Differential postural effects of plantar–flexor muscle fatigue under normal, altered and improved vestibular and neck somatosensory conditions

Sex differences concerning the effects of ankle muscle fatigue on static postural control and spinal proprioceptive input at the ankle

Aims

The main aim of this study was to determine sex differences in postural control changes with ankle muscle fatigue during a standing forward leaning (FL) task under different vision conditions. The secondary aim was to examine sex differences in the effect of fatigue on soleus (SOL) H-reflex amplitude, a measure of motoneuron excitability with activation of Ia afferents.

Methods

Fifteen healthy young adult males (mean age: 28.0 years) and 16 healthy young adult females (mean age: 26.1 years) were asked to perform four consecutive FL tasks [30 s; two with eyes open (EO) and two with eyes closed (EC)] before, and immediately following a fatiguing exercise consisting of alternating ankle plantarflexion (6 s) and dorsiflexion (2 s) maximal isometric contractions, and at 5 and 10 min of recovery. Center of pressure (COP) sway variables (mean position, standard deviation, ellipse area, average velocity, and frequency), an ankle co-contraction index, and a ratio of SOL H-reflex to the maximum amplitude of the compound muscle action potential (M-max) were obtained during the FL tasks. A rating of perceived fatigue (RPF) was also documented at the different time points.

Results

Time to task failure (reduction of 50% in maximal voluntary isometric contraction torque of ankle plantar flexors) and the increase in RPF value were not significantly different between males and females. Both sex groups showed similar and significant increases (p < 0.05) in mean COP sway velocity with no significant changes in co-contraction indices. No significant effects of fatigue and related interactions were found for SOL H/M-max ratio.

Discussion

The absence of a significant sex difference in postural control change (sway and co-contraction) with fatigue could be explained by similar perceived (RPF) and performance fatigability (exercise duration) between males and females in the present study. Fatigue did not lead to significant changes in SOL spinal motoneuron excitability with activation of Ia afferents.

Persistent cancer-related fatigue after breast cancer treatment predicts postural sway and post-exertional changes in sit-to-stand strategy

Background

Chemotherapy-induced peripheral neuropathy (CIPN) is considered a primary mechanism of imbalance among women diagnosed with breast cancer. Recent evidence, however, suggests that cancer-related fatigue (CRF) may also influence balance.

Purpose

Examine the contributions of CRF and CIPN to static and dynamic balance before and after a period of fatiguing exercise.

Methods

This is a secondary analysis of data examining functional differences between women with breast cancer with and without persistent CRF. Postural sway was measured during static standing and the rising phase of an instrumented sit-to-stand (ISTS) before and after exercise. Regression analyses were performed to determine how CRF and severity of CIPN predicted sway and how much variance was attributable to each.

Results

Greater CRF predicted increased pre-, p=.04, and post-exertional, p=.02, static sway in the anterior-posterior plane. CRF accounted for 10.5% and 9.5% of the variance in pre- and post-exertional sway (respectively) compared to the 0.9% and 1.4% accounted for by CIPN severity which was not a significant predictor. After exercise, greater CRF predicted smaller, more conservative, anterior weight shifting during the ISTS, p=.01, and accounted for 6.6% of the variance in sway compared to 3% attributed to CIPN which was not a significant predictor.

Limitations

This analysis is limited by its small and demographically homogenous sample.

Conclusions

These results suggest that CRF may influence balance independent of CIPN symptoms. While CIPN remains a risk factor for imbalance in this population, CRF warrants consideration in clinical practice and research as a mechanism of postural instability.

Sex differences in the effect of muscle fatigue on static postural control under different vision and task conditions

