Time-dependent adaptations to posture and movement characteristics during the development of repetitive reaching induced fatigue

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.

Few sex-specific effects of fatigue on muscle synergies in a repetitive pointing task

Previous studies have identified some sex differences in how individual muscles change their activation during repetitive multi-joint arm motion-induced fatigue. However, little is known about how indicators of multi-muscle coordination change with fatigue in males and females. Fifty-six (29 females) asymptomatic young adults performed a repetitive, forward-backward pointing task until scoring 8/10 on a Borg CR10 scale while surface electromyographic activity of upper trapezius, anterior deltoid, biceps brachii, and triceps brachii was recorded. Activation coefficient, synergy structure, and relative weight of each muscle within synergies were calculated using the non-negative matrix factorization method. Two muscle synergies were extracted from the fatiguing task. The synergy structures were mostly preserved after fatigue, while the activation coefficients were altered. A significant Sex × Fatigue interaction effect showed more use of the anterior deltoid in males especially before fatigue in synergy 1 during shoulder stabilization (p = 0.04). As for synergy 2, it was characterized by variations in the relative weight of biceps, which was higher by 16 % in females compared to males (p = 0.04), and increased with fatigue (p = 0.03) during the elbow flexion acceleration phase and the deceleration phase of the backward pointing movement. Findings suggest that both sexes adapted to fatigue similarly, using fixed synergy structures, with alterations in synergy activation patterns and relative weights of individual muscles. Results support previous findings of an important role for the biceps and anterior deltoid in explaining sex differences in patterns of repetitive motion-induced upper limb fatigue.

Fatigue task-dependent effect on spatial distribution of lumbar muscles activity

This study aims to identify how spatial distribution of lumbar muscle activity is modulated by different fatigue tasks. Twenty healthy adults performed two different isometric trunk extension endurance tasks (the modified Sorensen test and the inverted modified Sorensen test) until exhaustion. During these tasks, bilateral superficial lumbar muscle activity was recorded using high-density electromyography. The spatial distribution of activation within these muscles was obtained using the centroid coordinates in the medio-lateral and cranio-caudal directions. The effects of task and endurance time (left and right sides) were investigated using repeated measures ANOVA. Results revealed a significant lateral shift of the centroid throughout the fatigue tasks on both sides and no difference between tasks. Significant task × time interaction effects were found for the cranio-caudal direction on both sides showing a significantly more caudal location of the centroid in the modified Sorensen test compared to the inverted test at the beginning of the tasks. Our findings suggest that spatial distribution of lumbar muscle activity is task-dependent in a pre-fatigue stage while an alternative but similar muscle recruitment strategy is used in both tasks to maintain performance in the later stages of muscle fatigue.

Effect of Periodic Voluntary Interventions on Trapezius Activation and Fatigue During Light Upper Limb Activity

OBJECTIVE

The effects of diverse periodic interventions on trapezius muscle fatigue and activity during a full day of computer work were investigated.

BACKGROUND

Musculoskeletal disorders, including trapezius myalgia, may be associated with repeated exposure to prolonged low-level activity, even during light upper-extremity tasks including computer work.

METHODS

Thirty healthy adults participated in a study that simulated two 6-hour workdays of computer work. One workday involved imposed periodic passive and active interventions aimed at disrupting trapezius contraction monotony (Intervention day), whereas the other workday did not (Control day). Trapezius muscle activity was quantified by the 3-dimensional acceleration of the jolt movement of the acromion produced by electrically induced muscle twitches. The spatio-temporal distribution of trapezius activity was measured through high-density surface electromyography (HD-EMG).

RESULTS

The twitch acceleration magnitude in one direction was significantly different across measurement periods (p = 0.0156) on Control day, whereas no significant differences in any direction were observed (p > 0.05) on Intervention day. The HD-EMG from Intervention day showed that only significant voluntary muscle contractions (swing arms, Jacobson maneuver) induced a decrease in the muscle activation time and an increase in the spatial muscle activation areas (p < 0.01).

CONCLUSION

Disruption of trapezius monotonous activity via brief voluntary contractions effectively modified the ensuing contraction pattern (twitch acceleration along one axis, active epochs reduction, and larger spatial distribution). The observed changes support an associated reduction of muscle fatigue.

APPLICATION

This study suggests that disruptive intervention activity is efficient in reducing the impact of trapezius muscle fatigue.

The influence of proximal motor strategies on pianists' upper-limb movement variability

Repetitive movements are considered a risk factor for developing practice-related musculoskeletal disorders. Intra-participant kinematic variability might help musicians reduce the risk of injury during repetitive tasks. No research has studied the effects of proximal motion (i.e., trunk and shoulder movement) on upper-limb movement variability in pianists. The first objective was to determine the effect of proximal movement strategies and performance tempo on both intra-participant joint angle variability of upper-limb joints and endpoint variability. The second objective was to compare joint angle variability between pianist's upper-limb joints. As secondary objectives, we assessed the relationship between intra-participant joint angle variability and task range of motion (ROM) and documented inter-participant joint angle variability. The upper body kinematics of 9 expert pianists were recorded using an optoelectronic system. Participants continuously performed two right-hand chords (lateral leap motions) while changing movements based on trunk motion (with and without) and shoulder motion (counter-clockwise, back-and-forth, and clockwise) at two tempi (slow and fast). Trunk and shoulder movement strategies collectively influenced variability at the shoulder, elbow and, to a lesser extent, the wrist. Slow tempi led to greater variability at wrist and elbow flexion/extension compared to fast tempi. Endpoint variability was influenced only along the anteroposterior axis. When the trunk was static, the shoulder had the lowest joint angle variability. When trunk motion was used, elbow and shoulder variability increased, and became comparable to wrist variability. ROM was correlated with intra-participant joint angle variability, suggesting that increased task ROM might result in increased movement variability during practice. Inter-participant variability was approximately six times greater than intra-participant variability. Pianists should consider incorporating trunk motion and a variety of shoulder movements as performance strategies while performing leap motions at the piano, as they might reduce exposure to risks of injury.

