Movement reorganization to compensate for fatigue during sawing

Inter- and intra-athlete technique variability of conventional new ball swing bowling in elite and pre-elite Australian male fast bowlers

This study aimed to investigate inter- and intra-athlete technique variability in pre-elite and elite Australian fast bowlers delivering new ball conventional swing bowling. Ball grip angle and pelvis, torso, shoulder, elbow, wrist, upper arm, forearm, and hand kinematics were investigated at the point of ball release for inswing and outswing deliveries. Descriptive evaluations of group and individual data and k-means cluster analyses were used to assess inter- and intra-bowler technique variability. Inter-athlete technique and ball grip variability were identified, demonstrating that skilled bowlers use individualised strategies to generate swing. Functional movement variability was demonstrated by intra-athlete variability in successful swing bowling trials. Bowlers demonstrated stable technique parameters in large proximal body segments of the pelvis and torso, providing a level of repeatability to their bowling action. Greater variation was observed in bowling arm kinematics, allowing athletes to manipulate the finger and ball position to achieve the desired seam orientation at the point of ball release. This study demonstrates that skilled bowlers use individualised techniques and grips to generate swing and employ technique variations in successive deliveries. Coaches should employ individualised training strategies and use constraints-led approaches in training environments to encourage bowlers to seek adaptive movement solutions to generate swing.

Utilising redundancy in musculoskeletal systems for adaptive stiffness and muscle failure compensation: a model-free inverse statics approach

Vertebrates possess a biomechanical structure with redundant muscles, enabling adaptability in uncertain and complex environments. Harnessing this inspiration, musculoskeletal systems offer advantages like variable stiffness and resilience to actuator failure and fatigue. Despite their potential, the complex structure presents modelling challenges that are difficult to explicitly formulate and control. This difficulty arises from the need for comprehensive knowledge of the musculoskeletal system, including details such as muscle arrangement, and fully accessible muscle and joint states. Whilst existing model-free methods do not need explicit formulations, they also underutilise the benefits of muscle redundancy. Consequently, they necessitate retraining in the event of muscle failure and require manual tuning of parameters to control joint stiffness limiting their applications under unknown payloads. Presented here is a model-free local inverse statics controller for musculoskeletal systems, employing a feedforward neural network trained on motor babbling data. Experiments with a musculoskeletal leg model showcase the controller's adaptability to complex structures, including mono and bi-articulate muscles. The controller can compensate for changes such as weight variations, muscle failures, and environmental interactions, retaining reasonable accuracy without the need for any additional retraining.

Validation of the Borg CR10 Scale for the evaluation of shoulder perceived fatigue during work-related tasks

Work-related upper extremity disorders (WRUEDs) are highly prevalent and costly. Development of fatigue is thought to be one of the causes of WRUEDs. Perceived fatigue can be assessed with the Borg CR Scale® (Borg CR10). The objective was to validate the Borg CR10 for the evaluation of shoulder perceived fatigue during lifting tasks. Seventy adults in working age performed three rhythmic lifting tasks with two loads (15% and 30% of maximal voluntary contraction). Using generalized repeated-measures ANOVA (Generalized Estimating Equations), statistically significant Task and Load effects (p < 0.001), as well as Task × Load interaction effects (p < 0.0001) were observed on Borg CR10, without any influence of sex. The Borg CR10 is a valid tool to assess shoulder perceived fatigue as it can discriminate between tasks of different difficulty levels in term of complexity, height, and resistance, regardless of sex.

The effects of umbrella handle shape and grip type on muscle activation and postural variability under windy conditions

This study investigated how different shapes of umbrella handles and grip types influence muscle activation and postural variability under windy conditions. Seventeen adult participants were enrolled in this study, and different handle shapes (cylindrical, ellipsoidal, and triangular prism-shaped), grip types (four- and five-finger grip), and wind strengths were tested. Activation of the forearm and upper arm muscles was recorded using surface electromyography. Postural variability and ratings of (1) perceived difficulty of use and (2) perceived grip-posture variability were measured. The results indicated that activation of the finger flexor muscle increased under windy conditions, whereas postural variability was not affected. Weak wind (3.2-7.5 m/s) conditions caused greater perceived postural variability and activation of the wrist extensor muscle. The ellipsoidal handle shape had lower endpoint postural variability when held with a five-finger grip and also had lower perceived postural variability and difficulty compared to that with the cylindrical shape. Our findings can be used to select appropriate umbrella handle designs based on grip type and wind conditions.

Effects of fatigue on intramuscle force-stabilizing synergies

We applied the recently introduced concept of intramuscle synergies in spaces of motor units (MUs) to quantify indexes of such synergies in the tibialis anterior during ankle dorsiflexion force production tasks and their changes with fatigue. We hypothesized that MUs would be organized into robust groups (MU modes), which would covary across trials to stabilize force magnitude, and the indexes of such synergies would drop under fatigue. Healthy, young subjects (n = 15; 8 females) produced cyclical, isometric dorsiflexion forces while surface electromyography was used to identify action potentials of individual MUs. Principal component analysis was used to define MU modes. The framework of the uncontrolled manifold (UCM) was used to analyze intercycle variance and compute the synergy index, ΔVZ. Cyclical force production tasks were repeated after a nonfatiguing exercise (control) and a fatiguing exercise. Across subjects, fatigue led, on average, to a 43% drop in maximal force and fewer identified MUs per subject (29.6 ± 2.1 vs. 32.4 ± 2.1). The first two MU modes accounted for 81.2 ± 0.08% of variance across conditions. Force-stabilizing synergies were present across all conditions and were diminished after fatiguing exercise (1.49 ± 0.40) but not control exercise (1.76 ± 0.75). Decreased stability after fatigue was caused by an increase in the amount of variance orthogonal to the UCM. These findings contrast with earlier studies of multieffector synergies demonstrating increased synergy index under fatigue. We interpret the results as reflections of a drop in the gain of spinal reflex loops under fatigue. The findings corroborate an earlier hypothesis on the spinal nature of intramuscle synergies.NEW & NOTEWORTHY Across multielement force production tasks, fatigue of an element leads to increased indexes of force stability (synergy indexes). Here, however, we show that groups of motor units in the tibialis anterior show decreased indexes of force-stabilizing synergies after fatiguing exercise. These findings align intramuscle synergies with spinal mechanisms, in contrast to the supraspinal control of multimuscle synergies.

Power Spectrum of Acceleration and Angular Velocity Signals as Indicators of Muscle Fatigue during Upper Limb Low-Load Repetitive Tasks

Muscle fatigue is a risk factor for developing musculoskeletal disorders during low-load repetitive tasks. The objective of this study was to assess the effect of muscle fatigue on power spectrum changes of upper limb and trunk acceleration and angular velocity during a repetitive pointing task (RPT) and a work task. Twenty-four participants equipped with 11 inertial measurement units, that include acceleration and gyroscope sensors, performed a tea bag filling work task before and immediately after a fatiguing RPT. During the RPT, the power spectrum of acceleration and angular velocity increased in the movement and in 6-12 Hz frequency bands for sensors positioned on the head, sternum, and pelvis. Alternatively, for the sensor positioned on the hand, the power spectrum of acceleration and angular velocity decreased in the movement frequency band. During the work task, following the performance of the fatiguing RPT, the power spectrum of acceleration and angular velocity increased in the movement frequency band for sensors positioned on the head, sternum, pelvis, and arm. Interestingly, for both the RPT and work task, Cohens' d effect sizes were systematically larger for results extracted from angular velocity than acceleration. Although fatigue-related changes were task-specific between the RPT and the work task, fatigue systematically increased the power spectrum in the movement frequency band for the head, sternum, pelvis, which highlights the relevance of this indicator for assessing fatigue. Angular velocity may be more efficient to assess fatigue than acceleration. The use of low cost, wearable, and uncalibrated sensors, such as acceleration and gyroscope, in industrial settings is promising to assess muscle fatigue in workers assigned to upper limb repetitive tasks.