The main aim of this study was to compare the effects of ankle plantar flexors fatigue on postural control between healthy young adult males and females. The secondary aim was to determine the effects of vision on the fatigue-induced postural changes. Ten healthy young males and nine females were asked to perform quiet standing (QS) and standing forward lean (FL) tasks with eyes open (EO) and closed (EC) before and immediately following exercise, and throughout a 15-min recovery period. A sustained isometric exercise of ankle plantar flexors was performed until participants were no longer able to maintain a target torque of 50% of maximal voluntary isometric contraction (MVIC). Mean anteroposterior (AP) and mediolateral (ML) positions of the center of pressure (COP), mean COP sway velocity, and 95% ellipse area of COP sway were measured. Ankle plantar flexors fatigue had significant effects on all dependent variables, except for sway area. A fatigue X sex interaction was found for sway velocity with the most challenging task condition (FL-EC), where males showed a significant increase in sway velocity up to 15 min following exercise, whereas females did not. Fatigue X vision interactions for AP position were also found, with the withdrawal of vision leading to a greater backward shift during recovery for both the QS (5 to 15 min) and FL (5 to 10 min) tasks. Our findings suggest the use of different postural control strategies with ankle fatigue between males and females, and also a contribution of vision to compensate for fatigue-induced instability that is not dependent on task difficulty.

Physiological Mechanisms of Exercise and Its Effects on Postural Sway: Does Sport Make a Difference?

While the effect of a variety of exercises on postural balance control has been extensively studied, less attention has been paid to those requiring sport-specific skills. Therefore there is a need to analyze the literature and elucidate changes in postural balance control after exercises performed in conditions close to a particular sport. This scoping review aims (i) To map the literature that addresses postural sway aspects of acute responses to general and sport-specific exercises and their underlying physiological mechanisms, and (ii) To identify gaps in the existing literature and propose future research on this topic. The main literature search conducted on MEDLINE, Web of Science, Scopus, PubMed, and Cochrane Library databases was completed by SpringerLink, Elsevier, and Google Scholar. A total of 60 articles met the inclusion criteria. Findings identified that among a variety of studies evaluating the effects of exercise on postural balance control, only few of them were conducted under sport-specific conditions (i.e., while shooting in biathlon or pentathlon, and after simulated or match-induced protocols in combat and team sports). Therefore, more research is still needed to address this gap in the literature and aim research at investigation of postural sway response to sport-specific exercises. Further analysis of the literature showed that the type, intensity and duration of exercise play a key role in increased postural sway. Whole body and localized muscular fatigue of the trunk, neck and lower limbs is considered to be a main factor responsible for the magnitude of balance impairment in an initial phase of recovery and speed of its readjustment to a pre-exercise level. Other likely factors affecting postural stability are hyperventilation and deterioration of sensorimotor functions, though some contribution of muscle damage, dehydration, hyperthermia or dizziness cannot be excluded. A better understanding of the physiological mechanisms of balance impairment after exercises performed under simulated fatigue induced protocol, close to conditions specific to a particular sport, has implications for designing smart exercise programs tailored to individual needs to improve athlete performance with high demands on postural stability and/or decrease their risk of injuries.

Rating of perceived exertion (RPE) in studies of fatigue-induced postural control alterations in healthy adults: Scoping review of quantitative evidence

BACKGROUND

Amongst the literature researching the effects of exercise-induced fatigue on postural control in healthy adults, many studies have used the Borg scales to document the rating of perceived exertion (RPE) and have shown a broad range of RPE values. Our main aim was to map fatigue-induced RPE values in included publications. Secondary aims were to summarize the preference and purpose for the use of Borg scales within the included publications and to explore the potential associations between fatigue-induced RPE values and postural control changes.

METHODS

Five databases (Ovid Medline, PubMed, CINAHL, Scopus, and SPORTDiscus) were systematically searched for synthesizing data among the publications that reported RPE values on the Borg RPE- and Category-Ratio (CR) 10 scales and also found fatigue effects on postural control in healthy adults. Spearman's rank correlations were conducted to assess potential associations between fatigue-induced RPE values and maximal postural control changes across the included publications (group data).

RESULTS

45 of 51 studies included in this review reported maximal RPE values following exercise and ranged from 10.4-20 (6-20 Borg RPE) or 0.9-10 (CR10) indicating "very light" or "very weak" to "maximal" exertions. The 6-20 Borg and CR10 scales were mainly used to assess cardiovascular and muscular exertion, respectively. The scales were used mostly to estimate fatigue levels (n = 45), and to a lesser extent to produce a specific exercise intensity (n = 5) and as the criterion for exercise termination (n = 1). In general, there was no significant association between RPE and postural control changes across studies.