Size and structure of motor variability in young and old adults performing a rhythmic, repetitive tapping task

The size of motor variability increases with fatigue in repetitive upper limb tasks, and the structure of variability differs with old age. However, the combined influences of old age and fatigue on the size and structure of movement-to-movement variability are unclear. Eighteen young and sixteen old adults performed a fatiguing repetitive tapping task while seated using their dominant arm. Optoelectronic motion capture served to measure upper body angles via forward kinematics. Movement-to-movement variability was measured by the size at joints (standard deviation: SD) and by the structure of the uncontrolled manifold (variance: V_UCM, V_ORT; synergy index: ΔV_z) in the first and final minutes of the task for the early, middle, and late forward movement phases. Outcomes were analyzed by Age*Condition*Phase general estimating equations. Old adults had lower humerothoracic abduction/adduction and flexion/extension SD, wrist flexion/extension SD, V_UCM, and V_ORT, mainly in the early movement phase (p < 0.014). With fatigue, humerothoracic flexion/extension SD increased in young adults only and humerothoracic abduction/adduction SD, wrist pronation/supination SD, wrist flexion/extension SD, and V_UCM increased in both groups. ΔV_z was positive and did not differ with age or fatigue (p > 0.014). Results indicate that fatigue adjustments were mainly in the frontal plane, old age did not affect the ratio of good vs. bad variability, and motor synergy was preserved during fatigue despite less motor flexibility in old age.

Exploring the relationship between kinematic variability and fatigue development during repetitive lifting

To investigate the variability-fatigue and repeaters-replacers hypotheses, motor variability (MV) and indicators of fatigue were assessed during repetitive lifting. Eighteen participants performed sequential repetitive bouts of lifting divided into a short bout, and three phases of a prolonged bout until volitional fatigue (or until a 1-h time limit). Whole-body kinematics were collected to calculate variability in three-dimensional joint angles and in continuous relative phase (CRP) of sagittal joint angle couplings, which were summed for the upper and lower body, and whole-body. Excellent individual consistency (ICC = 0.95-0.97) was demonstrated across lifting bouts as fatigue developed. Therefore, strong evidence was obtained for MV as an individual trait in support of the repeaters-replacers hypothesis. Associations were found for endurance and baseline effort with lower body variability, while no associations were found for rate of fatigue. Thus, some support was found for the variability-fatigue hypothesis which suggests that repeaters are less fatigue-resistant than replacers.

Detecting subject-specific fatigue-related changes in lifting kinematics using a machine learning approach

Individual responses to fatigue have been observed in lifting kinematics, suggesting a subject-specific approach is necessary for fatigue identification. One-class support vector machines (OCSVM) may provide an objective method to classify fatigue-related kinematic changes during repetitive lifting. Participants completed a repetitive lifting protocol while motion capture recorded lifting motions. Subject-specific kinematics from participants' first 35% of lifts trained OCSVM decision boundaries. The remaining lifts were separated into test sets and classified against the decision boundary to identify the percentage of outlier lifts within each test set. Spearman's correlation assessed if the test sets' percentage of outlier lifts increased concurrently with participants' rating of perceived exertion (RPE). Significant positive associations were found for participants who demonstrated evidence of fatigue, while no significant associations were found for participants who did not demonstrate evidence of fatigue. These results demonstrate the prospective efficacy of an outlier detection tool for fatigue detection during repetitive lifting.Practitioner Summary: An objective subject-specific fatigue detection method is desired for workplace tasks, such as lifting. An outlier detection machine learning approach was identified when lifting movement patterns changed from baseline throughout a repetitive lifting protocol. Participants who demonstrated an increase in outlier movement patterns had a concurrent increase in self-reported fatigue.

Uncontrolled Manifold Analysis of the Effects of Different Fatigue Locations on Kinematic Coordination During a Repetitive Upper-Limb Task

Fatigue at individual joints is known to affect interjoint coordination during repetitive multijoint tasks. However, how these coordination adjustments affect overall task stability is unknown. Twelve participants completed a repetitive pointing task at rest and after fatigue of the shoulder, elbow, and trunk. Upper-limb and trunk kinematics were collected. Uncontrolled manifold framework was applied to a kinematic model to link elemental variables to endpoint fingertip position. Mixed and one-way analysis of variances determined effects (phase and fatigue location) on variance components and synergy index, respectively. The shoulder fatigue condition had the greatest impact in causing increases in variance components and a decreased synergy index in the late phase of movement, suggesting more destabilization of the interjoint task caused by shoulder fatigue.

A Comprehensive Evaluation of Spine Kinematics, Kinetics, and Trunk Muscle Activities During Fatigue-Induced Repetitive Lifting

OBJECTIVE

Spine kinematics, kinetics, and trunk muscle activities were evaluated during different stages of a fatigue-induced symmetric lifting task over time.

