Brain areas associated with force steadiness and intensity during isometric ankle dorsiflexion in men and women

Effects of visual feedback and force level on bilateral ankle-dorsiflexion force control

This study investigated the potential effects of visual feedback and force level on bilateral force control capabilities in the lower limbs. Thirty-nine healthy young adults performed bilateral ankle-dorsiflexion isometric force control tasks for different visual feedback conditions, including continuous visual feedback (CVF) and withdrawal of visual feedback (WVF), indicating the removal of visual feedback on force outputs during the task and force level conditions (i.e., 10 % and 40 % of the maximum voluntary contraction). Bilateral force control capabilities were estimated using force accuracy, variability, regularity, and absolute power in 0-4 Hz and interlimb coordination by cross-correlation with time lag and uncontrolled manifold (UCM) variables. Correlation analyses determined the relationship between changes in bilateral force control capabilities and interlimb coordination from the CVF to WVF conditions. The findings revealed better bilateral force control capabilities in the CVF condition as indicated by less force error, variability, regularity, absolute power in 0-4 Hz, and advanced interlimb force coordination. From CVF to WVF conditions, increased bad variability correlated with greater force control deficits. These findings suggest that visuomotor processing is an important resource for successful fine motor control in the lower limbs.

Sex differences in the fractal dynamics of force control during maximal handgrip

This work examines the temporal structure of force fluctuations during maximal handgrip with detrended fluctuation analysis (DFA α). Here, we assess the influence of fatigue and sex on force complexity during unimanual handgrip for the fatigued and the contralateral, non-fatigued hand. Participants randomly completed experimental sessions requiring fatiguing handgrip contractions or control measurements only. Maximal unimanual forces of both hands were measured before and after the fatigue trial or a time-matched control visit. DFA revealed substantially lower alpha values for females (PRE = 1.15, POST = 1.25) compared to males (PRE = 1.30, POST = 1.33) regardless of fatigue (p < 0.01, d = 0.738) for the dominant hand with a similar pattern observed for the contralateral, non-fatigued hand (p = 0.045, d = 0.561). Females also showed greater alpha changes (Δ = 0.09) versus males (Δ = 0.01) following fatigue (p = 0.028, ηp2 = 0.151). The data provide evidence of reduced force complexity during successive maximal handgrip contractions for females, but not males. Our findings highlight task-specific factors involving force control and demonstrate the utility of complexity analyses to provide insights regarding the influence of sex on motor control strategies.

Advanced Force Coordination of Lower Extremities During Visuomotor Control Task in Soccer Players

Purpose: This study is aimed to determine specific bilateral lower extremity motor performances and coordination patterns in soccer players with healthy controls using the bilateral force control paradigm. We hypothesized that soccer players would show more advanced bilateral force control performances than untrained controls. Methods: Participants were 13 university soccer players and 13 healthy controls. Each group performed bilateral ankle dorsiflexion force control tasks across two vision conditions (i.e., vision and no-vision) and two targeted force levels (i.e., 10% and 40% maximum voluntary contraction). We calculated force accuracy, variability, and symmetry to assess force control performances. To estimate bilateral force coordination, we calculated Pearson's correlation coefficients between feet-within a single trial and considered uncontrolled manifold variables across multiple trials. In the no-vision condition, we performed secondary analyses for initial force control patters after removal of visual feedback. Results: There were no significant group differences in bilateral force accuracy and variability but, compared to the control group, soccer players showed higher force symmetry between left and right ankle dorsiflexion forces. For force coordination between feet, soccer players revealed more negative values of the correlation coefficient and greater good variability from the uncontrolled manifold analysis than those for the control group. The secondary analysis revealed no significant group differences in the time until force drift and amount of force adjustments. Conclusions: Soccer players have more compensatory and flexible interlimb force coordination strategies between feet.

Relationship between disturbances of CO2 homeostasis and force output characteristics during isometric knee extension

To determine whether disturbances of CO₂ homeostasis alter force output characteristics of lower limb muscles, participants performed four isometric knee extension trials (MVC_30%, 10s each with 20-s rest intervals) in three CO₂ conditions (normocapnia [NORM], hypercapnia [HYPER], and hypocapnia [HYPO]). Respiratory frequency and tidal volume were matched between CO₂ conditions. In each MVC_30%, the participants exerted a constant force (30% of maximum voluntary contraction [MVC]). The force coefficient of variation (Fcv) during each MVC_30% and MVC before and after the four MVC_30% trials were measured. For the means of the four trials, Fcv was significantly lower in HYPER than in HYPO. However, within HYPER, a significant positive correlation was found between the increase in end-tidal CO₂ partial pressure and the increase in Fcv. MVCs in NORM and HYPO decreased significantly over the four trials, while no such reduction was observed in HYPER. These results suggest that perturbed CO₂ homeostasis influences the force output characteristics independently of breathing pattern variables.