CONCLUSION

The broad range of RPE values and weak correlations may suggest that various fatigue levels can lead to postural control changes. However, one should be careful in comparing the extent of fatigue from RPE values and its potential effect on postural control in the light of many confounding factors.

Smartphone-based human fatigue level detection using machine learning approaches

Human muscle fatigue is the main result of diminishing muscle capability, leading to reduced performance and increased risk of falls and injury. This study provides a classification model to identify the human fatigue level based on the motion signals collected by a smartphone. 24 participants were recruited and performed the fatiguing exercise (i.e. squatting). Upon completing each set of squatting, they walked for a fixed distance while the smartphone attached to their right shank and the gait data were associated with the Borg's Rating of Perceived Exertion (i.e. data label). Our machine-learning model of two (no- vs. strong-fatigue), three (no-, medium-, and strong-fatigue) and four (no-, low-, medium-, and strong-fatigue) levels of fatigue reached the accuracy of 91, 78, and 64%, respectively. The outcomes of this study may facilitate the accessibility of a fatigue-monitoring tool in the workplace, which improves the workers' performance and reduce the risk of falls and injury. Practitioner Summary: This study aimed to develop a machine-learning model to identify human fatigue level using motion data captured by a smartphone attached to the shank. Our results can facilitate the development of an accessible fatigue-monitoring system that may improve the workers' performance and reduce the risk of falls and injury. Abbreviations: WMSD: work-related musculoskeletal disorders; IMU: inertial measurement unit; RPE: rating of perceived exertion; SVM: support vector machine; IRB: institutional review board; SOM: self-organizing map; LDA: linear discriminant analysis; PCA: principal component analysis; FT: fourier transformation; RBF: radial basis function; CUSUM: cumulative sum; ROM: range of motion; MVC: maximum voluntary contractions.

Fatigue effects on the viscoelastic behavior of men and women in a landing task: a Mass-Spring-Damper modeling approach

The aim of this study was to compare the fatigue effect on the viscoelastic behavior of the musculoskeletal system between the two sexes in a landing task. Stiffness and shock absorption capacity were studied from ground reaction force data using a mass-spring-damper model. A parametric simulation was also performed to demonstrate the effect of different values of model parameters on the vertical ground reaction force. Following fatigue, the shock absorption capacity reduced in men. According to the parametric simulation, this reduction can result in lower rate of force development which may make men less susceptible to impact injuries than women.

Sensorimotor Strategies in Individuals With Poststroke Hemiparesis When Standing Up Without Vision

This study investigated the sensorimotor strategies for dynamic balance control in individuals with stroke by restricting sensory input that might influence task accomplishment. Sit-to-stand movements were performed with restricted vision by participants with hemiparesis and healthy controls. The authors evaluated the variability in the position of participants’ center of mass and velocity, and the center-of-pressure position, in each orthogonal direction at the lift-off point. When vision was restricted, the variability in the mediolateral center-of-pressure position decreased significantly in individuals with hemiparesis, but not in healthy controls. Participants with hemiparesis adopted strategies that explicitly differed from those used by healthy individuals. Variability may be decreased in the direction that most requires accuracy. Individuals with hemiparesis have been reported to have asymmetrical balance deficits, and that meant they had to prioritize mediolateral motion control to prevent falling. This study suggests that individuals with hemiparesis adopt strategies appropriate to their characteristics.

Effects of Plantar Flexor Muscles Fatigue on Postural Control during Quiet Stance and External Perturbation in Healthy Subjects

BACKGROUND

The maintenance of postural control is a key component in dynamic physical activity, especially during muscle fatigue and against external forces. Despite many studies in this field, there is no consensus regarding the effects of plantar flexor muscles fatigue on postural control during different postural tasks.

OBJECTIVE

To evaluate the effects of plantar flexor muscles fatigue on postural control during quiet stance and external perturbation in healthy subjects.

MATERIAL AND METHODS

Twenty four healthy individuals (20-35 years) participated this interventional study. The foot center of pressure data was measured using a single force platform, and then the postural control parameters, including the center of pressure displacement and velocity in the anterior-posterior and medial-lateral direction and also path length calculated under two conditions; quiet and perturbed stance, before and after plantar flexor muscles fatigue.

