0.60, P = 0.049) and 60º (ApEn, r = − 0.64, P = 0.035; DFA α, ρ = 0.68, P = 0.015).ConclusionThe lack of correlation between {text{m}dot{text{V}}text{O}}_{{2}} and complexity at 30º was likely due to low relative task demands, given the similar kinetics of {text{m}dot{text{V}}text{O}}_{{2}} and torque complexity. An inverse correlation between {text{m}dot{text{V}}text{O}}_{{2}} and knee-extensor torque complexity occurs during high-intensity contractions at intermediate, but not short, muscle lengths.

Recurrence quantification analysis of force signals to assess neuromuscular fatigue in men and women

Recurrence quantification analysis (RQA) is a nonlinear method providing information on the temporal structure of time series. RQA has been extensively used to explore various noisy and nonstationary physiological signals. However, the application of RQA to force signals acquired during voluntary fatiguing contractions performed until exhaustion remain to be investigated. We aimed to explore the sensitivity of the percentage of determinism (DET), an RQA predictability measure, to detect changes of force signal complexity induced by fatigue and recovery. Changes in force signal complexity were compared between women and men to explore the ability of DET measures to detect different fatigue profiles. Nineteen women and nineteen men performed intermittent isometric contractions of knee extensors at 50% of maximal voluntary contraction (MVC) until exhaustion. Participants performed MVC before, during and after the fatiguing task to assess neuromuscular fatigue. Recovery measurements were performed three minutes after exhaustion. Particular attention has been given to the selection of the input parameters of RQA and to the influence of nonstationarity. A detailed methodology is provided to apply RQA to force signals. At the whole group level, complexity decreased with fatigue then increased after recovery. Greater fatigability of men was associated with a faster loss of complexity (i.e. faster increase of DET) of force signals. After recovery, complexity returned to baseline value only for women. These findings confirm that RQA is suited to explore force signal temporal structure and is able to reveal changes of complexity induced by fatigue and recovery by taking into account sex differences.

Acute Effect of Sodium Bicarbonate Supplementation on Symptoms of Gastrointestinal Discomfort, Acid-Base Balance, and Performance of Jiu-Jitsu Athletes.

The purpose of the present study was to verify the acute effect of sodium bicarbonate supplementation on symptoms of gastrointestinal discomfort, acid-base balance and intermittent isometric handgrip test performance in Jiu-Jitsu athletes. Ten male (22.2 ± 3.9 years; 174 ± 0.07 cm; 74.5 ± 8.9 kg) jiu-jitsu athletes participated in this counterbalanced double-blind crossover study. Two protocols, a) supplementation with 0.3 g.kg-1 of body weight of sodium bicarbonate, and b) supplementation with 0.045 g.kg-1 of body weight of placebo substance, were employed. Gastrointestinal tolerability was assessed by the questionnaire. Blood samples were collected at three time points (baseline, pre-ISO, and post-ISO) to determine the responses of potential hydrogenionic (pH), bicarbonate (HCO3-), base excess (EB) and lactate concentrations. The maximum voluntary contraction test and the intermittent isometric contraction test were also performed. As a result, none of the athletes reported significant gastrointestinal discomfort (p > 0.05). HCO3-, pH, and EB at the pre-ISO and post-ISO moments were significantly higher for the sodium bicarbonate protocol. Lactate concentrations were significantly higher for both post-ISO protocols (p = 0.000). There was no significant difference in the performance of the maximum voluntary contraction test and the intermittent isometric contraction test (p > 0.05). Thus, we conclude that sodium bicarbonate supplementation does not generate adverse responses resulting in gastrointestinal discomfort, and does not benefit performance yet promotes a state of metabolic alkalosis.

Acupuncture and Dry Needling in the Reduction of Peripheral Acute Fatigue Induced in the Biceps Braquii: Protocol for a Single-blinded Randomized Controlled Clinical Trial

The present study aims to propose a protocol to verify the efficacy and acute effects of traditional Chinese acupuncture, dry needling, and the rest in peripheral acute fatigue (PAF) induced by intermittent isometric contractions of the nondominant biceps brachii (BB) of nonphysically active men in a randomized, single-blind clinical trial assessed with surface electromyography, contraction time in seconds, infrared thermal imaging, and visual analog scale applied to the PAF. These instruments will evaluate the median frequency, endurance time, temperature (°C), and perceived fatigue in BB of the volunteers. The measurements will be collected in four moments (Test 0, 01, 02, and 03) divided between the beginning and the end of two sets of exercises (Exercises 01 and 02) of intermittent isometric contractions. Trial identifierNCT03448120 in www.clinicaltrials.gov.