Brain activity associated with quadriceps strength deficits after anterior cruciate ligament reconstruction

Prolonged treatment resistant quadriceps weakness after anterior cruciate ligament reconstruction (ACL-R) contributes to re-injury risk, poor patient outcomes, and earlier development of osteoarthritis. The origin of post-injury weakness is in part neurological in nature, but it is unknown whether regional brain activity is related to clinical metrics of quadriceps weakness. Thus, the purpose of this investigation was to better understand the neural contributions to quadriceps weakness after injury by evaluating the relationship between brain activity for a quadriceps-dominated knee task (repeated cycles of unilateral knee flexion/extension from 45° to 0°), , and strength asymmetry in individuals returned to activity after ACL-R. Forty-four participants were recruited (22 with unilateral ACL reconstruction; 22 controls) and peak isokinetic knee extensor torque was assessed at 60°/s to calculate quadriceps limb symmetry index (Q-LSI, ratio of involved/uninvolved limb). Correlations were used to determine the relationship of mean % signal change within key sensorimotor brain regions and Q-LSI. Brain activity was also evaluated group wise based on clinical recommendations for strength (Q-LSI < 90%, n = 12; Q-LSI ≥ 90%, n = 10; controls, all n = 22 Q-LSI ≥ 90%). Lower Q-LSI was related to increased activity in the contralateral premotor cortex and lingual gyrus (p < .05). Those who did not meet clinical recommendations for strength demonstrated greater lingual gyrus activity compared to those who met clinical recommendations Q-LSI ≥ 90 and healthy controls (p < 0.05). Asymmetrically weak ACL-R patients displayed greater cortical activity than patients with no underlying asymmetry and healthy controls.

Cortical activation in robot-assisted dynamic and static resistance training combining VR interaction: An fNIRS based pilot study

BACKGROUND

There are few isometric training systems based on upper limb rehabilitation robots. Its efficacy and neural mechanism are not well understood.

OBJECTIVE

This study aims to investigate the cortex activation of dynamic resistance and static (isometric) training based on upper limb rehabilitation robot combined with virtual reality (VR) interaction by using functional near-infrared spectroscopy (fNIRS).

METHODS

Twenty subjects were included in this study. The experiment adopts the block paradigm design. Experiment in dynamic and static conditions consisted of three trials, each consisting of task (60 s)-rest (40 s). The neural activities of the sensorimotor cortex (SMC), premotor cortex (PMC) and prefrontal cortex (PFC) were measured. The cortex activation and functional connectivity (FC) were analyzed.

RESULTS

Both the dynamic and static training can activate SMC, PMC, and PFC. In SMC and PMC, the activation of static training was stronger than dynamic training, there were significant differences between the two modes of each region of interest (ROI) (p <  0.05) (SMC: p = 0.022, ES = 0.72, PMC: p = 0.039, ES = 0.63). Besides, the FC between all ROIs of the static training was stronger than that of the dynamic training.

CONCLUSION

The static training based on upper limb rehabilitation robot may better facilitate the cortical activation associated with motor control.

Sex-Specific Brain Activations during Single-Leg Exercise

Background

Females have an increased incidence of musculoskeletal injuries compared to males. Sex differences in neuromuscular control has been widely studied regarding the dynamics and muscle activity during preplanned movements. While muscle activation patterns and movement biomechanics are understood to differ between sexes, it is not well understood how sex influences brain activity for lower extremity movement. Since the brain plays a vital role for voluntary movement and joint stability, it is important to understand the sex differences in brain function in order to better understand neuromuscular control associated with increased musculoskeletal injury risk in female.

Hypothesis/Purpose

The purpose of this study is to understand the differences in brain activation patterns between sexes during a simple active knee extension-flexion movement. It was hypothesized that females would demonstrate higher cortical activation in the somatosensory areas compared to males as a compensatory strategy.

Study Design

Cross-Sectional Study.