RESULTS

The statistical analysis demonstrated that mean displacement and velocity of the center of pressure in the anterior-posterior direction and also path length increased after the fatigue protocol in the perturbed condition. However, fatigue had no significant effects on postural control parameters in the quiet standing condition.

CONCLUSION

These results indicated that the effects of muscle fatigue on postural control depend on the difficulty of the task and the relevance of proprioceptive information. The postural control system appears to use distinct control strategies in different situations such as quiet and perturbed stance conditions, and these strategies may be differentially altered by fatigue. In conclusion, due to the potential risk of loss of balance, it is important to take the role of plantar flexor muscle fatigue into account during more difficult postural tasks.

A prophylactic effect of local vibration on quadriceps muscle fatigue in non-athletic males: a randomized controlled trial study

[Purpose] This study was conducted to investigate the immediate prophylactic effects of local vibration on quadriceps muscle fatigue in young non-athletic males. [Participants and Methods] Thirty healthy young males were randomly assigned to vibration and sham control groups. Participants in the local vibration group received a single session vibration (2 minutes, 30 Hz). They also in the control group received a 2-minute vibration, while the vibration system was off. MVC, RMS and median frequency of EMG findings and time to reach fatigue were measured. [Results] Time to reach fatigue and MVC in the local vibration group was significantly high than those in the sham control group. [Conclusion] It seems that the prior local muscle vibration may be useful to attenuate some signs and symptoms of muscle fatigue.

Effect of Gastrocnemius Kinesio Taping on Countermovement Jump Performance and Vertical Stiffness Following Muscle Fatigue

Context: Kinesio tape (KT) is a widely used intervention in the fields of sports and rehabilitation. However, its effects on lower-extremity behavior during functional activities are not entirely known. Objective: To test the hypothesis that application of KT can change performance and vertical stiffness (VS) during the countermovement jump (CMJ) before and after a fatigue protocol. Design: A predesign and postdesign to study the effect of KT in 2 situations, prefatigue and postfatigue. In each fatiguing condition, there were 2 conditions with and without KT application on the gastrocnemius muscle. Setting: Biomechanics laboratory. Participants: Fifty healthy, nonathlete participants (26 females and 24 males). Interventions: KT application on the gastrocnemius muscle and a fatigue protocol to induce fatigue in plantar flexor muscles. Main Outcome Measures: Various jumping parameters, including jump height, maximum force, maximum power, rate of force development, eccentric lower-limb stiffness, and VS, were calculated using the vertical ground reaction force data. Eccentric lower-limb stiffness and VS were calculated using a mass-spring model. The studied variables can present lower-extremity elastic behavior and performance during CMJ. Results: The results of a 2-way repeated-measures analysis of variance showed no significant effect for KT application. Fatigue resulted in lower values of jump height, maximum power, and VS. Moreover, male subjects showed greater values of jump height, maximum force, and maximum power than did females. Conclusions: The main finding of this study was that gastrocnemius KT has no effect on performance or elastic behavior of the lower-extremity during CMJ. Moreover, KT cannot reduce the adverse effect of fatigue in a functional activity such as CMJ. It seems that gastrocnemius KT is not effective for demanding activities.

Stabilisation times after transitions to standing from different working postures

Transitioning to standing after maintaining working postures may result in imbalance and could elicit a fall. The objective of this study was to quantify the magnitude of imbalance using a stabilisation time metric. Forty-five male participants completed three replications of conditions created by one of four working postures (bent at waist, squat, forward kneel, reclined kneel) and three durations within posture. Participants transitioned to quiet standing at a self-selected pace. Stabilisation time, based on changes in centre of pressure velocity, was used to indicate the initiation of steady state while standing. Stabilisation time was significantly affected by static postures but not duration within posture. The largest stabilisation times resulted from transitions initiated from a bent at waist posture. The smallest were associated with the kneeling postures, which were not significantly different from each other. Findings may lead to recommendations for redesign of tasks, particularly in high-risk environments such as construction. Statement of Relevance: Task performance on the jobsite often requires individuals to maintain non-erect postures. This study suggests that working posture affects stabilisation during transition to a standing position. Bending at the waist and squatting resulted in longer stabilisation times, whereas both kneeling postures evaluated resulted in greater imbalance but for a shorter duration.