BACKGROUND

Due to neuromuscular adaptations, postural behaviors of workers during lifting tasks are affected by fatigue. Comprehensive aspects of these adaptations remain to be investigated.

METHOD

Eighteen volunteers repeatedly lifted a box until perceived exhaustion. Body center of mass (CoM), trunk and box kinematics, and feet center of pressure (CoP) were estimated by a motion capture system and force-plate. Electromyographic (EMG) signals of trunk/abdominal muscles were assessed using linear and nonlinear approaches. The L5-S1 compressive force (Fc) was predicted via a biomechanical model. A two-way multivariate analysis of variance (MANOVA) was performed to examine the effects of five blocks of lifting cycle (C1 to C5) and lifting trial (T1 to T5), as independent variables, on kinematic, kinetic, and EMG-related measures.

RESULTS

Significant effects of lifting trial blocks were found for CoM and CoP shift in the anterior-posterior direction (respectively p < .001 and p = .014), trunk angle (p = .004), vertical box displacement (p < .001), and Fc (p = .005). EMG parameters indicated muscular fatigue with the extent of changes being muscle-specific.

CONCLUSION

Results emphasized variations in most kinematics/kinetics, and EMG-based indices, which further provided insight into the lifting behavior adaptations under dynamic fatiguing conditions.

APPLICATION

Movement and muscle-related variables, to a large extent, determine the magnitude of spinal loading, which is associated with low back pain.

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.

A Study on sEMG-Based Motor Variability and Functional Connectivity of the Upper Limb Depending on Weight Distributions in a Handle of a Cordless Stick-Type Vacuum Cleaner

This study investigated the muscle activities, motor variability, and functional connectivity of the upper limb as a function of weight distributions in a handle of a cordless stick-type vacuum cleaner. Eighteen female college students with experience of vacuum cleaner-use participated in testing. Five handles with different centers of mass (CM) were prepared (centroid, top-rear, top-front, bottom-front, and bottom-rear), and electromyography for the muscles of the upper limb were measured during vacuuming. The results showed that the %MVC values of the Extensor Carpi Ulnaris (p = 0.0038) and Deltoid Middle (p = 0.0094) increased but that of the Biceps Brachii (p = 0.0001) decreased, as the CM moved from the top to bottom area of the handle. The motor variability of the Extensor Carpi Ulnaris (p = 0.0335) and Brachioradialis (p = 0.0394) significantly varied depending on the CM locations but failed to show significance in the post-hoc analyses. Lastly, the functional connectivity values of the muscle pairs such as the Extensor Carpi Ulnaris–Deltoid Middle (p = 0.0016), Extensor Carpi Ulnaris–Upper Trapezius (p = 0.0174), Brachioradialis–Biceps Brachii (p = 0.0356), and Biceps Brachii–Upper Trapezius (p = 0.0102) were significantly altered as a function of the CM locations. The lowest functional connectivity was found with the handle of which CM was at centroid.

Sex-specific muscle activation and oxygenation kinetics during a repetitive forward pointing task

We compared the minute-by-minute muscle activity and oxygenation responses to a repetitive arm motion-induced fatiguing task between the sexes in order to address the literature gap on these time-dependent fatigue responses. Twenty-six (13 females) healthy adults performed a repetitive pointing task (RPT) with the arm moving forward/backward at shoulder height until reaching 8/10 (Borg CR10) for neck/shoulder perceived exertion (RPE). Neck/shoulder RPE, oxygenation and electromyography were recorded every minute and compared between first and second half of the task and between the sexes. Greater changes in oxygen supply and activation amplitude occurred during the second half of the task. Despite similar time to fatigue-terminal (p > 0.05), females showed greater anterior deltoid activation amplitude at all time points than males, and only the males showed increases in anterior and posterior deltoid activation amplitudes. In females, middle (ρ = -0.34, p = 0.04) and posterior (ρ = -0.44, p = 0.01) deltoid amplitudes were negatively correlated with perceived exertion during the first half of the task. Results suggest that reduced modulation of anterior deltoid activation amplitude in females may reflect a sub-optimal fatigue-mitigation mechanism compared with males and may help explain their greater susceptibility to neck/shoulder musculoskeletal disorders. Novelty: Despite similar fatigability and trapezius oxygenation, females showed greater deltoid activation throughout the task. Deltoid activation increased in males but not in females. The results support the important role of the deltoid in sex-specific neck/shoulder injury mechanisms.

A Real-Time Algorithm to Estimate Shoulder Muscle Fatigue Based on Surface EMG Signal For Static and Dynamic Upper Limb Tasks

Despite prevention efforts, the prevalence of workrelated upper extremity musculoskeletal disorders (WRUED) is increasing. A limit in the development of preventive interventions is the lack of devices that can measure and process sEMG signals in order to provide real-time reliable information on muscular fatigue of the upper limb in relation to the physical demands of the work. In this paper, the development and evaluation of a real-time muscle fatigue detection algorithm based on sEMG will be presented. The proposed algorithm uses the median frequency of sEMG power spectrum density (PSD) obtained with the Continuous Wavelet Transform (CWT) as an indicator of the muscle fatigue level. To extend the algorithm's efficiency to dynamic tasks, a muscle contraction detection module is added in order to remove the segments when the muscle is not contracting. To assess the algorithm's performance, eight healthy adults performed simple static and dynamic shoulder tasks using different loads. The results of the proposed time-frequency method (i.e. CWT) were first compared to those of the traditional Short Time Fourier Transform (STFT). It was shown that the CWT performs better than the STFT in both static and dynamic loading conditions. The validity of the algorithm's output as a muscle fatigue indicator was verified by comparing the output's decrease rate with different loads. As expected, the algorithm's fatigue indicator decreased faster over time with heavier loads. It was also shown that the initial muscle fatigue estimation output is independent of the load. Finally, we studied the proposed muscle contraction detection module's efficiency to overcome issues associated with dynamic tasks. We observed a substantial improvement of the smoothness of the fatigue indicator's evolution by using of the muscle contraction detection module.