Methods

Thirteen males and seventeen females who were healthy and physically active participated in this study (Male: 23.7±3.8 years, 74.5±13.5 kg, 172.3±6.4 cm; Female: 20.6±1.6 years, 65.4±12.8 kg, 163±6.1 cm). Functional magnetic resonance imaging data were obtained during a simple left knee extension-flexion exercise with their own leg weight while lying on the MRI table. The blood oxygen level dependent (BOLD) signals were compared between sexes.

Results

There was significantly greater activation in the visual cortices and premotor cortex in females compared to males during the studied movement. Males demonstrated significantly greater activation in the right cerebellum.

Conclusion

The results revealed sex differences in BOLD signal during simple knee extension-flexion movement. The results suggest that sex may be a biological factor in understanding brain activity associated with knee motor control.

Level of Evidence

Level 3.

Relation between Cortical Activation and Effort during Robot-Mediated Walking in Healthy People: A Functional Near-Infrared Spectroscopy Neuroimaging Study (fNIRS)

Force and effort are important components of a motor task that can impact rehabilitation effectiveness. However, few studies have evaluated the impact of these factors on cortical activation during gait. The purpose of the study was to investigate the relation between cortical activation and effort required during exoskeleton-mediated gait at different levels of physical assistance in healthy individuals. Twenty-four healthy participants walked 10 m with an exoskeleton that provided four levels of assistance: 100%, 50%, 0%, and 25% resistance. Functional near-infrared spectroscopy (fNIRS) was used to measure cerebral flow dynamics with a 20-channel (plus two reference channels) device that covered most cortical motor regions bilaterally. We measured changes in oxyhemoglobin (HbO₂) and deoxyhemoglobin (HbR). According to HbO₂ levels, cortical activation only differed slightly between the assisted conditions and rest. In contrast, bilateral and widespread cortical activation occurred during the two unassisted conditions (somatosensory, somatosensory association, primary motor, premotor, and supplementary motor cortices). A similar pattern was seen for HbR levels, with a smaller number of significant channels than for HbO₂. These results confirmed the hypothesis that there is a relation between cortical activation and level of effort during gait. This finding should help to optimize neurological rehabilitation strategies to drive neuroplasticity.

Neuromuscular recruitment strategies of the vastus lateralis according to sex

AIM

Despite males typically exhibiting greater muscle strength and fatigability than females, it remains unclear if there are sex-based differences in neuromuscular recruitment strategies e.g. recruitment and modulation of motor unit firing rate (MU FR) at normalized forces and during progressive increases in force.

METHODS

The study includes 29 healthy male and 31 healthy female participants (18-35 years). Intramuscular electromyography (iEMG) was used to record individual motor unit potentials (MUPs) and near-fibre MUPs from the vastus lateralis (VL) during 10% and 25% maximum isometric voluntary contractions (MVC), and spike-triggered averaging was used to obtain motor unit number estimates (MUNE) of the VL.

RESULTS

Males exhibited greater muscle strength (P < .001) and size (P < .001) than females, with no difference in force steadiness at 10% or 25% MVC. Females had 8.4% and 6.5% higher FR at 10% and 25% MVC, respectively (both P < .03), while the MUP area was 33% smaller in females at 10% MVC (P < .02) and 26% smaller at 25% MVC (P = .062). However, both sexes showed similar increases in MU size and FR when moving from low- to mid-level contractions. There were no sex differences in any near-fibre MUP parameters or in MUNE.

CONCLUSION

In the vastus lateralis, females produce muscle force via different neuromuscular recruitment strategies to males which is characterized by smaller MUs discharging at higher rates. However, similar strategies are employed to increase force production from low- to mid-level contractions. These findings of similar proportional increases between sexes support the use of mixed sex cohorts in studies of this nature.

Acute effects of ankle plantar flexor force-matching exercises on postural strategy during single leg standing in healthy adults

BACKGROUND

Ankle plantar flexor force steadiness, assessed by measuring the fluctuation of the force around the submaximal target torque, has been associated with postural stability.

RESEARCH QUESTION

To investigate whether a force-matching exercise, where submaximal steady torque is maintained at the target torque, can modulate postural strategy immediately.

METHODS

Twenty-eight healthy young adults performed ankle plantar flexor force-matching exercises at target torques of 5%, 20%, and 50% of maximum voluntary contraction (MVC), in a randomized crossover trial. Participants with their ankle in a neutral position were instructed to maintain isometric contraction at each target torque, as measured by a dynamometer, for 20 s with 3 sets of 5 contractions. Before and after the force-matching exercises, the anterior-posterior velocities and standard deviation of the center of pressure (COP) on the stable platform and the tilt angle of the unstable platform during 20-seconds single-leg standing were measured. The velocities and standard deviations of the COP and tilt angle before and after the exercises were compared using paired t-tests.