Ischemic Preconditioning Blunts Loss of Knee Extensor Torque Complexity with Fatigue.

ABSTRACTIntroductionNeuromuscular fatigue reduces the temporal structure, or complexity, of muscle torque output, purportedly through an effect on motor unit behavior. Ischemic preconditioning (IPC), an emerging ergogenic aid, has been demonstrated to have a potent effect on muscular output and endurance. We therefore tested the hypothesis that IPC would attenuate the fatigue-induced loss of muscle torque complexity.MethodsTen healthy participants (6 males/4 females) performed intermittent isometric knee extension contractions (6 s contraction, 4 s rest) to task failure at 40% maximal voluntary contraction. Contractions were preceded by either IPC (three bouts of 5 min proximal thigh occlusion at 225 mm Hg, interspersed with 5 min rest) or SHAM (as IPC, but occlusion at only 20 mm Hg) treatments. Torque and EMG signals were sampled continuously. Complexity and fractal scaling were quantified using approximate entropy (ApEn) and the detrended fluctuation analysis (DFA) α scaling exponent. Muscle oxygen consumption (mV˙O2) was determined using near-infrared spectroscopy.ResultsIPC increased time to task failure by 43% ± 13% (mean ± SEM, P = 0.047). Complexity decreased in both trials (decreased ApEn, increased DFA α; both P < 0.001), although the rate of decrease was significantly lower after IPC (ApEn, −0.2 ± 0.1 vs –0.4 ± 0.1, P = 0.013; DFA α, 0.2 ± 0.1 vs 0.3 ± 0.1, P = 0.037). Similarly, the rates of increase in EMG amplitude (P = 0.022) and mV˙O2 (P = 0.043) were significantly slower after IPC.ConclusionThese results suggest that the ergogenic effect of IPC observed here is of neural origin and accounts for the slowing of the rates of change in torque complexity, EMG amplitude, and mV˙O2 as fatigue develops.

Investigating the effect of fatigue on muscle microvasculature blood flow during intermittent isometric contraction.

Localized muscle fatigue (LMF) decreases muscular strength, while affects the performance and potentially increases the risk of musculoskeletal disorders (MSD). An important mechanism in recovering from muscle fatigue is blood flow (BF). The BF response to muscle contraction and fatigue is highly dynamic and difficult to predict, as it depends on both metabolic demand and intramuscular pressure. The aim of this study was to measure both fatigue and BF during intermittent exertion of the first dorsal interosseous (FDI) muscle, in order to better characterize the relationship between BF and LMF during muscle contraction and rest. This study utilized Diffuse Correlation Spectroscopy (DCS) for BF measurement within the microvasculature of the FDI muscle. Exertion levels (EL) for intermittent fatiguing contraction were set to 20%, 30%, and 40% of an individual's maximum voluntary contraction (MVC). Our results showed that as an individual fatigued, relative BF rates increased, on average, by ~66% during exertion periods and ~330% during rest periods. Differences between exerting and resting BF increased over time for every EL (p<0.04), increasing by up to 11 times the baseline BF. At the same levels of muscle capacity (%MVC), resting BF was also found to increase with EL consistently. Our findings highlight BF dependence on both EL and history of muscle contraction. These results imply a variable recovery rate based on both the current state of contraction, (i.e., exertion vs. rest), and the muscle contraction history. The outcome of our study may facilitate the estimation of BF, thus, the muscle recovery rate, which can be implemented in the fatigue models to improve the prediction of muscle capacity to generate force/power.