Sensory Substitution for Balance Control Using a Vestibular-to-Tactile Device

Postural control is essential for most activities of daily living. The impairment of this function can be extremely disabling. This work was stimulated by the testimony of a bilateral partial foot amputee who describes his difficulty in maintaining balance while washing his hair in the shower. We postulated that if the postural control system could not rely on accurate and reliable somatosensory inputs from the foot and ankle, as is probably the case following bilateral foot amputation due to the loss of the foot afferents and efferents, the weight of visual and vestibular cues would increase. We therefore assessed if a vestibular-to-tactile sensory substitution device could compensate for this impairment. Two separate experiments were conducted. Experiment 1: The effect of a vestibular-to-tongue tactile biofeedback balance system on the postural stability of this amputee was tested (on a force platform) and compared with a non-amputated, matched control group. The results showed that use of the biofeedback reduced centre of foot (CoP) displacement in all subjects but more spectacularly in the amputee. Experiment 2: The effect of the biofeedback was tested in 16 young healthy adults following a protocol of ankle muscle fatigue (known to alter ankle neuromuscular function and to perturb the control of bipedal posture). The results showed a significant decrease in CoP displacement compared with the control, non-biofeedback condition and a significantly greater effect of the biofeedback in the fatigue than the non-fatigue condition. Taken together, the results of these two studies suggest that an individual with double partial foot amputation was able to improve his balance control thanks to the use of a vestibular-to-tongue tactile biofeedback balance system and that young healthy individuals were able to take advantage of it to reduce the postural destabilisation induced by plantar-flexor muscle fatigue. Further studies are however necessary to confirm this in larger numbers of impaired persons as well as to assess the effectiveness in dynamic situations.

Sensory Re-Weighting in Human Bipedal Postural Control: The Effects of Experimentally-Induced Plantar Pain

The present study was designed to assess the effects of experimentally-induced plantar pain on the displacement of centre of foot pressure during unperturbed upright stance in different sensory conditions of availability and/or reliability of visual input and somatosensory input from the vestibular system and neck. To achieve this goal, fourteen young healthy adults were asked to stand as still as possible in three sensory conditions: (1) No-vision, (2) Vision, and (3) No-vision - Head tilted backward, during two experimental conditions: (1) a No-pain condition, and (2) a condition when a painful stimulation was applied to the plantar surfaces of both feet (Plantar-pain condition). Centre of foot pressure (CoP) displacements were recorded using a force platform. Results showed that (1) experimentally-induced plantar pain increased CoP displacements in the absence of vision (No-vision condition), (2) this deleterious effect was more accentuated when somatosensory information from the vestibular and neck was altered (No-vision - Head tilted backward condition) and (3) this deleterious effect was suppressed when visual information was available (Vision condition). From a fundamental point of view, these results lend support to the sensory re-weighting hypothesis whereby the central nervous system dynamically and selectively adjusts the relative contributions of sensory inputs (i.e. the sensory weightings) in order to maintain balance when one or more sensory channels are altered by the task (novel or challenging), environmental or individual conditions. From a clinical point of view, the present findings further suggest that prevention and treatment of plantar pain may be relevant for the preservation or improvement of balance control, particularly in situations (or individuals) in which information provided by the visual, neck proprioceptive and vestibular systems is unavailable or disrupted.