RESULTS

The tilt angle velocity of an unstable platform significantly decreased after the force-matching exercise at a target torque of 5% MVC (p = 0.029), whereas it was unchanged after the exercises at target torques of 20% and 50% MVC. The standard deviations of the tilt angle of unstable platform test did not change significantly after any exercise. Furthermore, no significant differences were observed in the COP velocities or standard deviations on the stable platform test after any exercise.

SIGNIFICANCE

Our findings suggest that repeated exertion training at low-intensity contractions can affect postural stability in an unstable condition. Particularly, force-matching exercise at very low-intensity torque, such as 5% of MVC, may be an effective method to improve postural control in the unstable condition, but not in a stable condition.

Impairments in grip and pinch force accuracy and steadiness in people with osteoarthritis of the hand: A case-control comparison

BACKGROUND

Symptomatic hand osteoarthritis (OA) is severely disabling condition. Limited evidence has focused on force control measures in this population.

OBJECTIVES

It was the aim of the present study to determine whether force matching accuracy and steadiness are impaired in people with hand OA. In addition, the relationship between force control measures (accuracy and steadiness) and measures of hand function and pain in people with symptomatic hand OA was explored.

DESIGN

Case-control study.

METHOD

Sixty-two participants with symptomatic hand OA and 26 healthy pain-free controls undertook an isometric grip and pinch force matching task at 50 % of their maximum voluntary contraction. Average pain hand pain was recorded. In addition, the Disability of the Arm Shoulder and Hand Questionnaire (DASH), and the Functional Index of Hand Osteoarthritis were collected.

RESULTS

Grip force-matching accuracy and steadiness were significantly impaired in the hand OA group compared to controls (P < 0.05). Pinch force-matching error was greater in people with hand OA (P < 0.05), however, pinch force steadiness was not different between groups. There was a learning effect in people with hand OA, with resolution of force matching impairments with task repetition. A small positive correlation was identified between grip force control and the DASH. No association was found between other measures of force control and self-reported measures of function or pain.

CONCLUSIONS

People with hand OA presented with greater impairments in measures of submaximal force control. These were correlated with self-reported hand function but not pain. Future studies may wish to examine whether objective measures of functional performance are related to force-matching error and steadiness.

Revealing Sex Differences During Upper and Lower Extremity Neuromuscular Fatigue in Older Adults Through a Neuroergonomics Approach

Background: Sex differences in neuromuscular fatigue is well-documented, however the underlying mechanisms remain understudied, particularly for the aging population. Objective: This study investigated sex differences in fatigability of the upper and lower extremity of older adults using a neuroergonomics approach. Methods: Thirty community-dwelling older adults (65 years or older; 15 M, 15 F) performed intermittent submaximal fatiguing handgrip and knee extension exercises until voluntary exhaustion on separate days. Muscle activity from prime muscles of the hand/arm and knee extensors were monitored using electromyography, neural activity from the frontal, motor, and sensory areas were monitored using functional near infrared spectroscopy, and force output were obtained. Results: While older males were stronger than females across both muscle groups, they exhibited longer endurance times and greater strength loss during knee extension exercises. These lower extremity findings were associated with greater force complexity over time and concomitant increase in left motor and right sensory motor regions. While fatigability during handgrip exercises was comparable across sexes, older females exhibited concurrent increases in the activation of the ipsilateral motor regions over time. Discussion: We identified differences in the underlying central neural strategies adopted by males and females in maintaining downstream motor outputs during handgrip fatigue that were not evident with traditional ergonomics measures. Additionally, enhanced neural activation in males during knee exercises that accompanied longer time to exhaustion point to potential rehabilitation/exercise strategies to improve neuromotor outcomes in more fatigable older adults.

Corticomotor excitability of gluteus maximus and hip extensor strength: The influence of sex

PURPOSE

To compare hip extensor strength and corticomotor excitability (CME) of gluteus maximus (GM) between males and females. A secondary purpose was to determine if CME of GM is predictive of hip extensor strength.