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.

Impact of bed height on the biomechanics of healthcare professionals during chest compressions on the neonate: a descriptive pilot study

OBJECTIVES

The biomechanics of the healthcare professionals (HCPs) performing the life-saving intervention of chest compressions in the neonatal population is poorly understood. The aim of this pilot study was to describe the variations in body position at a self-selected and a predetermined bed height during neonatal chest compressions. Measures of joint angles, time to postural sway and number of postural adjustments were chosen as indices for the stability of the HCP's position.

SETTING

Data were collected at a simulation-based research centre in which the patient care environment was replicated.

PARTICIPANTS

HCPs with varying roles working in the neonatal intensive care unit and holding a current Neonatal Resuscitation Program Provider certification were recruited for this study.

INTERVENTIONS

Fifteen HCPs performed two trials of chest compressions, each lasting 2 min, at a predetermined bed height and a self-selected bed height. Trials were video recorded, capturing upper and lower body movements. Videos were analysed for time to postural sway and number of postural adjustments. Joint angles were measured at the start and end of each trial.

RESULTS

A statistically significant difference was found between the two bed height conditions for number of postural adjustments (p=0.02). While not statistically significant, time postural sway was increased in the choice bed height condition (85 s) compared with the predetermined bed height (45 s). After 30 s of chest compressions, mean shoulder and knee angles were smaller for choice bed height (p=0.03, 95% CI Lower=-12.14, Upper=-0.68 and p=0.05, 95% CI Lower=3.43, Upper=0.01, respectively). After 1 min and 45 s of chest compressions, mean wrist angles were smaller in the choice bed height condition (p=0.01, 95% CI Lower=-9.20, Upper=-1.22), stride length decreased between the 30 s and 1 min 45 s marks of the chest compressions in the predetermined height condition (p=0.02).

The association of reactive balance control and spinal curvature under lumbar muscle fatigue

Background

Although low back fatigue is an important intervening factor for physical functioning among sedentary people, little is known about its possible significance in relation to the spinal posture and compensatory postural responses to unpredictable stimuli. This study investigates the effect of lumbar muscle fatigue on spinal curvature and reactive balance control in response to externally induced perturbations.

Methods

A group of 38 young sedentary individuals underwent a perturbation-based balance test by applying a 2 kg load release. Sagittal spinal curvature and pelvic tilt was measured in both a normal and Matthiass standing posture both with and without a hand-held 2 kg load, and before and after the Sørensen fatigue test.

Results

Both the peak anterior and peak posterior center of pressure (CoP) displacements and the corresponding time to peak anterior and peak posterior CoP displacements significantly increased after the Sørensen fatigue test (all at p < 0.001). A lumbar muscle fatigue led to a decrease of the lumbar lordosis in the Matthiass posture while holding a 2 kg load in front of the body when compared to pre-fatigue conditions both without a load (p = 0.011, d = 0.35) and with a 2 kg load (p = 0.000, d = 0.51). Also the sacral inclination in the Matthiass posture with a 2 kg additional load significantly decreased under fatigue when compared to all postures in pre-fatigue conditions (p = 0.01, d = 0.48). Contrary to pre-fatigue conditions, variables of the perturbation-based balance test were closely associated with those of lumbar curvature while standing in the Matthiass posture with a 2 kg additional load after the Sørensen fatigue test (r values in range from −0.520 to −0.631, all at p < 0.05).

Conclusion

These findings indicate that lumbar muscle fatigue causes changes in the lumbar spinal curvature and this is functionally relevant in explaining the impaired ability to maintain balance after externally induced perturbations. This emphasizes the importance for assessing both spinal posture and reactive balance control under fatigue in order to reveal their interrelations in young sedentary adults and predict any significant deterioration in later years.

Changes in muscle activation, oxygenation, and morphology following a fatiguing repetitive forward reaching task in young adult males and females

We sought to evaluate sex-specific 1) muscle activation patterns, hemodynamics, and swelling responses to short-cycle repetitive fatigue; 2) relationships between muscular responses and perceived fatigability. Asymptomatic participants (N = 26, 13 females) completed a repetitive pointing task until 8/10 on the Borg CR10 scale. Upper trapezius (UT), supraspinatus (SUPRA), and biceps brachii (BIC) muscle activation, activation variability (CV), median power frequency (MdPF) and thickness, and UT oxygenation were recorded. Males had higher BIC CV, UT and SUPRA MdPF, and UT and BIC thickness. Longer time to fatigue-terminal was correlated to greater SUPRA activation increase (ρ = 0.624) and BIC MdPF decrease (ρ = -0.674) in males, while in females it was correlated to greater (ρ = -0.657) and lower (ρ = 0.683) decrease of SUPRA and BIC CV, respectively. Male's greater increase in SUPRA thickness correlated to greater increase in UT thickness and tissue oxygenation index, and to lower increase of UT deoxyhemoglobin. Females' greater decrease of SUPRA MdPF correlated to greater decrease of UT MdPF, while greater UT activation increase was related to lower UT thickness increase. Results suggest that despite comparable time to fatigue-terminal, males have greater force-generating capacity and neuromuscular reliance on recruitment and excitation rates, while females have greater reliance on activation variability. Further, there are relationships between hemodynamic and swelling patterns in males, while there are relationships between activation and swelling patterns in females. Although there were no differences in experimental task-induced changes, there are sex-specific relationships between muscular patterns and perceived fatigability, which may help explain sex-specific mechanisms of musculoskeletal disorders.