Hip Abductor Muscle Fatigue Induces Different Strategies During Disrupted Postural Control

Background

Ankle sprain is one of the most common injuries in sport, and hip abductor muscle weakness has recently been reported as a predisposing factor. Currently, the influence of hip abductor muscle fatigue on ankle joint control has not been elucidated during an ankle disturbed balance exercise. This study aimed to determine the influence of hip abductor muscle fatigue on ankle joint control during a disturbed balance task, and to consider inter-individual variability in the kinematic and neuromuscular reorganizations implemented.

Methods

Twenty-six healthy subjects (13 males; 13 females) performed a unipedal postural balance task with eyes closed before and after a fatiguing exercise (up to a 50% decrease in strength) of the hip abductor muscles. Subjects completed balance task while equipped with an ankle destabilization device that allows inversion/eversion movements. Electromyographic (EMG) activity of the gastrocnemius lateralis (GastL), peroneus longus (PL) and brevis, tibialis anterior, and gluteus medius were recorded during task. Kinematics (e.g., frontal foot angulation) of the ankle complex were determined using inertial measurement units.

Results

In the overall group, no significant time, sex or interaction effect was observed for kinematic and EMG variables. However, when considering individual responses to hip fatigue, 14 subjects decreased the standard deviation of frontal angulation (−30%) suggesting enhancement of ankle joint control, while 12 subjects increased it (+46%). Normalized EMG for PL and GastL muscles changed with fatigue for both these groups. However, variations were significantly different between groups (p = 0.027 for PL and p = 0.006 for GastL). Indeed, the contribution of ankle muscles increased for the enhanced-stability group while no change for the impaired-stability group.

Conclusion

These results highlight that subject adopt different neuromuscular and kinematic ankle strategies to control ankle destabilization in response to hip abductor muscle fatigue. Frontal foot angulation variability seemed to be a valuable marker to detect the type of strategy employed. The strategy adopted by the impaired-stability group might have important implications when analyzing risk factors for ankle sprains. Further studies should consider individual responses to fatigue, to understand which factor could predispose athletes to use of one or other strategy.

Sex-specific effects of localized muscle fatigue on upper body kinematics during a repetitive pointing task

Background

Females are reported to have a higher risk of musculoskeletal disorders than males. Repetitive motions can lead to muscle fatigue, which may play a mediator role in the development of musculoskeletal disorders. However, sex differences in adaptations to localized fatigue at different joints are poorly understood. We examined the sex-specific effects of fatigue location on shoulder, elbow and spinal joint angles, and angular variabilities during a repetitive pointing task.

Methods

Seven males and ten females performed a 30-s standing repetitive pointing task with their right upper limb when they were non-fatigued (NF), elbow-fatigued (EF), shoulder-fatigued (SF) and trunk-fatigued (TF), while trunk and upper body tridimensional kinematic data was recorded. Joint angles and angular variabilities of shoulder, elbow, upper thoracic spine, lower thoracic spine, and lumbar spine were calculated.

Results

Results showed that shoulder angles changed the most after EF in males, but after SF in females. The similarities between sexes were that SF increased the variabilities at upper (lateral flexion: 0.15° greater than NF, rotation: 0.26° greater than all other conditions) and lower thoracic spine (lateral flexion: 0.13° greater than NF, rotation: averagely 0.1° greater than all other condition) in both sexes. TF altered upper thoracic spine variability (0.36° smaller than SF), lower thoracic spine angle (lateral flexion: 3.00° greater than NF, rotation: 1.68° greater than SF), and lumbar angle (averagely 1.8° smaller than all other conditions) in both sexes. However, females had greater lower thoracic spine angle (lateral flexion: 8.3° greater, p = 0.005) as well as greater upper (rotation: 0.53° greater, p = 0.006) and lower thoracic spine (rotation: 0.5° greater, p = 0.007; flexion: 0.6° greater, p = 0.014) angular variabilities than males.

Conclusions

Results suggest that females’ fatigue responses focused on the trunk and spine. Results highlight a few sex differences in adapting to localized muscle fatigue, which may help explain how sex differences in repetitive motion-related injuries differ between joints.

The use of a three-dimensional dynamic arm support prevents the development of muscle fatigue during repetitive manual tasks in healthy individuals

Work-related upper extremity disorders are costly to society due to resulting medical costs, presenteeism and absenteeism. Although their aetiology is likely multifactorial, physical workplace factors are known to play an important role in their development. Promising options for preventing work-related upper extremity disorders include assistive technologies such as dynamic arm supports designed to follow the movement of the arm while compensating for its weight. The objective of this study was to assess the effects of a dynamic arm support on perceived exertion, muscle activity and movement patterns of the upper limb during repetitive manual tasks in healthy individuals. Thirty healthy right-handed individuals were allocated either a static or a dynamic task to perform with and without a dynamic arm support. During the task, surface electromyographic activity (anterior and middle deltoid, upper trapezius) and upper limb kinematics (elbow, shoulder, sternoclavicular) were measured using surface EMG and inertial sensors. Results showed that the dynamic arm support significantly reduced perceived exertion during the tasks and limited the development of muscular fatigue of the anterior and middle deltoid as demonstrated by EMG signal mean epoch amplitudes and median frequency of the EMG power spectrum. The dynamic arm support also prevented a decrease in shoulder elevation and an increase in total shoulder joint excursion during static and dynamic task, respectively. These results denote the potential benefits of dynamic arm supports in work environments. Further studies should focus on their efficacy, acceptability and implementability in work settings.

Shoulder electromyography-based indicators to assess manifestation of muscle fatigue during laboratory-simulated manual handling task

Muscle fatigue is a risk factor for developing shoulder musculoskeletal disorders. The aim of this study was to identify shoulder electromyographic indicators that are most indicative of muscle fatigue during a laboratory simulated manual handling task. Thirty-two participants were equipped with electromyographic electrodes on 10 shoulder muscles and moved boxes for 45-minutes. The modified rate of perceived exertion (mRPE) was assessed every 5-minutes and multivariate linear regressions were performed between myoelectric manifestation of fatigue (MMF) and the mRPE scores. During a manual handling task representative of industry working conditions, spectral entropy, median frequency, and mobility were the electromyographic indicators that explained the largest percentage of the mRPE. Overall, the deltoids, biceps and upper trapezius were the muscles that most often showed significant changes over time in their electromyographic indicators. The combination of these three indicators may improve the accuracy for the assessment of MMF during manual handling. Practitioner Summary: To date, muscle fatigue has primarily been assessed during tasks done to exhaustion, which are not representative of typical working conditions. During a manual handling task representative of industry working conditions, EMG-derived spectral entropy, and median frequency, both extracted from time-frequency analysis, and mobility extracted from time domain, were the best indicators of the manifestation of muscle fatigue.