Task-specificity Of Corticospinal Excitability: The Influence Of Contractile Properties

Transcranial magnetic stimulation (TMS) is increasingly applied to investigate neurophysiological function during exercise interventions. Although often assessed in testing modalities that are different than the intervention, neurophysiological function may be task-specific. PURPOSE: To compare neurophysiological function between an isometric squat (SQT) and knee extension (KE). METHODS: Twenty-two young adults (2 women, 20 right-footed, age: 25±5yrs, BMI: 25.9±3.1, VO2: 46.2±8.8 ml·kg-1·min-1) performed isometric SQT (N=7) or KE (N=15), with hip-, knee- and ankle-joints fixed at 90° as part of a larger study, exposing participants to operational stress for a 5-day period. Lower extremity strength and muscle activity (RMS) were recorded during maximum voluntary contractions (MVCs), using a linear force transducer and electromyography (EMG) sensors placed over the vastus lateralis, respectively. Motor-evoked-potential (MEP)-based stimulus response curves (SRC) were derived using TMS and a double cone coil placed over the dominant motor cortex leg hotspot during intermittent isometric contractions at 15% MVC. Forty stimuli were applied for two rounds of SRC, with stimulator output (SO) ranging from 5-100% in 5% increments and random order. Since neurophysiological function did not differ across days, grand averaged responses were compared using multivariate ANOVAs or Mann-Whitney U. RESULTS: Greater force and muscle activity were evident for KE compared to SQT (Force: 1303.9±407.0 vs. 812.8±189.5N, p=0.01; EMGRMS: 0.04±0.003 vs. 0.06±0.01, p=0.03). KE corticospinal excitability was twice as high compared to SQT (1.4±0.7 vs. 0.7±0.4mV, p=0.04), but similar when normalized to muscle activity (KE: 25.3±13.9 vs. SQT: 17.4±10.7 mV·EMGRMS-1, p=0.21). No difference was evident in SRCV50 and SRCSLOPE (54±9 vs. 62±14%, p=0.12 and 5.0±1.9 vs. 4.2±1.1, p=0.35). CONCLUSION: Contractile and corticospinal excitability appear to be task-specific, discouraging the assessment of neurophysiological function in modalities that are different from the intervention. Contractile function further seems to influence corticospinal excitability, which may reflect the underlying differences in neuromechanics between the two movements. Supported by the Department of Defense W81XWH-16-PHTBIRP-CR3A.

Acute Effects Of Photobiomodulation Therapy On Muscle Force Recovery In Persons With Multiple Sclerosis

Symptomatic fatigue, as well as muscle fatigue, are common in persons with multiple sclerosis (PwMS), and both can negatively affect quality of life (QOL). Photobiomodulation Therapy (PBMT), comprising light in a 600-1000 nm bandwidth, is an emerging therapeutic modality thought to enhance mitochondrial function. There is evidence that PBMT can improve muscle fatigue, inflammation, and psychological status and thus might be beneficial for PwMS. PURPOSE: To investigate PBMT on muscle force recovery after a fatiguing contraction in PwMS. A secondary aim was to determine if self-reported symptom outcomes influenced the response to PBMT. METHODS: Randomized double-blinded repeated-measures design. Ambulatory males and females (n=17, 14 females) with relapsing-remitting MS were evaluated pre- and post-PBMT in 4 visits over 4 weeks. The muscle function measurements consisted of 3 maximal voluntary contractions (MVCs) of Tibialis Anterior (TA) muscle followed by 2 min. of intermittent isometric contraction at 45% MVC. PBMT was then administered to the TA muscle belly at 1 of 4 energies: 40J, 80J, 120J or placebo (active device: 3 pulsed wavelengths at the same time including 640nm, 875nm, and 905nm, placebo device: single pulsed wavelength at 640 nm). After PBMT, recovery MVC’s were obtained. Baseline psychosocial measurements comprising depression (CES-D), fatigue (MFIS) and QOL (PROMIS) were obtained during visit 1. McNemar’s test was used to test for each light dose compared to placebo and Spearman’s correlations were compared with Fisher’s r to z transformation. RESULTS: All subjects were able to recover strength to within 12% of their initial strength during the 120J visit. However, 6/17 subjects did not recover to the same degree after the placebo. This difference in force recovery between high (120J) dose and placebo was significant with p=0.03. Muscle recovery correlations with depression and QOL differed between 120J vs. Placebo (0.35 vs. -0.44, p=0.03 and -0.09 vs. 0.67, p=0.03, respectively). CONCLUSIONS: PBMT at 120J may improve muscle force recovery in PwMS. Those reporting greater depression, or lower QOL may benefit most from PBMT. Supported by a pilot grant from the National MS Society

