The Effect of Cycling While Typing on Patterns of Upper Body Muscle Activation.

OBJECTIVE: To investigate the effect of one's sex and pedaling intensity on upper body muscle activation patterns during typing while cycling. BACKGROUND: Females are at a higher risk for computer work-related musculoskeletal disorders, and mobile workstations have been suggested to induce healthier muscular patterns compared with sitting. However, the neuromuscular characteristics of performing computer work in a cycling workstation have not been investigated. METHOD: Twenty-two participants (10 females) completed a 60-min computer typing task while pedaling on a cycle ergometer at two different intensities (25%, 4% heart rate reserve). Surface electromyography (EMG) was recorded from seven muscles of the upper body. Effects of time, sex, and intensity were assessed for muscle activation (RMS), activation variability (CV), and normalized mutual information (NMI) via generalized estimating equations. RESULTS: With time, neck/shoulder CV increased in males during higher pedaling intensity, whereas in females it decreased during lower intensity. In females, RMS of neck/shoulder and NMI of neck/shoulder muscle pairs were lower, whereas forearm RMS was 34.2% higher with higher intensity cycling compared with lower intensity. Lower back RMS decreased 28.3% in the initial half of the task in females, but in males it increased 14.4% in the later half. CONCLUSION AND APPLICATION: Cycling workstation showed a sex- and intensity-specific EMG response. These differing responses should be considered when implementing the use of cycling workstation and may be important for preventing/managing sex-specific muscle fatigue and musculoskeletal disorders.

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.

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.

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.

Characterization and visualization of the liposecretion process taking place during ceiling culture of human mature adipocytes.

OBJECTIVE: To investigate and further characterize the process of mature adipocyte dedifferentiation. Our hypothesis was that dedifferentiation does not involve mitosis but rather a phenomenon of liposecretion. METHODS: Mature adipocytes were isolated by collagenase digestion of human adipose tissue samples. Ceiling cultures were established using our six-well plate model. Cells were treated with cytosine ß-d-arabinofuranoside (AraC) or vincristine (VCR), two agents blocking cell division, and were compared with vehicle. Liposecretion events were visualized by time-lapse microscopy, with and without AraC in adipocytes transducted with a baculovirus. Microscopic analyses were performed after labeling phosphorylated histone 3 and cyclin B1 in ceiling cultures. RESULTS: Treatment with AraC almost entirely prevented the formation of fibroblasts up to 12 days of ceiling culture. Similar results were obtained with VCR. The antimitotic effectiveness of the treatment was confirmed in fibroblast cultures from the adipose tissue stromal-vascular fraction by proliferation assays and colony-forming unit experiments. Using time-lapse microscopy, we visualized liposecretion events in which a large lipid droplet was rapidly secreted from isolated mature adipocytes. The same phenomenon was observed with AraC. This was observed in conjunction with histone 3 phosphorylation and cyclin B1 segregation to the nucleus. CONCLUSION: Our results support the notion that dedifferentiation involves rapid secretion of the lipid droplet by the adipocytes with concomitant generation of fibroblast-like cells that subsequently proliferate to generate the dedifferentiated adipocyte population during ceiling culture. The presence of mitotic markers suggests that this process involves cell cycle progression, although cell division does not occur.

Focus on dedifferentiated adipocytes: characteristics, mechanisms, and possible applications.

It is largely believed that after undergoing differentiation, adipocytes can no longer divide. Yet, using ceiling culture, it was demonstrated in vitro that some adipocytes are able to regain proliferative abilities by becoming fibroblast-like cells called dedifferentiated adipocytes. Mature adipocytes are abundant, can be easily isolated, and represent a homogenous cell population. Because of these advantageous characteristics, dedifferentiated adipocytes are clinically attractive in tissue engineering as a potential treatment resource for conditions such as type 2 diabetes, cardiac and kidney diseases, as well as autoimmune diseases. The aim of this review article is to summarize current knowledge on adipocyte dedifferentiation by accurately describing dedifferentiated adipocyte characteristics such as morphological appearance, gene expression, antigen signature, pluripotency, and functionality. Current hypotheses possibly explaining the biological mechanisms and cellular reprogramming of the dedifferentiation process are summarized. Dedifferentiated adipocytes show a stem cell-like antigen profile and genome signature which add to their proliferative capacities and their ability to re-differentiate into diverse cell lineages. The dedifferentiation process likely involves liposecretion, that is, the rapid secretion of the cell's lipid droplet. Dedifferentiated adipocytes may allow development of new uses in tissue engineering.