METHOD

Thirty-two healthy individuals participated (15 males and 17 females). CME of GM was assessed using the input-output curve (IOC) procedure acquired from transcranial magnetic stimulation (average slope). Hip extensor strength was measured by a dynamometer during a maximal voluntary isometric contraction. Independent t-tests were used to compare CME of GM and peak hip extensor torque between males and females. Linear regression analysis was used to determine whether peak hip extensor torque was predicted by CME of GM.

RESULT

Compared to males, females demonstrate lower peak hip extensor torque (4.42 ± 1.11 vs. 6.15 ± 1.72 Nm/kg/m2, p < 0.01) and lower CME of GM (1.36 ± 1.07 vs. 2.67 ± 1.30, p < 0.01). CME of GM was a significant predictor of peak hip extensor torque for males and females combined (r² = 0.36, p < 0.001).

CONCLUSION

Our findings support the premise that corticomotor excitability plays a role in the ability of a muscle to generate torque.

Sex differences in the modulation of the motor unit discharge rate leads to reduced force steadiness

The purpose of this study was to evaluate the relationship between the variability in the motor unit inter-pulse interval and force steadiness at submaximal and maximal force outputs between the sexes. Twenty-four male and 24 female participants were recruited to perform isometric dorsiflexion contractions at 20, 40, 60, 80, and 100% maximum voluntary contraction. Tibialis anterior myoelectric signal was recorded by an intramuscular electrode. Females had lower force steadiness (coefficient of variation of force (CoV-Force), 27.3%, p < 0.01) and a greater coefficient of variation of motor unit action potential inter-pulse interval (CoV-IPI), compared with males (9.6%, p < 0.01). There was no significant correlation between the normalized CoV-IPI and CoV-Force (r = 0.19, p > 0.01), but there was a significant repeated measures correlation between the raw scores for root-mean-square force error and the standard deviation of motor unit discharge rate (r = 0.65, p < 0.01). Females also had a greater incidence of doublet discharges on average across force levels (p < 0.01). The sex differences may result from motor unit behaviours (i.e., doublet and rapid discharges, synchronization, rate coding or recruitment), leading to lower force steadiness and greater CoV-IPI in females. Novelty: Sex differences in force steadiness may be due to neural strategies. Females have lower force steadiness compared with males. Greater incidence of doublet discharges in females may result in lesser force steadiness.

Relationship between postural sway on an unstable platform and ankle plantar flexor force steadiness in community-dwelling older women

BACKGROUND

Force steadiness is evaluated as force variability during constant force exertion around a target level. Ankle plantar flexor force steadiness is reported to be related to postural sway on an unstable platform in healthy young adults; however, this relationship in older adults is unclear.

RESEARCH QUESTION

This study aimed to investigate whether ankle plantar flexor force steadiness was related to postural sway on stable and unstable platforms in older adults.

METHODS

Twenty-six community-dwelling older women participated in this study (72 ± 6 years). Maximal isometric strength and force steadiness at 5%, 20 %, and 50 % of the maximal strength of ankle plantar flexion were assessed. Postural sway in the anteroposterior direction during bipedal standing was measured on stable and unstable platforms.

RESULTS

The results showed that force steadiness at any intensity level and maximal isometric strength were not related to postural sway on the stable platform. Force steadiness at 20 % of maximal strength alone was significantly correlated with postural sway on the unstable platform (ρ = 0.441, p < 0.05).

SIGNIFICANCE

These results indicate that the ability to control muscle force could be important for postural stability on an unstable platform in older adults.

Sex differences in soleus muscle H-reflex and V-wave excitability

NEW FINDINGS

 What is the central question of this study? How do H-reflex and V-wave excitability compare between men and women engaging in similar levels of physical activity?  What is the main finding and its importance? H-reflex excitability is lower in women than in men because of their greater level of antagonist co-activation during sustained plantar flexion isometric exercise. In addition, supraspinal drive is similar between men and women independently of their differences in H-reflex excitability and antagonist muscle co-activation.