Influence of muscle fatigue on motor task performance of the hand and wrist: A systematic review

Muscle fatigue is represented as a reduction in force production capability; however, fatigue does not necessarily result in performance impairments. As the distal upper limb serves as the end effector when interacting or manipulating objects, it is important to understand how muscle fatigue may impact motor functionality. The aim of this study was to systematically review the literature to identify how various aspects of motor performance of the distal upper limb are impaired following muscle fatigue. Four databases were searched using 23 search terms describing the distal upper limb, muscle fatigue, and various performance metrics. A total of 4561 articles were screened with a total of 28 articles extracted and critically appraised. Evidence extracted indicates that muscle fatigue results in unique impairments based on the type of motor performance being evaluated. Furthermore, much data suggests that muscle fatigue does not result in consistent, predictable performance impairments, particularly while performing submaximal tasks. Additionally, magnitude of fatigue does not directly correlate with reductions in performance outcomes at the hand and wrist. Fatiguing protocols used highlighted the importance of fatigue specificity. When fatiguing and performance tasks are similar, performance impairment is likely to be observed. The numerous muscles found in the hand and wrist, often considered redundant, play a critical role in maintaining task performance in the presence of muscle fatigue. The presence of motor abundance (e.g. multiple muscles with similar function) is shown to reduce the impairment in multiple performance metrics by compensating for reduced function of fatigued muscles. Continued exploration into various fatiguing protocols (i.e. maximal or submaximal) will provide greater insights into performance impairments in the distal upper limb.

Evidence of movement variability patterns during a repetitive pointing task until exhaustion

Human movement is characterized by its variability: the same task is never performed twice in exactly the same way. This variability is believed to play a functional role in movement performance and adaptability, as well as in preventing musculoskeletal damage. This article focuses on the time-evolution of movement variability throughout a repetitive pointing task until exhaustion. The kinematics of 13 subjects performing the pointing task is analyzed. Principal Component Analysis of joint angles identifies joint coordinations for each pointing cycle, and cycle-by-cycle comparison highlights movement variability. Non-supervised clustering reveals that subjects adopt successive coordination patterns at an intra-individual level. Inter-individual variability is characterized by the number and type of such patterns: from 3 to 5 patterns, mobilizing the trunk, the shoulder and the upper limbs differently. Movement variability exists even in a seemingly basic and constrained task. It appears in the very early stages of fatigue onset, and may correspond to adaptative coordination responses throughout task performance. This observation should encourage workstation designers to better account for movement variability in order to preserve operators' health and safety.

Sex-specific effects of a repetitive fatiguing task on stability: Analysis with motor equivalence model

Although studies showed that several internal factors affect task-specific stability, the sex-specific effects of fatigue on whole-limb stability during a semi-cycle repetitive pointing task remain unidentified. Synergy and Motor Equivalence concepts in the UCM framework have been developed to explain task-specific stability. The motor equivalence model quantifies the amount of deviation in the space of elemental variables that occurs in two directions; one that preserves the performance variable (good variance), and the other that affects it (bad variance). Synergy index (the difference between good and bad variance divided by the total variance > 0) represent stability in performing a task. Healthy adults (n = 26, 13F; age: 35.3 ± 10.6 yrs.) performed an RPT by moving their dominant arm between a proximal target and a distal target in a standing position until near fatigue (Borg CR10 rating 8/10). Tridimensional kinematics of trunk, upper arm, forearm, and hand segments were captured by high-resolution cameras every minute, and joint angles were extracted according to the ZX'Y″ Euler sequence. Results showed the synergy > 0 for both women and men, reflecting synergies stabilizing the endpoint coordinate in both Non-Fatigue and Fatigue conditions. Statistics (ANOVA) showed a significant Condition * Sex effect (p = 0.01), with higher good (by 0.19 ± 0.1 rad) and bad variances (by 0.15 ± 0.09 rad) in women compared to men after fatigue. Higher good and bad variability, with no change in women's performance could represent a less stable strategy, leading to the development of risk factors for neck-shoulder disorders.

One more time about motor (and non-motor) synergies

We revisit the concept of synergy based on the recently translated classical book by Nikolai Bernstein (On the construction of movements, Medgiz, Moscow 1947; Latash, Bernstein's Construction of Movements, Routledge, Abingdon 2020b) and progress in understanding the physics and neurophysiology of biological action. Two aspects of synergies are described: organizing elements into stable groups (modes) and ensuring dynamical stability of salient performance variables. The ability of the central nervous system to attenuate synergies in preparation for a quick action-anticipatory synergy adjustments-is emphasized. Recent studies have demonstrated synergies at the level of hypothetical control variables associated with spatial referent coordinates for effectors. Overall, the concept of synergies fits naturally the hierarchical scheme of control with referent coordinates with an important role played by back-coupling loops within the central nervous system and from peripheral sensory endings. Further, we review studies showing non-trivial changes in synergies with development, aging, fatigue, practice, and a variety of neurological disorders. Two aspects of impaired synergic control-impaired stability and impaired agility-are introduced. The recent generalization of the concept of synergies for non-motor domains, including perception, is discussed. We end the review with a list of unresolved and troubling issues.

Investigating the Effects of Mental Fatigue on Resistance Exercise Performance

Mental fatigue can impart negative effects on subsequent physical performance, although the mechanisms underlying these effects are not well understood. This study examined whether mental fatigue confers negative carryover effects on the performance of a set of biceps curls, while also investigating physiological and psychological mechanisms proposed to explain the predicted effect. A randomized, cross-over design was employed. On visit 1, participants (N = 10) performed a barbell biceps curl one-repetition maximum (1RM) test. On visits 2-3, participants performed 20 biceps curls at 50% of their 1RM, followed by their respective 10 min experimental manipulation (high vs. low cognitive exertion) and then a second set of biceps curls to exhaustion. Ratings of perceived exertion and electromyography of the biceps brachii, triceps brachii, upper trapezius, thoracic erector spinae and lumbar erector spinae were recorded during the physical task. The total number of repetitions completed was similar across the conditions. Results also failed to show between-condition differences for muscle activation and perceptions of exertion. Future research is needed to build an adequate knowledge base to determine whether there is an effect of mental fatigue on dynamic resistance-based task performance and, if so, identify the mechanisms explaining how and why.

The Effects of Set Structure Manipulation on Chronic Adaptations to Resistance Training: A Systematic Review and Meta-Analysis

BACKGROUND

The acute effects of resistance training (RT) set structure alteration are well established; however, less is known about their effects on chronic training adaptations.

OBJECTIVE

The aim of this systematic review and meta-analysis was to synthesise the available evidence on the effectiveness of traditional (TS), cluster (CS) and rest redistribution (RR) set structures in promoting chronic RT adaptations, and provide an overview of the factors which might differentially influence the magnitude of specific training adaptations between set structure types.

METHODS

This review was performed using the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines encompassing the literature search of five databases. Studies in English that compared muscular strength, endurance, and/or hypertrophy adaptations, as well as vertical jump performance, velocity and power at submaximal loads and shifts in the slopes of force-velocity profiles between TS and CS or RR set structures (i.e., alternative set structures) were included. Risk of bias assessment was performed using a modified Cochrane Collaboration's tool for assessing risk of bias in randomised trials. Random-effects meta-analyses and meta-regressions were performed where possible.