Sex differences in glenohumeral muscle activation and coactivation during a box lifting task.

Manual material handling is associated with shoulder musculoskeletal disorders, especially for women. Sex differences in glenohumeral muscle activity may contribute to women's higher injury risk by affecting shoulder load and stability. We assessed the effects of sex (25 women vs 26 men) and lifting load (6 kg vs 12 kg) on muscle activation during box lifting from hip to eye level. Surface and intramuscular electromyography were recorded from 10 glenohumeral muscles. Most muscles were more activated for the heavier box and for women. These effects were larger for 'prime movers' than for stabilisers and antagonists. Despite their apparently heterogeneous effects on muscle activity, sex and mass did not affect Muscle Focus, a metric of coactivation. This may be partly related to the limited sensitivity of the Muscle Focus. Nevertheless, sex differences in strength, more than in coactivation patterns, may contribute to the sex imbalance in the prevalence of musculoskeletal disorders. Practitioner summary: We studied sex differences in glenohumeral muscle activity in a lifting task to eye level. Women lifting a 6-kg box activated their muscles similarly to men lifting a 12-kg box, i.e. up to 48% of their maximum capacity. Interventions minimising shoulder load should be implemented, especially for women. Abbreviations: BB: biceps brachii; DeltA: anterior deltoid; DeltL: lateral deltoid; DeltP: posterior deltoid; DoF: degrees of freedom; ED: effect duration; EMG: electromyography; ES: effect size; Infra: infraspinatus; Lat: latissimus dorsi; MF: muscle focus; MMH: manual material handling; MVA: maximal voluntary activation; Pect: pectoralis major; Subscap: subscapularis; Supra: supraspinatus; TB: triceps brachii.

Sex differences in upper limb 3D joint contributions during a lifting task.

Sex-related differences in work technique may contribute to increasing the risk of musculoskeletal joint disorders among women. In lifting tasks, sex differences have been reported for the trunk and lower limb, although women present a higher prevalence of shoulder disorders. We investigated sex differences in the upper limb technique during a lifting task. Trunk and upper limb kinematics were recorded in 27 women and 27 men lifting a box (6 or 12 kg) from hip to eye level. Work technique was quantified through the three-dimensional contribution of each joint to overall box height. The glenohumeral joint showed a higher contribution in women with a 6 kg box and wrist and elbow joints did with a 12 kg box, compared to men at either 6 or 12 kg. Sex differences occurred systematically above shoulder level. Our results argue for careful consideration of sex during ergonomic intervention, particularly during the overhead task. Practitioner Summary: We investigated the sex-related differences in upper limb technique during lifting tasks. Results highlight a sex-specific kinematic strategy above the shoulder level on the glenohumeral joint and on the wrist and elbow joints. To help reduce women's shoulder disorders in overhead task, ergonomic interventions should account for those differences. Abbreviations: DoF: degree-of-freedom; WR/EL: wrist and elbow; GH: glenohumeral; SC/AC: sternoclavicular and acromioclavicular; TR/PE: pelvo-thoracic.

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.

Gender differences in fatigability and muscle activity responses to a short-cycle repetitive task.

PURPOSE: Epidemiological research has identified women to be more susceptible to developing neck-shoulder musculoskeletal disorders when performing low-force, repetitive work tasks. Whether this is attributable to gender differences in fatigability and motor control is currently unclear. This study investigated the extent to which women differ from men in fatigability and motor control while performing a short-cycle repetitive task. METHODS: 113 healthy young adults (58 women, 55 men) performed a standardized repetitive pointing task. The task was terminated when the subject's perceived exertion reached 8 on the Borg scale. The time to task termination, and changes in means and cycle-to-cycle variabilities of surface electromyography signals from start to end of the task, were compared between women and men, for the upper trapezius, anterior deltoid, biceps and triceps muscles. RESULTS: Women and men terminated the task after 6.5 (SD 3.75) and 7 (SD 4) min on average (p > 0.05). All four muscles showed an increase of 25-35 % in average muscle activity with fatigue (no significant sex differences). However, men exhibited a higher increase than women in trapezius muscle variability with fatigue (31 vs. 7 %; p < 0.05), and a decrease in biceps muscle variability where women had an increase (-23 vs. 12 %; p < 0.05). CONCLUSIONS: Our results suggest that women and men may not differ in the ability to perform repetitive tasks at low-to-moderate force levels. However, differences in motor control strategies employed in task performance may explain gender differences in susceptibility to developing musculoskeletal disorders when performing repetitive work for prolonged periods in occupational life.