A model-based estimation of critical torques reduces the experimental effort compared to conventional testing

PurposeCritical torque (CT) is an important fatigue threshold in exercise physiology and can be used to analyze, predict, or optimize performance. The objective of this work is to reduce the experimental effort when estimating CTs for sustained and intermittent isometric contractions using a model-based approach.Materials and methodsWe employ a phenomenological model of the time course of maximum voluntary isometric contraction (MVIC) torque and compute the highest sustainable torque output by solving an optimization problem. We then show that our results are consistent with the steady states obtained when simulating periodic maximum loading schemes. These simulations correspond to all-out tests, which are used to estimate CTs in practice. Based on these observations, the estimation of CTs can be formulated mathematically as a parameter estimation problem. To minimize the statistical uncertainty of the parameter estimates and consequently of the estimated CTs, we compute optimized testing sessions. This reduces the experimental effort even further.ResultsWe estimate CTs of the elbow flexors for sustained isometric contractions to be 28% of baseline MVIC torque and for intermittent isometric contractions consisting of a 3 s contraction followed by 2 s rest to be 41% of baseline MVIC torque. We show that a single optimized testing session is sufficient when using our approach.ConclusionsOur approach reduces the experimental effort considerably when estimating CTs for sustained and intermittent isometric contractions.

Sex differences in muscle activity emerge during sustained low-intensity contractions but not during intermittent low-intensity contractions.

Sex differences in motor performance may arise depending on the mode of contraction being performed. In particular, contractions that are held for long durations, rather than contractions that are interspersed with rest periods, may induce greater levels of fatigue in men compared to women. The purpose of this study was to examine fatigue responses in a cohort of healthy men (n = 7, age [mean] = 21.6 ± [SD] 1.1 year) and women (n = 7, age: 22.0 ± 2.0 year) during sustained isometric and intermittent isometric contractions. Two contraction protocols were matched for intensity (20% MVC) and total contraction time (600‐s). Biceps brachii EMG and elbow flexion torque steadiness were examined throughout each protocol, and motor nerve stimulation was used to quantify central and peripheral fatigue. Overall, there were few sex‐related differences in the fatigue responses during intermittent contractions. However, men exhibited progressively lower maximal torque generation (39% versus 27% decrease), progressively greater muscle activity (220% versus 144% increase), progressively greater declines in elbow flexion steadiness (354% versus 285% decrease), and progressively greater self‐perception of fatigue (Borg scale: 8.8 ± 1.2 versus 6.3 ± 1.1) throughout the sustained contractions. The mechanism underlying fatigue responses had a muscle component, as voluntary activation of the biceps brachii did not differ between sexes, but the amplitude of resting twitches decreased throughout the sustained contractions (m: 32%, w: 10% decrease). As generating large sustained forces causes a progressive increase in intramuscular pressure and mechanical occlusion—which has the effect of enhancing metabolite accumulation and peripheral fatigue—it is likely that the greater maximal strength of men contributed to their exacerbated levels of fatigue.

Strength and Electromyographic Responses of Upper and Lower Limbs During Maximal Intermittent Contractions in Males and Females.