RESULTS

17 studies met the inclusion criteria, none had more than one risk of bias item assessed as high risk. Pooled results revealed that none of the set structures were more effective at inducing strength (standardised mean difference (SMD) = - 0.06) or hypertrophy (SMD = - 0.03). TS were more effective at improving muscular endurance compared to alternative set structures (SMD = - 0.38), whereas alternative set structures tended to be more effective for vertical jump performance gains (SMD = 0.13), but this effect was not statistically significant (p = 0.190). Greater velocity and power outputs at submaximal loads (SMD = 0.18) were observed when using alternative set structures compared to TS. In addition, alternative set structures promoted greater shifts of the slope of force-velocity profiles towards more velocity dominant profiles compared to TS (SMD = 0.28). Sub-group analyses controlling for each alternative set structure independently showed mixed results likely caused by the relatively small number of studies available for some outcomes.

CONCLUSION

Modifying TS to an alternative set structure (CS or RR) has a negligible impact on strength and hypertrophy. Using CS and RR can lead to greater vertical jump performance, velocity and power at submaximal loads and shifts to more velocity dominant force-velocity profiles compared to training using TS. However, TS may provide more favourable effects on muscle endurance when compared to CS and RR. These findings demonstrate that altering TS to alternative set structures may influence the magnitude of specific muscular adaptations indicating set structure manipulation is an important consideration for RT program design.

PROTOCOL REGISTRATION

The original protocol was prospectively registered (CRD42019138954) with the PROSPERO (International Prospective Register of Systematic Reviews).

Consistent Hand Dynamics Are Achieved by Controlling Variabilities Among Joint Movements During Fastball Pitching

This study aimed to determine whether covariations among joint movements are utilized to stabilize hand orientation and movement and to determine which of the upper or lower extremities make effective use of the covariation. Joint angles during pitching were measured in 12 skilled baseball pitchers, using a motion capture system. The joint angles in 10 successful trials were used for the reconstructed motions. The reconstructed motion in the first condition was the same as for the measured motion. In the second condition, the reconstructed motion was generated with joint angles that were pseudo-randomly selected to artificially break off covariation in the measured joint-angle combination. In the third and fourth conditions, the reconstructed motions were generated with the same joint-angle combinations as the measured angles in the throwing arm and the stride leg, respectively, but pseudo-randomly selected in the other joint angles. Ten reconstructed motions were generated for each condition. Standard deviations (SDs) of hand orientation and movement direction were calculated and compared among the conditions. All SDs for the first condition were the smallest among the conditions, indicating that the movements in the measured condition used the covariation in joint angles to make the hand movement stable. The results also illustrated that some SDs in the fourth condition were smaller than those in the third condition, suggesting that the lower extremity made effective use of the covariation. These results imply that it is necessary not only to reduce variability in each joint but also to regulate joint movements to stabilize hand orientation and movement.

Joint moment trade-offs across the upper extremity and trunk during repetitive work

Individuals can coordinate small kinematic changes at several degrees of freedom simultaneously in the presence of fatigue, leaving it unclear how overall biomechanical demands at each joint are altered. The purpose of this study was to evaluate trade-offs in joint moments between the trunk, shoulder, and elbow during repetitive upper extremity work. Participants performed four simulated workplace tasks cyclically until meeting fatigue termination criteria. Emergent fatigue-induced adaptations to repetitive work resulted in task-dependent trade-offs in joint moments. In general, reduced shoulder moments were compensated for by increased elbow and trunk joint moment contributions. Although mean joint moment changes were modest (range: 1-3 Nm) across participants, a wide distribution of responses was observed, with standard deviations exceeding 10 Nm. Re-distributing biomechanical demands across joints may alleviate constant tissue loads and facilitate continued task performance with fatigue but may be at the expense of increasing demands at adjacent joints.

Redundancy and multifunctionality among spinal locomotor networks

c-Fos is used to identify system-wide neural activation with cellular resolution in vivo. However, c-Fos can only capture neural activation of one event. Targeted recombination in active populations (TRAP) allows the capture of two different c-Fos activation patterns in the same animal. So far, TRAP has only been used to examine brain circuits. This study uses TRAP to investigate spinal circuit activation during resting and stepping, giving novel insights of network activation during these events. The level of colabeled (c-Fos+ and TRAP+) neurons observed after performing two bouts of stepping suggests that there is a probabilistic-like phenomenon that can recruit many combinations of neural populations (synapses) when repetitively generating many step cycles. Between two 30-min bouts of stepping, each consisting of thousands of steps, only ∼20% of the neurons activated from the first bout of stepping were also activated by the second bout. We also show colabeling of interneurons that have been active during stepping and resting. The use of the FosTRAP methodology in the spinal cord provides a new tool to compare the engagement of different populations of spinal interneurons in vivo under different motor tasks or under different conditions.NEW & NOTEWORTHY The results are consistent with there being an extensive amount of redundancy among spinal locomotor circuits. Using the newly developed FosTRAP mouse model, only ∼20% of neurons that were active (labeled by Fos-linked tdTomato expression) during a first bout of 30-min stepping were also labeled for c-Fos during a second bout of stepping. This finding suggests variability of neural networks that enables selection of many combinations of neurons (synapses) when generating each step cycle.

Effect of Motor Development Levels on Kinematic Synergies During Two-Hand Catching in Children

The ability to coordinate different body parts under different constraints that are imposed by organism, environment, and tasks during motor development might be different in children. The aim of this study was to examine whether children with different motor development levels are different with regard to multijoint coordination during two-hand catching. Eighty-four children (age: 6.05 ±0.67 years) who were assessed on object control skills were recruited voluntarily. The biomechanical model was defined from 20 movements of seven segments (shoulders, elbows, wrists, and torso), and the principal component analysis was used to quantify the multijoint coordination and kinematic synergies during catching. The results showed that the redundancy of joints in two-hand catching is controlled by three kinematic synergies that defined the majority of the variance. The participants who were grouped based on their development levels did not show differences in the number and strength of synergies; however, they were different in the utilization of the kinematic synergies for successful catching. In conclusion, the number and the strength of the kinematic synergies during two-hand catching are not affected by the developmental levels and are related to the nature of the task.

Central and Peripheral Shoulder Fatigue Pre-screening Using the Sigma-Lognormal Model: A Proof of Concept

Background

Clinical tests for detecting central and peripheral shoulder fatigue are limited. The discrimination of these two types of fatigue is necessary to better adapt recovery intervention. The Kinematic Theory of Rapid Human Movements describes the neuromotor impulse response using lognormal functions and has many applications in pathology detection. The ideal motor control is modeled and a change in the neuromuscular system is reflected in parameters extracted according to this theory.

Objective

The objective of this study was to assess whether a shoulder neuromuscular fatigue could be detected through parameters describing the theory, if there is the possibility to discriminate central from peripheral fatigue, and which handwriting test gives the most relevant information on fatigue.

Methods

Twenty healthy participants performed two sessions of fast stroke handwriting on a tablet, before and after a shoulder fatigue. The fatigue was in internal rotation for one session and in external rotation during the other session. The drawings consisted of simple strokes, triangles, horizontal, and vertical oscillations. Parameters of these strokes were extracted according to the Sigma–Lognormal model of the Kinematic Theory. The evolution of each participant was analyzed through a U-Mann–Whitney test for individual comparisons. A Hotelling’s T²-test and a U-Mann–Whitney test were also performed on all participants to assess the group evolution after fatigue. Moreover, a correlation among parameters was calculated through Spearman coefficients to assess intrinsic parameters properties of each handwriting test.