Interrater and Intrarater Reliability and Validity of 3 Measurement Methods for Shoulder-Position Sense.

CONTEXT: Joint-position sense (JPS) plays a critical role in the stability of shoulder joint. Restoration of JPS is essential to improve rehabilitation outcomes in individuals with shoulder injury. However, the number of affordable and reliable shoulder-JPS measurement methods for everyday clinical practice is limited. OBJECTIVE: To estimate reliability and validity of 3 simple shoulder-JPS measurement methods. DESIGN: Cross-sectional study. PARTICIPANTS: 25 healthy men and women. MAIN OUTCOME MEASURE: Absolute-error scores of JPS in 3 ranges of shoulder flexion (low, mid, and high), measured with a laser pointer, an inclinometer, and a goniometer in 2 separate sessions (48 h apart). RESULTS: Overall interrater and intrarater intraclass correlation coefficients were .86 and .78 for the laser pointer, .67 and .70 for the inclinometer, and .60 and .50 for the goniometer, respectively. There was excellent reliability in the low range for the laser pointer and inclinometer methods, but fair to good and poor reliability in mid- and high ranges, respectively. All methods showed strong validity. CONCLUSION: The laser pointer and inclinometer JPS measurement methods are reliable and can be used by clinicians during rehabilitation of shoulder injuries.

First steps for integrating sex and gender considerations into basic experimental biomedical research.

In recent decades there has been an increasing recognition of the need to account for sex and gender in biology and medicine, in order to develop a more comprehensive understanding of biological phenomena and to address gaps in medical knowledge that have arisen due to a generally masculine bias in research. We have noted that as basic experimental biomedical researchers, we face unique challenges to the incorporation of sex and gender in our work, and that these have remained largely unarticulated, misunderstood, and unaddressed in the literature. Here, we describe some of the specific challenges to the incorporation of sex and gender considerations in research involving cell cultures and laboratory animals. In our view, the mainstreaming of sex and gender considerations in basic biomedical research depends on an approach that will allow scientists to address these issues in ways that do not undermine our ability to pursue our fundamental scientific interests. To that end, we suggest a number of strategies that allow basic experimental researchers to feasibly and meaningfully take sex and gender into account in their work.

The effect of walking while typing on neck/shoulder patterns.

PURPOSE: This project aimed to quantify the effects of modifying computer work posture on neck/shoulder patterns during a prolonged typing task. METHOD: Twenty healthy participants completed a 90-min typing task while sitting or walking on a treadmill. Electromyography (EMG) was recorded from eight upper body muscles and laser Doppler flowmetry (LDF) from two upper body sites. Effects of Time and Posture were assessed for EMG amplitude (RMS), variability (CoV), normalized mutual information (NMI), LDF and upper limb discomfort. RESULTS: Upper limb discomfort was higher during sitting and increased with time, from 0.86 ± 1.3 to 3.7 ± 3.1 out of 10. Interaction effects showed that EMG amplitude decreased over time for the lumbar erector spinae (LES) (from 6.3 ± 2.9 to 5.6 ± 3.2 % MIVC) and wrist extensor (from 12.4 ± 2.7 to 11.3 ± 3.5 % MIVC) during walking, but increased during sitting. Anterior Deltoid EMG amplitude was 64 % lower during walking while External Oblique EMG amplitude (43 %) and Lower Trapezius EMG variability (65 %) were higher during walking. Interaction effects showed higher LES CoV during walking compared to sitting (p = 0.019) in the beginning but not at the end of the task, and higher neck/shoulder NMI (p = 0.050) towards the end of the task during sitting compared to walking. CONCLUSION: Results suggest that walking while performing computer work may be effective in inducing healthier muscular patterns, possibly explaining the lower level of discomfort compared to sitting.

Sex differences in the shoulder joint position sense acuity: a cross-sectional study.