Carr, JC and Ye, X. Strength and electromyographic responses of upper and lower limbs during maximal intermittent contractions in males and females. J Strength Cond Res 36(9): 2403-2409, 2022-This study examined the strength and electromyographic (EMG) responses of upper vs. lower limb muscles during intermittent maximal contractions in both sexes. Twenty subjects ( n = 7 women) performed a fatiguing protocol (6, 30-second intermittent maximal isometric contractions with a 50% duty cycle) with either the elbow flexors or the knee extensors on separate visits. Bipolar surface EMG signals were detected from the biceps brachii and vastus lateralis muscles ( n = 5 women retained). Women maintained more of their maximal force than men (Δforce: men vs. women = -55.0 ± 12.8% vs. -43.3 ± 9.9%, p = 0.042). Although force loss was similar between the elbow flexors and knee extensors, the EMG responses showed greater reductions for the biceps brachii than those for the vastus lateralis (Δamplitude: biceps brachii vs. vastus lateralis: -32.0 ± 22.3% vs. -18.9 ± 28.9%; Δmedian frequency: biceps brachii vs. vastus lateralis: -31.1 ± 14.5% vs. -10.3 ± 17.0%). During a series of maximal intermittent isometric contractions with 30 seconds of recovery between work bouts, women are more fatigue resistant than men. In addition, the greater electrophysiological fatigue exhibited by the biceps brachii than that by the vastus lateralis suggests that high-intensity contractions involving elbow flexion will have a greater rate of fatigue progression than those involving knee extension.

Determinants of Climbing Performance: When Finger Flexor Strength and Endurance Count.

Marcolin, G, Faggian, S, Muschietti, M, Matteraglia, L, and Paoli, A. Determinants of climbing performance: when finger flexor strength and endurance count. J Strength Cond Res XX(X): 000-000, 2020-Aim of the study was: (a) to compare finger flexor strength and endurance among climbers and nonclimbers; (b) to predict climbers' level of ability using climbing-specific strength tests and prolonged fatigue protocols. 17 advanced climbers (ADV), 17 intermediate climbers (INT), and 15 nonclimbers (NOCLIMB) performed a maximal finger grip test on a climbing-specific device, a maximal handgrip test, 20 intermittent isometric maximal contractions (E1), a suspension test on a bar till exhaustion, and again 20 intermittent isometric maximal contractions (E2). Strength values were normalized to body weight (%BW). The handgrip test failed to discriminate ADV from INT. Maximal finger flexor strength differed among ADV (59.90 ± 9.42 %BW), INT (46.75 ± 8.40 %BW) and NOCLIMB (36.40 ± 6.51 %BW) (p < 0.0001; ηp: 0.586). ADV showed the best suspension time (58.55 ± 14.87 seconds) followed by INT (32.55 ± 16.87 seconds) and NOCLIMB (17.20 ± 14.30 seconds) (p < 0.0001; ηp: 0.563). ADV showed the best endurance performance in both E1 and E2. The highest correlations with climbers' ability scores were obtained with sport-specific tests (maximal finger strength, r = 0.60, p < 0.0001; bar suspension, r = 0.69, p < 0.0001) and at the highest level of fatigue (E2, r = 0.74, p < 0.0001). Strength and endurance gain more importance in determining climbers' ability if assessed with finger specific tests and after prolonged fatigue.

Effects of Nonstationarity on Muscle Force Signals Regularity During a Fatiguing Motor Task.

Physiological signals present fluctuations that can be assessed from their temporal structure, also termed complexity. The complexity of a physiological signal is usually quantified using entropy estimators, such as Sample Entropy. Recent studies have shown a loss of force signal complexity with the development of neuromuscular fatigue. However, these studies did not consider the stationarity of the force signals which is an important prerequisite of Sample Entropy measurements. Here, we investigated the effect of the potential nonstationarity of force signals on the kinetics of neuromuscular fatigue-induced change in force signal's complexity. Eleven men performed submaximal intermittent isometric contractions of knee extensors until exhaustion. Neuromuscular fatigue was assessed from changes in voluntary and electrically evoked contractions. Sample Entropy values were computed from submaximal force signals throughout the fatiguing task. The Dickey-Fuller test was used to statistically investigate the stationarity of force signals and the Empirical Mode Decomposition was applied to detrend these signals. Maximal voluntary force, central voluntary activation and muscle twitch decreased throughout the task (all ), indicating the development of global, central and peripheral fatigue, respectively. We found an increase in Sample Entropy with fatigue ( p = 0.024 ) when not considering the nonstationarity of force signals (i.e., 43% of nonstationary signals). After applying the Empirical Mode Decomposition, we found a decrease in Sample Entropy with fatigue ( p = 0.002 ). These findings confirm the presence of nonstationarity in force signals during submaximal isometric contractions which influences the kinetics of Sample Entropy with neuromuscular fatigue.