Results

Central and peripheral parameters were statistically different before and after fatigue with a possibility to discriminate them. Participants had various responses to fatigue. However, when considering the group, parameters related to the motor program execution showed significant increase in the handwriting tests after shoulder fatigue. The test of simple strokes permits to know more specifically where the fatigue comes from, whereas the oscillations tests were the most sensitive to fatigue.

Conclusion

The results of this study suggest that the Sigma–Lognormal model of the Kinematic Theory is an innovative approach for fatigue detection with discrimination between the central and peripheral systems. Overall, there is a possibility to implement the setting for clinics and sports personalized follow-up.

Effects of different fatigue locations on upper body kinematics and inter-joint coordination in a repetitive pointing task

Studies have shown that muscle fatigue can lead to posture, joint angle, inter-joint coordination and variability alterations. However, the three-dimensional kinematic effects of localized muscular fatigue on a multijoint movement remain unclear. Healthy young adults (N = 17, 10 females) performed a standing repetitive pointing task when they were non-fatigued, and after localized muscle fatigue was induced at the elbow, the shoulder, and the trunk using isometric protocols performed until exhaustion. Joint angles and angular standard deviation (SD) of trunk, shoulder and elbow, and continuous relative phase (CRP) and CRP SD between trunk and shoulder, and shoulder and elbow were computed and compared between fatigue conditions. Results showed that trunk lateral flexion SD increased after fatigue of the elbow (0.1°, p = 0.04), shoulder (0.1°, p = 0.04) and trunk (0.1°, p<0.01). However, fatigue at different muscles brought different kinematic changes. Shoulder fatigue induced the greatest overall changes, with angular changes at all three joints. Trunk fatigue increased the shoulder horizontal abduction SD, elbow flexion SD and trunk-shoulder CRP. Elbow fatigue induced angular changes at trunk, shoulder and elbow, but did not affect CRP or CRP SD. This study highlights the crucial role of trunk variability in compensating for localized muscle fatigue during a repetitive upper limb task performed while standing.

Kinematic Synergy of Multi-DoF Movement in Upper Limb and Its Application for Rehabilitation Exoskeleton Motion Planning

It is important for rehabilitation exoskeletons to move with a spatiotemporal motion patterns that well match the upper-limb joint kinematic characteristics. However, few efforts have been made to manipulate the motion control based on human kinematic synergies. This work analyzed the spatiotemporal kinematic synergies of right arm reaching movement and investigated their potential usage in upper limb assistive exoskeleton motion planning. Ten right-handed subjects were asked to reach 10 target button locations placed on a cardboard in front. The kinematic data of right arm were tracked by a motion capture system. Angular velocities over time for shoulder flexion/extension, shoulder abduction/adduction, shoulder internal/external rotation, and elbow flexion/extension were computed. Principal component analysis (PCA) was used to derive kinematic synergies from the reaching task for each subject. We found that the first four synergies can explain more than 94% of the variance. Moreover, the joint coordination patterns were dynamically regulated over time as the number of kinematic synergy (PC) increased. The synergies with different order played different roles in reaching movement. Our results indicated that the low-order synergies represented the overall trend of motion patterns, while the high-order synergies described the fine motions at specific moving phases. A 4-DoF upper limb assistive exoskeleton was modeled in SolidWorks to simulate assistive exoskeleton movement pattern based on kinematic synergy. An exoskeleton Denavit-Hartenberg (D-H) model was established to estimate the exoskeleton moving pattern in reaching tasks. The results further confirmed that kinematic synergies could be used for exoskeleton motion planning, and different principal components contributed to the motion trajectory and end-point accuracy to some extent. The findings of this study may provide novel but simplified strategies for the development of rehabilitation and assistive robotic systems approximating the motion pattern of natural upper-limb motor function.

Effect of fatigue and the absence of visual feedback on shoulder motor control in an healthy population during a reaching task

INTRODUCTION

The main role of the upper limb is to position the hand in order to carry out varied activities requiring coordinated multi-joint movement, which requires mobility and stability at the glenohumeral joint. This is made possible by the interaction between active and passive structures as well as the integration of information coming from multiple systems. This interaction can be compromised by factors such as muscle fatigue and lack of visual feedback, leading to decreased performance. Several studies have investigated their isolated effect without looking at their combined effect.

OBJECTIVE

To measure the specific and the combined effects of shoulder muscles fatigue and of lack of visual feedback on shoulder motor control during a reaching task with the arm in an elevated position.

METHODS

60 healthy participants were randomly assigned to one of four experimental groups: 1) control with visual feedback; 2) control without visual feedback; 3) fatigue with visual feedback; 4) fatigue without visual feedback. Subjects had to perform 10 trials of a reaching task in the KINARM robotic arm. Kinematic variables of interest were time taken to complete the task, final error, initial angle of deviation and area under curve. Non-parametric ANOVAs were used.

RESULTS

Analyses showed that there were statistically significant differences (p < 0,01) for the time taken to complete the task (1.15 s compared to 0.70 s), the area under the curve (0.015m² compared to 0.009m²) and the final error (0.025 m compared to 0,011 m) between those who had visual feedback and those who did not. No statistically significant fatigue or feedback X fatigue interaction effects were found for all kinematic variables.

CONCLUSION

Findings show that lack of visual feedback had an impact on the reaching task performance while fatigue did not. In addition, fatigue did not increase the effect of the lack of visual feedback.

Assessing the brain 'on the line': An ecologically-valid assessment of the impact of repetitive assembly line work on hemodynamic response and fine motor control using fNIRS

To investigate neural correlates of repetitive assembly tasks in ecologically-valid empirical settings, this study measured bilateral prefrontal (PFC) and motor activations when participants performed a carburetor assembly task using functional near-infrared spectroscopy (fNIRS). Participants worked for one hour at a typical (low-) pace and at an accelerated high-pace. Before and after the task, a test was conducted to assess motion stability and fine motor control. The behavioral data revealed decreased motion stability after the assembly work in both conditions, with a significantly higher reduction after the high-pace task. The fNIRS data also revealed reduced activations in bilateral prefrontal and motor regions in both conditions over time. However, the low-pace task led to significantly greater activity decreases compared with the high-pace. Activity decrease in prefrontal and motor regions within the low pace also significantly related to minimal motion stability impairment, suggesting that the brain activation decreases in this and, potentially, findings of higher alpha in past repetitive-task studies using EEG, may be a result of not fatigue but worker adaptation or increasing efficiency.

Postural and muscular adaptations to repetitive simulated work

Complex repetitive tasks are common in the workplace and have been associated with upper extremity disorders. The purpose of this study was to examine the progressive effects of highly repetitive work on joint kinematics and muscle activity of the trunk and upper extremity. Fifteen healthy men performed 60 one-minute cycles of 4 simulated automotive-related tasks. Electromyography of eight muscles and kinematics of the trunk and right upper extremity were collected. Data were analysed at 12-min intervals and divided into a complete work cycle. The time to complete the work cycle decreased by 6.3 s over the trials. Peak shoulder flexion decreased and peak elbow flexion increased during the work cycle. Muscle activity magnitude and variability was influenced by time during the repetitive tasks. This study found adaptations to highly repetitive but light work in only 1 h; redistributing muscle demands within the shoulder over time may reduce muscle fatigue development. Practitioner Summary: While the work was not strenuous, we were able to demonstrate muscular and postural adaptations in a single hour of simulated work. By evaluating both the whole work cycle and the sub-tasks, we aim to develop new methods for evaluating the risk of complex tasks in prolonged repetitive work.