Spinal movement variability associated with low back pain: A scoping review

Objective

To identify suggestions for future research on spinal movement variability (SMV) in individuals with low back pain (LBP) by investigating (1) the methodologies and statistical tools used to assess SMV; (2) characteristics that influence the direction of change in SMV; (3) the methodological quality and potential biases in the published studies; and (4) strategies for optimizing SMV in LBP patients.

Methods

We searched literature databases (CENTRAL, Medline, PubMed, Embase, and CINAHL) and comprehensively reviewed the relevant papers up to 5 May 2020. Eligibility criteria included studies investigating SMV in LBP subjects by measuring trunk angle using motion capture devices during voluntary repeated trunk movements in any plane. The Newcastle-Ottawa risk of bias tool was used for data quality assessment. Results were reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews.

Results

Eighteen studies were included: 14 cross-sectional and 4 prospective studies. Seven linear and non-linear statistical tools were used. Common movement tasks included trunk forward bending and backward return, and object lifting. Study results on SMV changes associated with LBP were inconsistent. Two of the three interventional studies reported changes in SMV, one of which was a randomized controlled trial (RCT) involving neuromuscular exercise interventions. Many studies did not account for the potential risk of selection bias in the LBP population.

Conclusion

Designers of future studies should recognize that each of the two types of statistical tools assesses functionally different aspects of SMV. Future studies should also consider dividing participants into subgroups according to LBP characteristics, as three potential subgroups with different SMV characteristics were proposed in our study. Different task demands also produced different effects. We found preliminary evidence in a RCT that neuromuscular exercises could modify SMV, suggesting a rationale for well-designed RCTs involving neuromuscular exercise interventions in future studies.

Relative variability in muscle activation amplitude, muscle oxygenation, and muscle thickness: Changes with dynamic low-load elbow flexion fatigue and relationships in young and older females

In repetitive upper limb activities, variability of muscle activity (a feature of motor variability) is linked to upper limb fatigability. Prior studies suggest that the variability response may change with age and could be related to the oxygen saturation of hemoglobin within the muscle. We determined, in female adults, how age affects adjustments in movement-to-movement variability of electromyograph (EMG) amplitude (RMS), oxygenation, and thickness with fatigue, and explored how these responses were related. Fifteen young (23.3 ± 3.1 years) and ten older (62.8 ± 6.9 years) females completed repeated trials of low-load, isokinetic, concentric/eccentric elbow flexion until maximal torque ≤ 70% of baseline. Movement-to-movement variability of EMG RMS in concentric phases of movement was quantified by the coefficient of variation (EMG CV), and muscle oxygenation and thickness (MTH) were quantified using near-infrared spectroscopy, and B-mode ultrasonography. Age*Time and Spearman ρ analyses were conducted. Age did not affect fatigability or Time-related changes in muscular measures (p > 0.05). Biceps brachii and brachialis EMG CV decreased, biceps brachii HbO₂ decreased and did not fully recover, and biceps brachii and brachialis MTH increased. Higher initial brachialis EMG CV was related to less blunted oxygenation in young females (p = 0.021). Oxygenation responses were related to altered anterior deltoid EMG CV in young females but altered biceps brachii and brachialis EMG CV in older females. Age was not associated with changes in EMG CV, oxygenation, or thickness at similar performance fatigability in the concentric/eccentric elbow flexion task studied. Adjustments in biceps brachii oxygenation were linked to changes in EMG CV more local to the site of fatigue with older age.

Is sex a proxy for mechanical variables during an upper limb repetitive movement task? An investigation of the effects of sex and of anthropometric load on muscle fatigue

BACKGROUND

Women report more work-related pain and neck/shoulder musculoskeletal disorders than men. For the same absolute workload, due to lower strength, females generally work at a higher relative intensity, which could induce more fatigue. However, the arm's anthropometric load (AL) of men is higher. Therefore, simply lifting their arm could be more fatiguing. Sex as a variable is formed of many constructs, and analyses can become muddied by their differing responses to fatigue. No studies have considered AL, when comparing how fatigue affects men and women. The purpose was to determine if including the arm's AL in the statistical analysis would impact findings of sex-specific effects of shoulder fatigue on muscle EMG.

METHODS

Fifty-five (29m/26f) participants completed a repetitive pointing task (RPT) at shoulder height until they reported fatigue of 8+ on the BORG CR-10 scale. Muscle activities were measured using surface electrodes placed over the anterior deltoid (AD) and upper trapezius (UT) muscles. Muscle activity amplitude was quantified using root mean square (RMS). First- and last-minute data were used to assess change from no-fatigue (NF) to fatigue-terminal (FT) conditions. AL was calculated using sex-specific body parameter equations. General estimating equations (GEE) were used to determine the effects of sex and fatigue on RMS values, while including AL in the GEE.