BACKGROUND: Work-related musculoskeletal disorders (WMSD) is the most expensive form of work disability. Female sex has been considered as an individual risk factor for the development of WMSD, specifically in the neck and shoulder region. One of the factors that might contribute to the higher injury rate in women is possible differences in neuromuscular control. Accordingly the purpose of this study was to estimate the effect of sex on shoulder joint position sense acuity (as a part of shoulder neuromuscular control) in healthy individuals. METHODS: Twenty-eight healthy participants, 14 females and 14 males were recruited for this study. To test position sense acuity, subjects were asked to flex their dominant shoulder to one of the three pre-defined angle ranges (low, mid and high-ranges) with eyes closed, hold their arm in that position for three seconds, go back to the starting position and then immediately replicate the same joint flexion angle, while the difference between the reproduced and original angle was taken as the measure of position sense error. The errors were measured using Vicon motion capture system. Subjects reproduced nine positions in total (3 ranges × 3 trials each). RESULTS: Calculation of absolute repositioning error (magnitude of error) showed no significant difference between men and women (p-value ≥ 0.05). However, the analysis of the direction of error (constant error) showed a significant difference between the sexes, as women tended to mostly overestimate the target, whereas men tended to both overestimate and underestimate the target (p-value ≤ 0.01, observed power = 0.79). The results also showed that men had a significantly more variable error, indicating more variability in their position sense, compared to women (p-value ≤ 0.05, observed power = 0.78). DISCUSSION: Differences observed in the constant JPS error suggest that men and women might use different neuromuscular control strategies in the upper limb. In addition, higher JPS variability observed in men might be one of the factors that could contribute to their lower rate of musculoskeletal disorders, compared to women. CONCLUSIONS: The result of this study showed that shoulder position sense, as part of the neuromuscular control system, differs between men and women. This finding can help us better understand the reasons behind the higher rate of musculoskeletal disorders in women, especially in the working environments.

Is sitting worse than static standing? How a gender analysis can move us toward understanding determinants and effects of occupational standing and walking.

The Yant Award was established in 1964 to honor the contributions of William P. Yant, the first president of the American Industrial Hygiene Association. It is presented annually for outstanding contributions in industrial hygiene or allied fields to an individual residing outside the United States. The 2014 award recipient is Dr. Karen Messing, Professor emeritus, Department of Biological Sciences, Université du Québec à Montréal and Researcher, CINBIOSE Research Centre. Gender (socially determined) differences in occupations, employment, and working conditions, task assignments, and work methods that affect exposure to health risks are increasingly documented. Interactions of (biologically influenced) sex differences with workplace parameters may also influence exposure levels. During field studies, ergonomists learn a lot about gender and sex that can be important when generating and testing hypotheses about the mechanisms that link workplace exposures to health outcomes. Prolonged standing is common in North America; almost half (45%) of Québec workers spend more than three-quarters of their working time on their feet and 40% of these cannot sit at will. This posture has been linked to chronic back pain and musculoskeletal disorders (MSDs) in the lower limbs, but many health professionals suggest workers should stand rather than sit at work. We ask: (1) Given the fact that roughly the same proportion of men and women stand at work, what does a gender-sensitive analysis add to our ability to detect and thus prevent work-related MSDs?; (2) How does ergonomics research inform gender-sensitive analysis of occupational health data?; and (3) What do researchers need to know to orient interventions to improve general working postures? We have sought answers to these questions through collaborative research with specialists in epidemiology, occupational medicine, biomechanics, and physiology, carried out in partnership with public health organisations, community groups, and unions. We conclude that failure to characterize prolonged static standing and to apply gender-sensitive analysis can confuse assessment of musculoskeletal and circulatory effects of working postures. We suggest that prolonged static sitting and standing postures can and should be avoided by changes to workplace organization and environments. Research is needed to define optimal walking speeds and arrive at optimal ratios of sitting, standing, and walking in the workplace.

Dietary intakes in the nutritional management of gestational diabetes mellitus.