Central fatigue mechanisms are responsible for decreases in hand proprioceptive acuity following shoulder muscle fatigue

Muscle fatigue is a complex phenomenon, consisting of central and peripheral mechanisms which contribute to local and systemic changes in motor performance. In particular, it has been demonstrated that afferent processing in the fatigued muscle (e.g., shoulder), as well as in surrounding or distal muscles (e.g., hand) can be altered by fatigue. Currently, it is unclear how proximal muscle fatigue affects proprioceptive acuity of the distal limb. The purpose of the present study was to assess the effects of shoulder muscle fatigue on participants' ability to judge the location of their hand using only proprioceptive cues. Participants' (N = 16) limbs were moved outwards by a robot manipulandum and they were instructed to estimate the position of their hand relative to one of four visual reference targets (two near, two far). This estimation task was completed before and after a repetitive pointing task was performed to fatigue the shoulder muscles. To assess central versus peripheral effects of fatigue on the distal limb, the right shoulder was fatigued and proprioceptive acuity of the left and right hands were tested. Results showed that there was a significant decrease in the accuracy of proprioceptive estimates for both hands after the right shoulder was fatigued, with no change in the precision of proprioceptive estimates. A control experiment (N = 8), in which participants completed the proprioceptive estimation task before and after a period of quiet sitting, ruled out the possibility that the bilateral changes in proprioceptive accuracy were due to a practice effect. Together, these results indicate that shoulder muscle fatigue decreases proprioceptive acuity in both hands, suggesting that central fatigue mechanisms are primarily responsible for changes in afferent feedback processing of the distal upper limb.

Joint-Level Analyses of the Back Squat With and Without Intraset Rest

Purpose: To provide a joint-level analysis of traditional (TS) and cluster (CS) set structure during the back-squat exercise. Methods: Eight men (24 [3] y, 177.3 [7.9] cm, 82.7 [11.0] kg, 11.9 [3.5] % body fat, and 150.3 [23.0] kg 1-repetition maximum [1RM]) performed the back-squat exercise (80%1RM) using TS (4 × 6, 2-min interset rest) and CS (4 × [2 × 3], 30-s intraset rest, 90-s interset rest), randomly. Lower-limb kinematics were collected by motion capture, as well as kinetic data by bilateral force platforms. Results: CS attenuated the loss in mean power (TS -21.6% [3.9%]; CS -12.4% [7.5%]; P = .042), although no differences in gross movement pattern (sagittal-plane joint angles) within and between conditions were observed (P ≥ .05). However, joint power produced at the hip increased from repetition (REP) 1 through REP 6 during TS, while a decrease was noted at the knee. A similar pattern was observed in the CS condition but was limited to the hip. Joint power produced at the hip increased from REP 1 through REP 3 but returned to REP 1 values before a similar increase through REP 6, resulting in differences between conditions (REP 4, P = .018; REP 5, P = .022). Conclusions: Sagittal-plane joint angles did not change in either condition, although CS elicited greater power. Differing joint power contributions (hip and knee) suggest potential central mechanism that may contribute to enhanced power output during CS and warrant further study. Practitioners should consider incorporating CS into training to promote greater power adaptations and to mitigate fatigue.

Algorithmically detectable directional changes in upper extremity motion indicate substantial myoelectric shoulder muscle fatigue during a repetitive manual task

Repetitive workplace tasks are associated with fatigue-induced changes to shoulder muscular strategies, potentially altering kinematics and elevating susceptibility to tissue overexposures. Accessible and reliable methods to detect shoulder muscle fatigue in the workplace are therefore valuable. Detectable changes in joint motion may provide a plausible fatigue identification method. In this investigation, the onset of the first kinematic changes, as identified by a symbolic motion representation (SMSR) algorithm, and the onset of substantial surface electromyography (sEMG) mean power frequency (MPF) fatigue were not significantly different, both occurring around 10% of task duration. This highlights the potential utility of SMSR identified directional changes in joint motion during repetitive tasks as a cue of substantial muscle fatigue, enabling ergonomics responses that can mitigate shoulder muscular fatigue accumulation and its associated deleterious physical effects. Practitioner Summary: The onset of substantial muscle fatigue during a repetitive dynamic task was assessed using kinematics and myoelectric-based techniques. Algorithmically detectable directional changes in upper extremity joint motion occurred with the onset of substantial muscle fatigue, highlighting the potential of this as a useful approach for workplace fatigue identification.

Muscular and kinematic adaptations to fatiguing repetitive upper extremity work

Repetitive work is common in the modern workplace and the effects are often studied using fatigue protocols; however, there is evidence that the manner in which fatigue develops impacts the kinematic and muscular response to reduced physical capacity. The purpose of this study was to simultaneously evaluate muscular and kinematic adaptations during fatiguing, repetitive work until exhaustion. We measured muscle activity in 13 muscles in the shoulder and trunk, and captured full body kinematics while participants completed simulated, repetitive work tasks. Every 12 min, reference data were collected to quantify fatigue. This sequence continued until they reached the termination criteria. Participants displayed significant signs of muscle fatigue, loss of strength and increased perceived fatigue (p < 0.05). Analysis revealed a significant effect of time on posture and muscle activity that was both task and time dependent, and variable both between and within individuals. Participants were able to compensate for reduced physical capacity and maintain task performance through coordinated compensation strategies.

Changes in movement variability and task performance during a fatiguing repetitive pointing task

Changes in neuromuscular strategies employed with fatigue during multi-joint movements are still poorly understood. Studies have shown that motor variability of individual joints increases when performing upper limb tasks to fatigue, while movement parameters related to the task goal remain constant. However, how the inter-limb coordination and its variability change during specific movement phases with fatigue is still unclear. The aim of this study was to assess the effects of neck-shoulder fatigue on shoulder and elbow kinematic variabilities, shoulder-elbow coordination and its variability, and endpoint characteristics during different phases of a forward pointing movement. Nineteen healthy young adults continuously performed a repetitive pointing task until fatigue (Borg rating of 8/10). Changes in elbow-shoulder coordination through the movement were assessed using the continuous relative phase and statistical nonparametric mapping methods. At the end of the task, muscle fatigue was evidenced by significant increases in anterior deltoid (+13%) and biceps brachii (+30%) activity. Shoulder horizontal abduction, elbow flexion variability and shoulder-elbow coordination variability were increased with fatigue at different moments of the movement cycle (shoulder: during the first 17% and most of the second half movement, elbow: from 73% to 91%, coordination: almost the whole movement). However, movement timing errors and endpoint spatial variability were mostly preserved, even with fatigue. We showed that increased variability with fatigue is not only observed in the fatigued joint (shoulder), but also in the elbow and shoulder-elbow coordination, and may have a goal of preserving global task performance.

Sex differences in kinematic adaptations to muscle fatigue induced by repetitive upper limb movements

Background

Muscle fatigue induced by repetitive movements contributes to the development of musculoskeletal disorders. Men and women respond differently to muscle fatigue during isometric single-joint efforts, but sex differences during dynamic multi-joint tasks have not been clearly identified. Moreover, most studies comparing men and women during fatigue development assessed endurance time. However, none evaluated sex differences in kinematic adaptations to fatigue during multi-joint dynamic tasks. The objective of the study was to compare how men and women adapt their upper body kinematics during a fatiguing repetitive pointing task.