RESULTS

There was no sex difference in time to reach fatigue. A significant main effect of sex on RMS was observed (χ²(1) = 4.17, p = 0.04) when including AL as a covariate. Females displayed a significantly higher percentage change in AD RMS from NF to FT, compared to males (p = 0.03), when AL was included in the GEE. No sex differences in UT were observed.

CONCLUSIONS

This sex difference emerged when AL was included as a covariate, suggesting that sex-associated anthropometric differences may contribute to sex differences in the fatigue response. Differences in the impact of AL on AD compared to UT could be explained by differences in their respective mechanical roles or muscle fiber content. Anthropometrics may be useful to include as covariates in future research to separate individual anthropometric differences from sex differences.

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.

Age-Dependent Control of Shoulder Muscles During a Reach-and-Lift Task

Aging affects fatigability and is a risk factor for incurring a fatigue-related injury in the neck/shoulder region. Age-related changes in the electromyographical features of motor control may be partly responsible. Young (N = 17) and older (N = 13) adults completed a reach-and-lift task at their self-selected speed, before and after a fatiguing task targeting the neck/shoulder. Electromyography amplitude (root mean square), amplitude variability (root mean square coefficient of variation [CV]), functional connectivity (normalized mutual information [NMI]), and functional connectivity variability (NMI CV) were extracted from several muscles and analyzed for effects and interactions of age using general estimating equation models. Root mean square CV and deltoid NMI CV increased from pre- to postfatigue (ps < .05). Upper trapezius-deltoid NMI decreased for young, but increased for older adults, while the opposite response was found for lower trapezius-deltoid NMI (ps < .05). Older adults seem to adapt to fatigue in reach-and-lift movement with a cranial shift in control of the scapula.

Does sEMG normalization change results on sex differences in the activation of the shoulder girdle muscles during a simulated work task?

The aim was to investigate if the method of normalizing surface electromyography (sEMG) can change results on sex differences in the muscular activation of the shoulder girdle muscles during a simulated work task. sEMG was recorded in 36 asymptomatic participants (17 females, 19 males) from four parts of trapezius and from serratus anterior muscles during a simulated work task. Four normalization methods, one involving maximal voluntary contractions (MVCs) and three involving submaximal voluntary contractions were applied. Sex differences in absolute and normalized amplitude of sEMG were analyzed. The normalization method had a significant influence on the observed sex differences. Females only showed higher sEMG amplitude than males when the sEMGs were normalized to MVC and to a submaximal contraction based on 20% of MVC for the upper trapezius (acromial fibers). Researchers and practitioners should be aware of the impact of the sEMG normalization method in sex differences investigation.

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.

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.

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.

Are there sex differences in muscle coordination of the upper girdle during a sustained motor task?

PURPOSE

The higher prevalence of work-related musculoskeletal disorders among women compared with men could be explained by sex-gender differences related to biological and physiological processes. The aim of this study was to evaluate sex differences in motor coordination during a sustained and repetitive motor task.

METHODS

Seventeen healthy females and 21 healthy males participated. The surface electromyography (sEMG) of the trapezius portions and serratus anterior were recorded. Root mean square (RMS) values were computed to assess the level of muscle activity. The standard deviation (SD) and coefficient of variation (CV) were computed as metrics of size of variability. The normalized mutual information (NMI) values were calculated as index of functional connectivity between muscles pairs.

RESULTS

Females had higher normalized RMS values for the upper trapezius (acromial fibers) and serratus anterior muscles compared with males. RMS decreased, SD and CV increased while NMI decreased for almost all muscle pairs over time.

CONCLUSION

The present work showed some signs of sex differences in muscle coordination of the shoulder girdle during a sustained motor task, performed with the upper limb positioned below of the shoulder level.

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.

Influence of remote pain on movement control and muscle endurance during repetitive movements

During fatiguing tasks, people adapt their movement strategies to offset effects of muscle fatigue. Painful stimuli may compete for cognitive resources during this process, impairing fatigue adaptation. This study determined how pain affected movement control and muscle endurance during a repetitive task and how pain catastrophizing moderated these effects. Twenty-two healthy young adults performed timed reaching movements until voluntary exhaustion on two separate days. On 1 day, subjects simultaneously experienced ischemic pain in the contralateral arm. Subjective pain, and effort were recorded at regular intervals. Timing errors, distance and speed were calculated for each movement. Detrended fluctuation analysis was used to quantify temporal persistence in each time series. Subjects made shorter, slower movements during the last compared to the first minute of fatigue on both days (p < 0.001). Deviations in movement speed were corrected faster in the no pain condition compared to the pain condition (p = 0.042), but only early during the condition. Time to fatigue was influenced by pain and the order of testing. Subjects performed the task longer on the second day whether the condition was pain or no pain. This effect was larger when the pain condition was first (3.4 compared to 1.1 min. increase). Subjects with high and low pain catastrophizing responded similarly to the painful stimuli. The results suggest that pain causes people to adopt more conservative movement strategies which can affect the fatigue rate, but these effects depend on familiarity with the painful stimulus and the fatiguing task.