The role of textured material in supporting perceptual-motor functions

Simple deformation of the skin surface with textured materials can improve human perceptual-motor performance. The implications of these findings are inexpensive, adaptable and easily integrated clothing, equipment and tools for improving perceptual-motor functionality. However, some clarification is needed because mixed results have been reported in the literature, highlighting positive, absent and/or negative effects of added texture on measures of perceptual-motor performance. Therefore the aim of this study was to evaluate the efficacy of textured materials for enhancing perceptual-motor functionality. The systematic review uncovered two variables suitable for sub-group analysis within and between studies: participant age (groupings were 18–51 years and 64.7–79.4 years) and experimental task (upright balance and walking). Evaluation of studies that observed texture effects during upright balance tasks, uncovered two additional candidate sub-groups for future work: vision (eyes open and eyes closed) and stability (stable and unstable). Meta-analysis (random effects) revealed that young participants improve performance by a small to moderate amount in upright balance tasks with added texture (SMD = 0.28, 95%CI = 0.46–0.09, Z = 2.99, P = 0.001; Tau² = 0.02; Chi² = 9.87, df = 6, P = 0.13; I² = 39.22). Significant heterogeneity was found in, the overall effect of texture: Tau² = 0.13; Chi² = 130.71, df = 26, P<0.0001; I² = 85.98%, pooled samples in upright balance tasks: Tau² = 0.09; Chi² = 101.57, df = 13, P<0.001; I² = 72.67%, and in elderly in upright balance tasks: Tau² = 0.16; Chi² = 39.42, df = 5, P<0.001; I² = 83.05%. No effect was shown for walking tasks: Tau² = 0.00; Chi² = 3.45, df = 4, P = 0.27, I² = 22.99%. Data provides unequivocal support for utilizing textured materials in young healthy populations for improving perceptual-motor performance. Future research is needed in young healthy populations under conditions where visual and proprioceptive information is challenged, as in high-speed movements, or where use of equipment mediates the performer-environment interaction or where dysfunctional information sources ‘compete’ for attention. In elderly and ailing populations data suggests further research is required to better understand contexts where texture can facilitate improved perceptual-motor performance.

Does skin stimulation compensate impairments in postural control after ankle plantar flexors fatigue?

It has been suggested that skin stimulation using tape glued to the skin over the Achilles tendon helps to improve postural control. It was recently shown that these effects might emerge only after muscle fatigue and not during non-fatigue standing. Here we tested the influence of skin stimulation on postural control before and after fatigue of ankle plantar flexors. Eighteen subjects stood quietly on a force plate while the center of pressure was monitored before and after fatigue of ankle plantar flexors muscles, with and without medical tape glued to the skin over the Achilles tendon. Our results suggested main effects of tape stimulation before fatigue (P<.05) and significant effect of tape stimulation after fatigue only for amplitude of center of pressure in the anteroposterior direction. The novelty of our study is that most of the center of pressure variables are improved by skin stimulation when the ankle plantar flexors are not fatigued. Therefore the benefits of tape stimulation during fatigue condition are questionable.

Do somatosensory conditions from the foot and ankle affect postural responses to plantar-flexor muscles fatigue during bipedal quiet stance?

The present study investigated the effects of somatosensory conditions at the foot and ankle on postural responses to plantar-flexor muscle fatigue during bipedal quiet stance. Twenty-two young healthy adults were asked to stand upright as still as possible with their eyes closed in three somatosensory conditions (normal, altered and improved) both prior to and after exercises inducing plantar-flexor muscle fatigue. In the normal condition, the postural task was executed on a firm support surface constituted by the force platform. In the altered condition, a 2-cm thick foam support surface was placed under the subjects' feet. In the improved condition, increased cutaneous feedback at the foot and ankle was provided by strips of athletic tape applied across both their ankle joints. Muscle fatigue was induced in the plantar-flexor muscles of both legs through the execution of a repeated standing heel raise exercise. Centre of foot pressure displacements were recorded using a force platform. Results showed that plantar-flexor muscle fatigue yielded increased centre of foot pressure displacements under normal foot and ankle sensory conditions. Furthermore, this effect was exacerbated under altered foot and ankle sensory conditions and mitigated under improved foot and ankle sensory conditions. Altogether, the present findings suggested an increased reliance on somatosensory information from the foot and ankle for controlling upright posture in the presence of plantar-flexor muscle fatigue.

Effects of common working postures on balance control during the stabilisation phase of transitioning to standing

Standing after maintaining working postures may result in imbalance and could elicit a fall. The objective of this study was to assess the magnitude of this imbalance. Forty-five male participants completed three replications of conditions created by four static postures and three durations within posture. Participants transitioned to quiet standing at a self-selected pace. Body segment location and displacement of the centre of pressure (COP) were recorded using a motion capture system and two forceplates, respectively. Balance control measures were calculated during the stabilisation phase. All balance control measures were significantly affected by static posture but not duration within posture. Bending over at waist generally caused the smallest changes in balance control measures, whereas the reclined kneeling posture resulted in the largest. Findings may lead to recommendations for redesign of tasks to reduce the use of certain working postures, particularly in high-risk environments such as construction. STATEMENT OF RELEVANCE: Task performance on the jobsite often requires individuals to maintain non-erect postures. This study suggests that the working posture chosen affects stabilisation during a transition to a standing position. Bending at the waist or squatting seems to have less of an affect on balance control measures, whereas both types of kneeling postures evaluated resulted in greater imbalance.