Methods

Forty men and 41 women performed repetitive pointing movements (one per second) between two targets while maintaining their elbow elevated at shoulder height. The task ended when participants rated a perceived level of fatigue of 8/10. Trunk, humerothoracic, and elbow angles were compared between the first and last 30 s of the experiment and between men and women. Linear positions of the index finger (distance from the target) and the elbow (arm elevation) as well as movement timing were documented as task performance measures.

Results

Men (7.4 ± 3.2 min) and women (8.3 ± 4.5 min) performed the repetitive pointing task for a similar duration. For both sex groups, trunk range of motion increased with fatigue while shoulder’s and elbow’s decreased. Moreover, participants modified their trunk posture to compensate for the decreased humerothoracic elevation. Movements at all joints also became more variable with fatigue. However, of the 24 joint angle variables assessed, only two Sex × Fatigue interactions were observed. Although average humerothoracic elevation angle decreased in both subgroups, this decrease was greater in men (standardized response mean [SRM] − 1.63) than in women (SRM − 1.44). Moreover, the movement-to-movement variability of humerothoracic elevation angle increased only in women (SRM 0.42).

Conclusion

Despite many similarities between men’s and women’s response to fatigue induced by repetitive pointing movements, some sex differences were observed. Those subtle differences may indicate that men’s shoulder muscles were more fatigued than women’s despite a similar level of perceived exertion. They may also indicate that men and women do not adapt the exact same way to a similar fatigue.

Electronic supplementary material

The online version of this article (10.1186/s13293-018-0175-9) contains supplementary material, which is available to authorized users.

Postural adaptations to unilateral knee joint hypomobility induced by orthosis wear during gait initiation

Balance control and whole-body progression during gait initiation (GI) involve knee-joint mobility. Single knee-joint hypomobility often occurs with aging, orthopedics or neurological conditions. The goal of the present study was to investigate the capacity of the CNS to adapt GI organization to single knee-joint hypomobility induced by the wear of an orthosis. Twenty-seven healthy adults performed a GI series on a force-plate in the following conditions: without orthosis ("control"), with knee orthosis over the swing leg ("orth-swing") and with the orthosis over the contralateral stance leg ("orth-stance"). In orth-swing, amplitude of mediolateral anticipatory postural adjustments (APAs) and step width were larger, execution phase duration longer, and anteroposterior APAs smaller than in control. In orth-stance, mediolateral APAs duration was longer, step width larger, and amplitude of anteroposterior APAs smaller than in control. Consequently, step length and progression velocity (which relate to the "motor performance") were reduced whereas stability was enhanced compared to control. Vertical force impact at foot-contact did not change across conditions, despite a smaller step length in orthosis conditions compared to control. These results show that the application of a local mechanical constraint induced profound changes in the global GI organization, altering motor performance but ensuring greater stability.

Inter-joint coordination changes during and after muscle fatigue

People produce multi-joint movements by organizing many degrees of freedom into a few major covarying relationships, indicating a high level of inter-joint coordination. These relationships can be identified using data decomposition analyses (e.g. principal components analysis, non-negative matrix factorization). The purpose of this study was to determine how movement coordination changes during muscle fatigue by analyzing the covariance structure of multi-joint movements. Sixteen (16) healthy adults completed a continuous, timed ratcheting task with the right arm for three 1-min intervals before, during, and after an intermittent shoulder fatigue protocol. Joint angles from the right arm and trunk were tracked for subsequent principal components analysis. Principal component waveforms were constructed from the original joint angles, and changes in the waveforms during fatigue were assessed using cross-correlations. The variance explained by the first four principal components reached a maximum of 90.5% in the second minute of the pre-test and decreased to a minimum of 86.0% in the last minute of fatigue (p = .033). In the last minute of the post-test, explained variance (87.1%) did not differ from any other pre, fatigue, or post-test time point (p > .23). These results suggest that inter-joint coordination decreased during fatigue. Changes in the movement patterns and principal component waveforms suggest that subjects adopted a more rigid movement strategy when fatigued. However, the rigid movement strategy was not observed during the post-test. The results suggest that people adopted a new pattern of inter-joint coordination while using novel kinematics.

Exploring the Relationship of Task Performance and Physical and Cognitive Fatigue During a Daylong Light Precision Task

OBJECTIVE

Our aim was to explore the relationship between fatigue and operation system performance during a simulated light precision task over an 8-hr period using a battery of physical (central and peripheral) and cognitive measures.

BACKGROUND

Fatigue may play an important role in the relationship between poor ergonomics and deficits in quality and productivity. However, well-controlled laboratory studies in this area have several limitations, including the lack of work relevance of fatigue exposures and lack of both physical and cognitive measures. There remains a need to understand the relationship between physical and cognitive fatigue and task performance at exposure levels relevant to realistic production or light precision work.

METHOD

Errors and fatigue measures were tracked over the course of a micropipetting task. Fatigue responses from 10 measures and errors in pipetting technique, precision, and targeting were submitted to principal component analysis to descriptively analyze features and patterns.

RESULTS

Fatigue responses and error rates contributed to three principal components (PCs), accounting for 50.9% of total variance. Fatigue responses grouped within the three PCs reflected central and peripheral upper extremity fatigue, postural sway, and changes in oculomotor behavior.

CONCLUSION

In an 8-hr light precision task, error rates shared similar patterns to both physical and cognitive fatigue responses, and/or increases in arousal level.

APPLICATION

The findings provide insight toward the relationship between fatigue and operation system performance (e.g., errors). This study contributes to a body of literature documenting task errors and fatigue, reflecting physical (both central and peripheral) and cognitive processes.

Proximal and distal muscle fatigue differentially affect movement coordination

Muscle fatigue can cause people to change their movement patterns and these changes could contribute to acute or overuse injuries. However, these effects depend on which muscles are fatigued. The purpose of this study was to determine the differential effects of proximal and distal upper extremity muscle fatigue on repetitive movements. Fourteen subjects completed a repetitive ratcheting task before and after a fatigue protocol on separate days. The fatigue protocol either fatigued the proximal (shoulder flexor) or distal (finger flexor) muscles. Pre/Post changes in trunk, shoulder, elbow, and wrist kinematics were compared to determine how proximal and distal fatigue affected multi-joint movement patterns and variability. Proximal fatigue caused a significant increase (7°, p < 0.005) in trunk lean and velocity, reduced humeral elevation (11°, p < 0.005), and increased elbow flexion (4°, p < 0.01). In contrast, distal fatigue caused small but significant changes in trunk angles (2°, p < 0.05), increased velocity of wrench movement relative to the hand (17°/s, p < 0.001), and earlier wrist extension (4%, p < 0.005). Movement variability increased at proximal joints but not distal joints after both fatigue protocols (p < 0.05). Varying movements at proximal joints may help people adapt to fatigue at either proximal or distal joints. The identified differences between proximal and distal muscle fatigue adaptations could facilitate risk assessment of occupational tasks.