ABSTRACT

We compared H-reflex and V-wave excitability between men and women engaging in similar levels of physical activity. We also explored whether differences in antagonist muscle co-activation between sexes might partially explain sexual dimorphism in the excitability of the H-reflex and V-wave. Fifty-seven young participants were included (29 men: 21.7 ± 2.3 years; 28 women: 22.4 ± 3.3 years). Soleus M- and H-recruitment curves were constructed on a tonic background muscle activation. V-waves were elicited during maximal voluntary contraction (MVC). Besides being stronger than women, men achieved greater H_max /M_max values and presented a steeper slope of the ascending limb of the H-reflex recruitment curve (P < 0.05). The current intensity required to elicit H_max was lower for men (P < 0.05). The co-activation of the tibialis anterior muscle during the sustained plantar flexions was greater in women (ratio between tibialis and soleus normalized EMG: 20.5 vs. 8.3%, P < 0.05). Covariance analysis showed that sexual dimorphism in H-reflex excitability was dissipated when controlling for antagonist co-activation. V-wave normalized amplitude was similar between sexes even after controlling for the effects of H_max /M_max and antagonist co-activation as covariates. Thus, women exhibit lower H-reflex excitability than men and this is dependent on their higher level of antagonist muscle co-activation. While sex differences in antagonist co-activation persist during MVCs, this is not the case for V-wave normalized amplitude. Thus, although the efficacy of the transmission between Ia afferent fibres to α-motoneurons is lower in women because of a greater level of antagonist co-activation, our findings are consistent with similar supraspinal drive between sexes.

Force Control and Motor Unit Firing Behavior Following Mental Fatigue in Young Female and Male Adults

Purpose: The neuromuscular mechanisms leading to impaired motor performance in the presence of mental fatigue remain unclear. It is also unknown if mental fatigue differentially impacts motor performance in males and females. The purpose of this study was to assess the impact of mental fatigue on force production and motor unit (MU) firing behavior in males and females.

Methods: Nineteen participants performed 10-s isometric dorsiflexion (DF) contractions at 20 and 50% maximum voluntary contraction (MVC) before, during, and after completing 22 min of the psychomotor vigilance task (PVT), to induce mental fatigue. The DF force and indwelling MU firing behavior of the tibialis anterior (TA) was measured before and immediately following the PVT and within the first and final minutes of the PVT.

Results: Force steadiness and motor unit firing rate (MUFR) variability did not change during or following the PVT at either contraction intensity (p ≥ 0.16). Overall, females had more variability than males in MUFR during the 20% MVCs (15.98 ± 2.19 vs. 13.64 ± 2.19%, p = 0.03), though no sex differences were identified during the 50% MVCs (p = 0.20). Mean MUFR decreased following mental fatigue in both sexes in the 20% MVC condition (14.79 ± 3.20 vs. 12.92 ± 2.53 Hz, p = 0.02), but only in males during the 50% MVC condition (18.65 ± 5.21 vs. 15.03 ± 2.60 Hz, p = 0.01).

Conclusions: These results suggest possible sex and contraction intensity-specific neuromuscular changes in the presence of mental fatigue.

The Effect of Shortening-induced Torque Depression on Fatigue-related Sex Differences

Residual torque depression (rTD) is the decrease in isometric (ISO) torque after active shortening of skeletal muscle compared with a purely ISO contraction performed at the same muscle length and level of activation. Performance fatigability is defined as any exercise-induced reduction in voluntary force or power, and females are typically more fatigue resistant than males at low-intensity ISO contractions.

PURPOSE

This study investigated performance fatigability in males and females during ISO contractions and ISO contractions after active shortening (rTD).

METHODS

Fourteen females (22 ± 2 yr) and 14 males (23 ± 2 yr) performed three baseline maximal voluntary contractions (MVCs) of the dorsiflexors. The MVCs were used to determine a 30% submaximal torque target, which participants matched as steadily as possible until task failure. The ISO fatigue task was performed at 10° plantar flexion. In the rTD session, the participants' ankle was rotated from 40° to 10° plantar flexion before performing the same fatigue task. MVCs were performed immediately after task failure, 30 s, and 1, 2, 3, 4, 5, 10, 20, and 30 min after task failure to track recovery.

RESULTS

The baseline MVC torque amplitude for males (32.1 ± 6.6 N·m) was 31% greater than that for females (22.3 ± 3.1 N·m; P < 0.001, ηp = 0.490). Females' time to task failure was 44% longer than that of males in the ISO state (P = 0.032, ηp = 0.164). However, there was no sex difference in the rTD state (P = 0.142).

CONCLUSION

It seems that the sex differences in fatigue resistance observed in a low-intensity ISO task are abolished in the ISO state after an active shortening contraction.

Is there sufficient evidence to explain the cause of sexually dimorphic behaviour in force steadiness?