Effects of plantar-flexor muscle fatigue on the magnitude and regularity of center-of-pressure fluctuations

Control of bipedal posture is highly automatized but requires attentional investment, the amount of which varies between participants and with postural constraints, such as plantar-flexor muscle fatigue. Elevated attentional demands for standing with fatigued plantar flexors have been demonstrated using a stimulus-response reaction-time paradigm. Recently, a direct relation between the regularity of center-of-pressure (COP) fluctuations and the amount of attention invested in posture was proposed, according to which more regular COP fluctuations are expected with muscle fatigue than without. To study this prediction, we registered anterior-posterior COP fluctuations for bipedal stance with eyes closed prior to and after a plantar-flexor muscle fatiguing exercise protocol in 16 healthy young adults. We quantified the magnitude of COP fluctuations with conventional posturography and its regularity with sample entropy. The magnitude of COP fluctuations increased significantly with fatigued plantar flexors. In addition, more regular COP fluctuations were observed with fatigued plantar flexors, as evidenced by significantly lower sample entropy values. These findings corroborated our hypotheses. Moreover, COP regularity assisted in qualifying the change in sway magnitude with fatigue. Whereas increased sway is customary taken to reflect impaired postural control, we interpret it as a functional, but attention-demanding adaptation to the alteration of important posture-specific information.

Age and gender moderate the effects of localized muscle fatigue on lower extremity joint torques used during quiet stance

This study examined the effects of localized muscle fatigue, age, and gender on lower extremity joint torques used during quiet stance. Thirty-two participants performed exercises designed to fatigue the ankle plantarflexors, knee extensors, torso extensors, or shoulder flexors. Body kinematics and ground reaction forces were obtained both before and after the exercises, and joint torques were derived via inverse dynamics. Single joint fatigue affected torque variability at all lower extremity joints, with similar changes for both age groups. Males and females exhibited increased ankle torque variability after different tasks, with males showing more variability after ankle fatigue and females after shoulder and lumbar fatigue. Correlations between peak torques and torque variability differed between males and females and between age groups in certain cases. The results of this study suggested that both age and gender moderate the effects of fatigue on postural control and should be considered when developing strategies to prevent occupational falls.

Changes in the relative contribution of each leg to the control of quiet two-legged stance following unilateral plantar-flexor muscles fatigue

We used unilateral plantar-flexor muscles fatigue to assess the capacity of the central nervous system to adapt quiet two-legged stance control to a unilateral ankle neuromuscular perturbation. Eighteen young healthy adults stood barefoot with their eyes closed and were asked to sway as little as possible. The Experimental group (n = 9) executed this postural task in two conditions, before (pre-test) and following the completion of a fatiguing exercise designed to induce a muscular fatigue in the plantar-flexor muscles of their dominant leg (post-test). For the Control group (n = 9), this fatiguing exercise was replaced with a standing rest period corresponding to the fatiguing exercise. Results of the Experimental group showed no significant difference between the weightbearing index measured in the pre-test condition and that observed in the post-test condition. Results further revealed that unilateral plantar-flexor muscles fatigue yielded different effects on the centre of foot pressure (CoP) displacements under the non-fatigued leg and under the fatigued leg: a wider surface area of the CoP displacements was observed under the non-fatigued than under the fatigued leg, and a higher mean speed of the CoP displacements was observed under the non-fatigued leg only in the post-test relative to the pre-test condition. These findings evidenced that the contribution of each leg to the control of quiet two-legged stance is modified as a result of muscle fatigue of unilateral plantar-flexor muscles. The greater contribution of the non-fatigued leg could be viewed as a fatigue-induced adaptive change in the control of quiet two-legged stance in response to an alteration of the unilateral ankle neuromuscular function induced by unilateral plantar-flexor muscles fatigue.