Motor adaptations to unilateral quadriceps fatigue during a bilateral pedaling task

This study was designed to investigate how motor coordination adapts to unilateral fatigue of the quadriceps during a constant-load bilateral pedaling task. We first hypothesized that this local fatigue would not be compensated within the fatigued muscles leading to a decreased knee extension power. Then, we aimed to determine whether this decrease would be compensated by between-joints compensations within the ipsilateral leg and/or an increased contribution of the contralateral leg. Fifteen healthy volunteers were tested during pedaling at 350 W before and after a fatigue protocol consisting of 15 minutes of electromyostimulation on the quadriceps muscle. Motor coordination was assessed from myoelectrical activity (22 muscles) and joint powers calculated through inverse dynamics. Maximal knee extension torque decreased by 28.3%±6.8% (P<.0005) immediately after electromyostimulation. A decreased knee extension power produced by the ipsilateral leg was observed during pedaling (-22.8±12.3 W, -17.0%±9.4%; P<.0005). To maintain the task goal, participants primarily increased the power produced by the non-fatigued contralateral leg during the flexion phase. This was achieved by an increase in hip flexion power confirmed by a higher activation of the tensor fascia latae. These results suggest no adjustment of neural drive to the fatigued muscles and demonstrate no concurrent ipsilateral compensation by the non-fatigued muscles involved in the extension pedaling phase. Although interindividual variability was observed, findings provide evidence that participants predominantly adapted by compensating with the contralateral leg during its flexion phase. Both neural (between legs) and mechanical (between pedals) couplings and the minimization of cost functions might explain these results.

Influence of Lumbar Muscle Fatigue on Trunk Adaptations during Sudden External Perturbations

Introduction: When the spine is subjected to perturbations, neuromuscular responses such as reflex muscle contractions contribute to the overall balance control and spinal stabilization mechanisms. These responses are influenced by muscle fatigue, which has been shown to trigger changes in muscle recruitment patterns. Neuromuscular adaptations, e.g., attenuation of reflex activation and/or postural oscillations following repeated unexpected external perturbations, have also been described. However, the characterization of these adaptations still remains unclear. Using high-density electromyography (EMG) may help understand how the nervous system chooses to deal with an unknown perturbation in different physiological and/or mechanical perturbation environments.

Aim: To characterize trunk neuromuscular adaptations following repeated sudden external perturbations after a back muscle fatigue task using high-density EMG.

Methods: Twenty-five healthy participants experienced a series of 15 sudden external perturbations before and after back muscle fatigue. Erector spinae muscle activity was recorded using high-density EMG. Trunk kinematics during perturbation trials were collected using a 3-D motion analysis system. A two-way repeated measure ANOVA was conducted to assess: (1) the adaptation effect across trials; (2) the fatigue effect; and (3) the interaction effect (fatigue × adaptation) for the baseline activity, the reflex latency, the reflex peak and trunk kinematic variables (flexion angle, velocity and time to peak velocity). Muscle activity spatial distribution before and following the fatigue task was also compared using t-tests for dependent samples.

Results: An attenuation of muscle reflex peak was observed across perturbation trials before the fatigue task, but not after. The spatial distribution of muscle activity was significantly higher before the fatigue task compared to post-fatigue trials. Baseline activity showed a trend to higher values after muscle fatigue, as well as reduction through perturbation trials. Main effects of fatigue and adaptation were found for time to peak velocity. No adaptation nor fatigue effect were identified for reflex latency, flexion angle or trunk velocity.

Conclusion: The results show that muscle fatigue leads to reduced spatial distribution of back muscle activity and suggest a limited ability to use across-trial redundancy to adapt EMG reflex peak and optimize spinal stabilization using retroactive control.

Referent control and motor equivalence of reaching from standing

Motor actions may result from central changes in the referent body configuration, defined as the body posture at which muscles begin to be activated or deactivated. The actual body configuration deviates from the referent configuration, particularly because of body inertia and environmental forces. Within these constraints, the system tends to minimize the difference between these configurations. For pointing movement, this strategy can be expressed as the tendency to minimize the difference between the referent trajectory (R_T) and actual trajectory (Q_T) of the effector (hand). This process may underlie motor equivalent behavior that maintains the pointing trajectory regardless of the number of body segments involved. We tested the hypothesis that the minimization process is used to produce pointing in standing subjects. With eyes closed, 10 subjects reached from a standing position to a remembered target located beyond arm length. In randomly chosen trials, hip flexion was unexpectedly prevented, forcing subjects to take a step during pointing to prevent falling. The task was repeated when subjects were instructed to intentionally take a step during pointing. In most cases, reaching accuracy and trajectory curvature were preserved due to adaptive condition-specific changes in interjoint coordination. Results suggest that referent control and the minimization process associated with it may underlie motor equivalence in pointing.

NEW & NOTEWORTHY

Motor actions may result from minimization of the deflection of the actual body configuration from the centrally specified referent body configuration, in the limits of neuromuscular and environmental constraints. The minimization process may maintain reaching trajectory and accuracy regardless of the number of body segments involved (motor equivalence), as confirmed in this study of reaching from standing in young healthy individuals. Results suggest that the referent control process may underlie motor equivalence in reaching.

The Effect of Propulsion Style on Wrist Movement Variability During the Push Phase After a Bout of Fatiguing Propulsion

BACKGROUND

Wheelchair propulsion has been linked to overuse injuries regardless of propulsion style. Many aspects of the arcing (ARC) and semicircular (SEMI) propulsion styles have been compared, but differences in intracycle movement variability, which have been linked to overuse injuries, have not been examined.

OBJECTIVE

To explore how ARC and SEMI affect changes in intracycle wrist movement variability after a fatiguing bout of propulsion.

DESIGN

Repeated measures crossover design.

SETTING

Wheelchair rollers and wheelchair fatigue course in a research laboratory.

PARTICIPANTS

Twenty healthy, nondisabled adult men without previous wheelchair experience.

INTERVENTIONS

Participants learned ARC and SEMI and used each to perform a wheelchair fatigue protocol.

MAIN OUTCOME MEASUREMENTS

Thirty seconds of propulsion on rollers were recorded by motion-capture cameras before and after a fatigue protocol for each propulsion style on 2 testing days. Angular wrist orientations (flexion/extension and radial/ulnar deviation) and linear wrist trajectories (mediolateral direction) were computed, and intracycle movement variability was calculated as standard deviations of the detrended and filtered values during the push phase beginning and end. Paired samples t tests were used to compare ARC and SEMI based on the percent changes from pre- to postfatigue protocol.

RESULTS

Both propulsion styles resulted in increased intracycle wrist movement variability postfatigue, but observed increases did not significantly differ between ARC and SEMI.

CONCLUSIONS

This study evinces that intersubject variability exceeded average changes in intracycle wrist movement variability for both propulsion styles. Neither propulsion style resulting in a greater change in intracycle movement variability may suggest that no single propulsion style is ideal for everyone. The large intersubject variability may indicate that the propulsion style resulting in the smallest increase in intracycle movement variability after a fatiguing bout of propulsion may differ for each person and may help explain why wheelchair users self-select to use different propulsion styles.

Intrinsic movement variability at work. How long is the path from motor control to design engineering?

For several years, increasing numbers of studies have highlighted the existence of movement variability. Before that, it was neglected in movement analysis and it is still almost completely ignored in workstation design. This article reviews motor control theories and factors influencing movement execution, and indicates how intrinsic movement variability is part of task completion. These background clarifications should help ergonomists and workstation designers to gain a better understanding of these concepts, which can then be used to improve design tools. We also question which techniques--kinematics, kinetics or muscular activity--and descriptors are most appropriate for describing intrinsic movement variability and for integration into design tools. By this way, simulations generated by designers for workstation design should be closer to the real movements performed by workers. This review emphasises the complexity of identifying, describing and processing intrinsic movement variability in occupational activities.