Effects of a cognitive dual task on variability and local dynamic stability in sustained repetitive arm movements using principal component analysis: a pilot study

In many daily jobs, repetitive arm movements are performed for extended periods of time under continuous cognitive demands. Even highly monotonous tasks exhibit an inherent motor variability and subtle fluctuations in movement stability. Variability and stability are different aspects of system dynamics, whose magnitude may be further affected by a cognitive load. Thus, the aim of the study was to explore and compare the effects of a cognitive dual task on the variability and local dynamic stability in a repetitive bimanual task. Thirteen healthy volunteers performed the repetitive motor task with and without a concurrent cognitive task of counting aloud backwards in multiples of three. Upper-body 3D kinematics were collected and postural reconfigurations-the variability related to the volunteer's postural change-were determined through a principal component analysis-based procedure. Subsequently, the most salient component was selected for the analysis of (1) cycle-to-cycle spatial and temporal variability, and (2) local dynamic stability as reflected by the largest Lyapunov exponent. Finally, end-point variability was evaluated as a control measure. The dual cognitive task proved to increase the temporal variability and reduce the local dynamic stability, marginally decrease endpoint variability, and substantially lower the incidence of postural reconfigurations. Particularly, the latter effect is considered to be relevant for the prevention of work-related musculoskeletal disorders since reduced variability in sustained repetitive tasks might increase the risk of overuse injuries.

Postural reconfiguration and cycle-to-cycle variability in patients with work-related musculoskeletal disorders compared to healthy controls and in relation to pain emerging during a repetitive movement task

BACKGROUND

Movement variability in sustained repetitive tasks is an important factor in the context of work-related musculoskeletal disorders. While a popular hypothesis suggests that movement variability can prevent overuse injuries, pain evolving during task execution may also cause variability. The aim of the current study was to investigate, first, differences in movement behavior between volunteers with and without work-related pain and, second, the influence of emerging pain on movement variability.

METHODS

Upper-body 3D kinematics were collected as 22 subjects with musculoskeletal disorders and 19 healthy volunteers performed a bimanual repetitive tapping task with a self-chosen and a given rhythm. Three subgroups were formed within the patient group according to the level of pain the participants experienced during the task. Principal component analysis was applied to 30 joint angle coordinates to characterize in a combined analysis the movement variability associated with reconfigurations of the volunteers' postures and the cycle-to-cycle variability that occurred during the execution of the task.

FINDINGS

Patients with no task-related pain showed lower cycle-to-cycle variability compared to healthy controls. Findings also indicated an increase in movement variability as pain emerged, manifesting both as frequent postural changes and large cycle-to-cycle variability.

INTERPRETATION

The findings suggested a relationship between work-related musculoskeletal disorders and movement variability but further investigation is needed on this issue. Additionally, the findings provided clear evidence that pain increased motor variability. Postural reconfigurations and cycle-to-cycle variability should be considered jointly when investigating movement variability and musculoskeletal disorders.

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.

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.

Effects of Dual Monitor Computer Work Versus Laptop Work on Cervical Muscular and Proprioceptive Characteristics of Males and Females

OBJECTIVE

The effects of performing a 90-minute computer task with a laptop versus a dual monitor desktop workstation were investigated in healthy young male and female adults.

BACKGROUND

Work-related musculoskeletal disorders are common among computer (especially female) users. Laptops have surpassed desktop computer sales, and working with multiple monitors has also become popular. However, few studies have provided objective evidence on how they affect the musculoskeletal system in both genders.

METHODS

Twenty-seven healthy participants (mean age = 24.6 years; 13 males) completed a 90-minute computer task while using a laptop or dual monitor (DualMon) desktop. Electromyography (EMG) from eight upper body muscles and visual strain were measured throughout the task. Neck proprioception was tested before and after the computer task using a head-repositioning test. EMG amplitude (root mean square [RMS]), variability (coefficients of variation [CV]), and normalized mutual information (NMI) were computed.

RESULTS

Visual strain ( p < .01) and right upper trapezius RMS ( p = .03) increased significantly over time regardless of workstation. Right cervical erector spinae RMS and cervical NMI were smaller, while degrees of overshoot (mean = 4.15°) and end position error (mean = 1.26°) were larger in DualMon regardless of time. Effects on muscle activity were more pronounced in males, whereas effects on proprioception were more pronounced in females.

CONCLUSION

Results suggest that compared to laptop, DualMon work is effective in reducing cervical muscle activity, dissociating cervical connectivity, and maintaining more typical neck repositioning patterns, suggesting some health-protective effects.

APPLICATION

This evidence could be considered when deciding on computer workstation designs.

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.

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.

Muscle Activity Adaptations to Spinal Tissue Creep in the Presence of Muscle Fatigue

Aim

The aim of this study was to identify adaptations in muscle activity distribution to spinal tissue creep in presence of muscle fatigue.

Methods

Twenty-three healthy participants performed a fatigue task before and after 30 minutes of passive spinal tissue deformation in flexion. Right and left erector spinae activity was recorded using large-arrays surface electromyography (EMG). To characterize muscle activity distribution, dispersion was used. During the fatigue task, EMG amplitude root mean square (RMS), median frequency and dispersion in x- and y-axis were compared before and after spinal creep.

Results

Important fatigue-related changes in EMG median frequency were observed during muscle fatigue. Median frequency values showed a significant main creep effect, with lower median frequency values on the left side under the creep condition (p≤0.0001). A significant main creep effect on RMS values was also observed as RMS values were higher after creep deformation on the right side (p = 0.014); a similar tendency, although not significant, was observed on the left side (p = 0.06). A significant creep effects for x-axis dispersion values was observed, with higher dispersion values following the deformation protocol on the left side (p≤0.001). Regarding y-axis dispersion values, a significant creep x fatigue interaction effect was observed on the left side (p = 0.016); a similar tendency, although not significant, was observed on the right side (p = 0.08).