Consequences of lower extremity and trunk muscle fatigue on balance and functional tasks in older people: a systematic literature review

Background

Muscle fatigue reduces muscle strength and balance control in young people. It is not clear whether fatigue resistance seen in older persons leads to different effects. In order to understand whether muscle fatigue may increase fall risk in older persons, a systematic literature review aimed to summarize knowledge on the effects of lower extremity and trunk muscle fatigue on balance and functional tasks in older people was performed.

Methods

Studies were identified with searches of the PUBMED and SCOPUS data bases.

Papers describing effects of lower extremity or trunk muscle fatigue protocols on balance or functional tasks in older people were included. Studies were compared with regards to study population characteristics, fatigue protocol, and balance and functional task outcomes.

Results

Seven out of 266 studies met the inclusion criteria. Primary findings were: fatigue via resistance exercises to lower limb and trunk muscles induces postural instability during quiet standing; induced hip, knee and ankle muscle fatigue impairs functional reach, reduces the speed and power of sit-to-stand repetitions, and produces less stable and more variable walking patterns; effects of age on degree of fatigue and rate of recovery from fatigue are inconsistent across studies, with these disparities likely due to differences in the fatigue protocols, study populations and outcome measures.

Conclusion

Taken together, the findings suggest that balance and functional task performance are impaired with fatigue. Future studies should assess whether fatigue is related to increased risk of falling and whether exercise interventions may decrease fatigue effects.

Hypnotizability-related effects of vestibular impairment on posture and locomotion

Body sway and locomotion are differentially modulated in high (highs) and low (lows) hypnotizable subjects undergoing alteration of visual and neck/leg proprioceptive inputs. The study's aim was to investigate whether partial impairment of vestibular information due to backward head extension affects postural (Study 1) and locomotor behavior (Study 2) differentially in highs and lows. Results showed that, at variance with the visual and proprioceptive modalities, vestibular inactivation did not induce major differences between the 2 groups, with the exception of improvement in walking straight across consecutive trials, which was observed only in highs. The article presents an overview of the structures and mechanisms possibly involved in the observed hypnotizability-related differences in motor control and suggests that hypnotic susceptibility might be a relevant factor in neuro-rehabilitative treatments because it accounts for part of the variability in the sensorimotor self.

Postural adaptation to unilateral hip muscle fatigue during human bipedal standing

Although previous studies have investigated the effects of bilateral muscle fatigue on bipedal postural control, whether and how the central nervous system could adapt to unilateral muscle fatigue for controlling bipedal stance remains to be investigated. The purpose of the present experiment was designed to address this issue by assessing the effect of unilateral muscle fatigue induced on the hip's abductors of the dominant leg on bipedal standing. Twenty-four young healthy adults stood barefoot, feet together, with their eyes closed and were asked to sway as little as possible. The experimental group (n=12) executed this postural task in two conditions, pre- and post-fatigue. In the post-fatigue condition, the measurements were performed immediately after a designated fatiguing exercise for the hip abductors of their dominant leg. For the control group (n=12), this fatiguing exercise was replaced by a laying rest period corresponding to the fatiguing exercise. The distribution of the body weight and plantar centre of foot pressure (CoP) displacements from the non-fatigued and fatigued leg were recorded along the mediolateral and anteroposterior axes using a plantar pressure data acquisition system. Results of the experimental group showed that unilateral muscle fatigue induced on the hip's abductors of the dominant leg had different effects on the plantar CoP displacements (1) under the non-fatigued and fatigued legs, yielding larger displacements under the non-fatigued leg only, and (2) in the anteroposterior and mediolateral axes, yielding larger displacements along the mediolateral axis only. These observations could not be accounted for by any asymmetrical distribution of the body weight on both legs which were similar for both pre- and post-fatigue conditions. The observed postural responses could be viewed as an adaptive process to cope with an unilateral alteration in the hip neuromuscular function induced by the fatiguing exercise for controlling bipedal stance. The increase in CoP displacements observed under the non-fatigued leg in the fatigue condition could reflect enhanced exploratory "testing of the ground" movements with sensors of the non-fatigued leg's feet, providing supplementary somatosensory inputs to the central nervous system to preserve/facilitate postural control in condition of altered neuromuscular function of the dominant leg's hip abductors induced by the fatiguing exercise.