Neuromuscular noise is a determining factor in the control of isometric force steadiness (FS), quantified as coefficient of variation (CV) of force around a preestablished target output. In this paper we examine sex-related differences of neural, muscular, and tendon influences on neuromuscular noise to understand FS in females and males. We use evidence from the literature to identify that CV of force is higher in females compared with males in the upper and lower body, with sex-related differences becoming less apparent with increasing age. Evaluation of sex-related physiology in tandem with results from FS studies indicate that differences in fibre type, contractile properties, and number of motor units (MUs) are unlikely contributors to differences in FS between females and males. MU type, behaviour of the population (inclusive of number of active MUs from the population), agonist–antagonist activity, maximal strength, and tendon mechanics are probable contributors to sexually dimorphic behaviour in FS. To clearly determine underlying causes of sex-related differences in FS, further study and reporting between females and males is required. Females and males are included in many studies; however, rich data on sexually dimorphic behaviour is lost when data are collapsed across sex or identified as nonsignificant without supporting values. This poses a challenge to identifying the underlying cause of females having higher CV of force than males. This review provides evidence of sexually dimorphic behaviour in FS and suggests that physiological differences between females and males effect neuromuscular noise, and in-turn contribute to sex-related differences in FS.

Neural basis of exertional fatigue in the heat: A review of magnetic resonance imaging methods

The central nervous system, specifically the brain, is implicated in the development of exertional fatigue under a hot environment. Diverse neuroimaging techniques have been used to visualize the brain activity during or after exercise. Notably, the use of magnetic resonance imaging (MRI) has become prevalent due to its excellent spatial resolution and versatility. This review evaluates the significance and limitations of various brain MRI techniques in exercise studies-brain volumetric analysis, functional MRI, functional connectivity MRI, and arterial spin labeling. The review aims to provide a summary on the neural basis of exertional fatigue and proposes future directions for brain MRI studies. A systematic literature search was performed where a total of thirty-seven brain MRI studies associated with exercise, fatigue, or related physiological factors were reviewed. The findings suggest that with moderate dehydration, there is a decrease in total brain volume accompanied with expansion of ventricular volume. With exercise fatigue, there is increased activation of sensorimotor and cognitive brain areas, increased thalamo-insular activation and decreased interhemispheric connectivity in motor cortex. Under passive hyperthermia, there are regional changes in cerebral perfusion, a reduction in local connectivity in functional brain networks and an impairment to executive function. Current literature suggests that the brain structure and function are influenced by exercise, fatigue, and related physiological perturbations. However, there is still a dearth of knowledge and it is hoped that through understanding of MRI advantages and limitations, future studies will shed light on the central origin of exertional fatigue in the heat.

The Relevance of Sex Differences in Performance Fatigability

Performance fatigability differs between men and women for a range of fatiguing tasks. Women are usually less fatigable than men, and this is most widely described for isometric fatiguing contractions and some dynamic tasks. The sex difference in fatigability is specific to the task demands so that one mechanism is not universal, including any sex differences in skeletal muscle physiology, muscle perfusion, and voluntary activation. However, there are substantial knowledge gaps about the task dependency of the sex differences in fatigability, the involved mechanisms, and the relevance to clinical populations and with advanced age. The knowledge gaps are in part due to the significant deficits in the number of women included in performance fatigability studies despite a gradual increase in the inclusion of women for the last 20 yr. Therefore, this review 1) provides a rationale for the limited knowledge about sex differences in performance fatigability, 2) summarizes the current knowledge on sex differences in fatigability and the potential mechanisms across a range of tasks, 3) highlights emerging areas of opportunity in clinical populations, and 4) suggests strategies to close the knowledge gap and understanding the relevance of sex differences in performance fatigability. The limited understanding about sex differences in fatigability in healthy and clinical populations presents as a field ripe with opportunity for high-impact studies. Such studies will inform on the limitations of men and women during athletic endeavors, ergonomic tasks, and daily activities. Because fatigability is required for effective neuromuscular adaptation, sex differences in fatigability studies will also inform on optimal strategies for training and rehabilitation in both men and women.