Conclusion

Combined muscle fatigue and creep deformation of spinal tissues led to changes in muscle activity amplitude, frequency domain and distribution.

Test-retest reliability of trunk motor variability measured by large-array surface electromyography

OBJECTIVE

The objective of this study was to evaluate the test-retest reliability of the trunk muscle activity distribution in asymptomatic participants during muscle fatigue using large-array surface electromyography (EMG).

METHODS

Trunk muscle activity distribution was evaluated twice, with 3 to 4 days between them, in 27 asymptomatic volunteers using large-array surface EMG. Motor variability, assessed with 2 different variables (the centroid coordinates of the root mean square map and the dispersion variable), was evaluated during a low back muscle fatigue task. Test-retest reliability of muscle activity distribution was obtained using Pearson correlation coefficients.

RESULTS

A shift in the distribution of EMG amplitude toward the lateral-caudal region of the lumbar erector spinae induced by muscle fatigue was observed. Moderate to very strong correlations were found between both sessions in the last 3 phases of the fatigue task for both motor variability variables, whereas weak to moderate correlations were found in the first phases of the fatigue task only for the dispersion variable.

CONCLUSION

These findings show that, in asymptomatic participants, patterns of EMG activity are less reliable in initial stages of muscle fatigue, whereas later stages are characterized by highly reliable patterns of EMG activity.

Local Muscle Fatigue and 3D Kinematics of the Cervical Spine in Healthy Subjects

The authors aimed to further explore the effects of local muscle fatigue on cervical 3D kinematics and the interrelationship between these kinematic characteristics and local muscle endurance capacity in the unimpaired cervical spine. Twenty healthy subjects (38 ± 10 years; 5 women) performed 2 × 10 maximal cervical flexion-extension movements. Isometric muscle endurance tests (prone/supine lying) were applied between sets to induce local muscle fatigue quantified by Borg scale rates of perceived exertion (RPE) and slope in mean power frequency (MPF; surface electromyography; m. sternocleidomastoideus, m. splenius capitis). Cervical motion characteristics (maximal range of motion [ROM], coefficient of variation of the 10 repetitive movements, mean angular velocity, conjunct movements in transversal and frontal plane) were calculated from raw 3D ultrasonic movement data. Average isometric strength testing duration for flexion and extension correlated to the cervical ROM (r = .49/r = .48; p < .05). However, Student's t test demonstrated no significant alterations in any kinematic parameter following local muscle fatigue (p > .05). Although subjects' cervical muscle endurance capacity and motor output seems to be conjugated, no impact of local cervical muscle fatigue on motor function was shown. These findings underline the importance of complementary measures to address muscular performance and kinematic characteristics in outcome assessment and functional rehabilitation of the cervical spine.

Adaptations to isolated shoulder fatigue during simulated repetitive work. Part I: Fatigue

Upper extremity muscle fatigue is challenging to identify during industrial tasks and places changing demands on the shoulder complex that are not fully understood. The purpose of this investigation was to examine adaptation strategies in response to isolated anterior deltoid muscle fatigue while performing simulated repetitive work. Participants completed two blocks of simulated repetitive work separated by an anterior deltoid fatigue protocol; the first block had 20 work cycles and the post-fatigue block had 60 cycles. Each work cycle was 60s in duration and included 4 tasks: handle pull, cap rotation, drill press and handle push. Surface EMG of 14 muscles and upper body kinematics were recorded. Immediately following fatigue, glenohumeral flexion strength was reduced, rating of perceived exertion scores increased and signs of muscle fatigue (increased EMG amplitude, decreased EMG frequency) were present in anterior and posterior deltoids, latissimus dorsi and serratus anterior. Along with other kinematic and muscle activity changes, scapular reorientation occurred in all of the simulated tasks and generally served to increase the width of the subacromial space. These findings suggest that immediately following fatigue people adapt by repositioning joints to maintain task performance and may also prioritize maintaining subacromial space width.

Adaptations to isolated shoulder fatigue during simulated repetitive work. Part II: Recovery

The shoulder allows kinematic and muscular changes to facilitate continued task performance during prolonged repetitive work. The purpose of this work was to examine changes during simulated repetitive work in response to a fatigue protocol. Participants performed 20 one-minute work cycles comprised of 4 shoulder centric tasks, a fatigue protocol, followed by 60 additional cycles. The fatigue protocol targeted the anterior deltoid and cycled between static and dynamic actions. EMG was collected from 14 upper extremity and back muscles and three-dimensional motion was captured during each work cycle. Participants completed post-fatigue work despite EMG manifestations of muscle fatigue, reduced flexion strength (by 28%), and increased perceived exertion (∼3 times). Throughout the post-fatigue work cycles, participants maintained performance via kinematic and muscular adaptations, such as reduced glenohumeral flexion and scapular rotation which were task specific and varied throughout the hour of simulated work. By the end of 60 post-fatigue work cycles, signs of fatigue persisted in the anterior deltoid and developed in the middle deltoid, yet perceived exertion and strength returned to pre-fatigue levels. Recovery from fatigue elicits changes in muscle activity and movement patterns that may not be perceived by the worker which has important implications for injury risk.