PURPOSE: Changes were examined in energy intakes and percentage of energy from macronutrients in response to nutritional intervention in women with gestational diabetes mellitus (GDM). METHODS: The study included 17 women with GDM and 27 women with normal glucose tolerance (controls). Women with GDM were followed by a multidisciplinary team; they received dietary counselling by a registered dietitian, and were prescribed diets with 40% to 45% energy from carbohydrate (CHO), 20% to 25% from protein, and 30% to 35% from fat. Dietary intakes were assessed with food frequency questionnaires before the intervention (26.9 ± 3.8 weeks) and after the intervention (32.6 ± 0.6 weeks). RESULTS: After the intervention, women with GDM reduced their total energy intake to reach lower values than did controls (P value for time-group interaction =0.05). A concomitant reduction in total CHO and glucose intakes in women with GDM led to significantly lower values compared with intakes in controls (P values for time-group interaction =0.001 for all). The post-intervention rate of weight gain in women with GDM was within the Institute of Medicine (IOM)-recommended values, while the post-intervention rate of weight gain in controls was above IOM-recommended values (0.30 ± 0.27 versus 0.61 ± 0.50 kg/week, P≤0.05). CONCLUSIONS: These results suggest that this multidisciplinary medical and nutritional intervention was effective in the achievement of prescribed macronutrient distribution and controlling gestational weight gain in Canadian women with GDM.

Restraint and seclusion: the perspective of service users and staff members.

BACKGROUND: Restrictive measures may have important physical and psychological consequences on all persons involved. The current study examined how these are perceived by persons with intellectual disabilities and staff. MATERIALS AND METHODS: Interviews were conducted with eight persons with intellectual disabilities who experienced a restrictive measure and their care providers. They were queried on their understanding of the restrictive measure, its impact on the relationship, their emotions and alternative interventions. RESULTS: Restrictive measures were experienced negatively by persons with intellectual disabilities and their care providers. Service users reported feeling sad and angry, whereas staff mentioned feeling anxious. Moreover, persons with intellectual disabilities appeared to understand the goal of restrictive measures (e.g. ensuring their own and others' safety) and identified alternative interventions (e.g. speaking with a staff member or taking a walk). CONCLUSION: This study sheds further light on how persons with intellectual disabilities and staff experience the application of restrictive measures. Debriefing sessions with service users and staff may help minimize negative consequences.

Females show less decline in contractile function than males after repeated all-out cycling.

Females demonstrate greater fatigue resistance during a range of exercise modalities; however, this may be confounded by the lower mechanical work completed. Accordingly, this study examined the sex-specific peripheral and central fatigue mechanisms during repeated all-out cycling and whether they are affected by total mechanical work performed. A total of 26 healthy young adults (12 females) performed 10 × 10 s all-out cycling interspersed by 30 s passive recovery. Metabolic responses, peripheral and central fatigue, were quantified via changes in pre- to post-exercise blood lactate, potentiated quadriceps twitch force (and contractile properties) evoked via supramaximal electrical stimulation of the femoral nerve, and voluntary activation of the knee extensors, respectively. During exercise, mechanical work, vastus lateralis muscle activation (via surface electromyography), and deoxygenation (via near-infrared spectroscopy) were recorded. Sex comparison analyses were performed before and after statistically controlling for total mechanical work (via ANCOVA). Mechanical work and muscle activation plateaued at similar sprint repetition (sprint 5) and voluntary activation change (pre vs. post) was similar between the sexes. Females, however, showed lower %work decrement (i.e., fatigability; P = 0.037) and peripheral responses as evident by lower reductions in quadriceps twitch force (P < 0.001) and muscle deoxygenation (P = 0.001). Adjusting for total mechanical work did not change these sex comparison results. We show that females' greater fatigue resistance during repeated all-out cycling may not be attributed to the greater total mechanical work performed but could be mediated by lower peripheral fatigue in the knee extensor muscles.

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.

Gender effects on the coordination of subdivisions of the trapezius muscle during a repetitive box-folding task.

This study aimed at investigating gender difference in the coordination of the subdivisions of the trapezius muscle during a repetitive box-folding movement task. Twenty-two healthy volunteers (11 males and 11 females) performed the repetitive box-folding task for 34 min. During the task, perceived exertion and surface electromyographic (EMG) signals from the upper, middle, and lower trapezius subdivisions were recorded. Absolute and normalised root mean square (RMS) values as well as normalised mutual information (NMI) values were calculated to assess, respectively, activation levels within muscle subdivisions and functional connectivity among subdivisions. Females compared with males were characterised by higher normalised RMS values in the upper trapezius (P < 0.05) and higher NMI values within the upper-middle subdivision pair (P < 0.05) during repetitive box-folding. The elevated normalised level of activation of the upper trapezius as well as the enhanced functional connectivity among upper-middle trapezius subdivisions underlined that females adopted a different motor strategy than males did during a dynamic repetitive task. Such differences within and among muscle subdivisions may not be a favourable trait and could partly contribute to the higher prevalence of musculoskeletal disorders reported in females.