Voluntary reduction of force variability via modulation of low-frequency oscillations

Visual feedback can influence the force output by changing the power in frequencies below 1 Hz. However, it remains unknown whether visual guidance can help an individual reduce force variability voluntarily. The purpose of this study, therefore, was to determine whether an individual can voluntarily reduce force variability during constant contractions with visual guidance, and whether this reduction is associated with a decrease in the power of low-frequency oscillations (0-1 Hz) in force and muscle activity. Twenty young adults (27.6 ± 3.4 years) matched a force target of 15% MVC (maximal voluntary contraction) with ankle dorsiflexion. Participants performed six visually unrestricted contractions, from which we selected the trial with the least variability. Following, participants performed six visually guided contractions and were encouraged to reduce their force variability within two guidelines (±1 SD of the least variable unrestricted trial). Participants decreased the SD of force by 45% (P < 0.001) during the guided condition, without changing mean force (P > 0.2). The decrease in force variability was associated with decreased low-frequency oscillations (0-1 Hz) in force (R ² = 0.59), which was associated with decreased low-frequency oscillations in EMG bursts (R ² = 0.35). The reduction in low-frequency oscillations in EMG burst was positively associated with power in the interference EMG from 35 to 60 Hz (R ² = 0.47). In conclusion, voluntary reduction of force variability is associated with decreased low-frequency oscillations in EMG bursts and consequently force output. We provide novel evidence that visual guidance allows healthy young adults to reduce force variability voluntarily likely by adjusting the low-frequency oscillations in the neural drive.

The aging neuromuscular system and motor performance

Age-related changes in the basic functional unit of the neuromuscular system, the motor unit, and its neural inputs have a profound effect on motor function, especially among the expanding number of old (older than ∼60 yr) and very old (older than ∼80 yr) adults. This review presents evidence that age-related changes in motor unit morphology and properties lead to impaired motor performance that includes 1) reduced maximal strength and power, slower contractile velocity, and increased fatigability; and 2) increased variability during and between motor tasks, including decreased force steadiness and increased variability of contraction velocity and torque over repeat contractions. The age-related increase in variability of motor performance with aging appears to involve reduced and more variable synaptic inputs that drive motor neuron activation, fewer and larger motor units, less stable neuromuscular junctions, lower and more variable motor unit action potential discharge rates, and smaller and slower skeletal muscle fibers that coexpress different myosin heavy chain isoforms in the muscle of older adults. Physical activity may modify motor unit properties and function in old men and women, although the effects on variability of motor performance are largely unknown. Many studies are of cross-sectional design, so there is a tremendous opportunity to perform high-impact and longitudinal studies along the continuum of aging that determine 1) the influence and cause of the increased variability with aging on functional performance tasks, and 2) whether lifestyle factors such as physical exercise can minimize this age-related variability in motor performance in the rapidly expanding numbers of very old adults.

Is better preservation of eccentric strength after stroke due to altered prefrontal function?

Ventrolateral prefrontal cortex (VLPFC) is part of a network that exerts inhibitory control over the motor cortex (MC). Recently, we demonstrated that VLPFC was more activated during imagined maximum eccentric than during imagined concentric contractions in healthy participants. This was accompanied with lower activation levels within motor regions during imagined eccentric contractions. The aim was to test a novel hypothesis of an involvement of VLPFC in contraction mode-specific modulation of force. Functional magnetic resonance imaging was used to examine differences in VLPFC and motor regions during the concentric and the eccentric phases of imagined maximum contractions in a selected sample of subjects with stroke (n = 4). The subjects were included as they exhibited disturbed modulation of force. The previously demonstrated pattern within VLPFC was evident only on the contralesional hemisphere. On the ipsilesional hemisphere, the recruitment in VLPFC was similar for both modes of contractions. The findings support a hypothesis of the involvement of VLPFC in contraction mode-specific modulation of maximum force production. A disturbance of this system might underlie the lack of contraction mode-specific modulation commonly found among stroke subjects, often expressed as an increased ratio between eccentric and concentric strength.

Maximal force and tremor changes across the menstrual cycle

PURPOSE

Sex hormones have profound effects on the nervous system in vitro and in vivo. The present study examines the effect of the menstrual cycle on maximal isometric force (MVC) and tremor during an endurance task.

METHODS

Nine eumenorrheic females participated in five study visits across their menstrual cycle. In each menstrual phase, an MVC and an endurance task to failure were performed. Tremor across the endurance task was quantified as the coefficient of variation in force and was assessed in absolute time and relative percent time to task failure.

RESULTS

MVC decreases 23% from ovulation to the mid luteal phase of the menstrual cycle. In absolute time, the mid luteal phase has the highest initial tremor, though the early follicular phase has substantially higher tremor than other phases after 150 s of task performance. In relative time, the mid luteal phase has the highest level of tremor throughout the endurance task.

CONCLUSIONS

Both MVC and tremor during an endurance task are modified by the menstrual cycle. Performance of tasks and sports which require high force and steadiness to exhaustion may be decreased in the mid luteal phase compared to other menstrual phases.