Gender differences in neck/shoulder muscular patterns in response to repetitive motion induced fatigue

Comparing gender differences in activation of the sternocleidomastoid muscle during the simulation of a repetitive industrial activity: a cross-sectional study

Objectives. Neck and shoulder problems are increasing among workers, particularly during low-level repetitive tasks. Considering the interaction between neck and shoulder muscles during these tasks, studying the sternocleidomastoid muscle (SCM) activity pattern is crucial. This study aimed to investigate SCM activity during a simulated repetitive task involving muscle fatigue and upper limb elevation. Methods. Nineteen females and 20 males aged 18-35 years with body mass index 18.5-28 participated. Exclusions included musculoskeletal problems within the past 7 days, circulatory/rheumatic/inflammatory disorders, routine engagement in overhead sports or expertise in repetitive movements. Surface electromyography was used to record SCM activity, normalized using maximum voluntary contractions and converted into the root mean square (rms) and median frequency (MF). Pre-fatigue and post-fatigue periods were the first and last 30 s of the task. The craniocervical flexion test assessed deep cervical flexors and their interaction with SCM activity. Results. The rms of SCMs increased bilaterally post fatigue, regardless of sex. Females showed lower MF values in the SCM left side than males, independent of task duration. Conclusion. There were no gender differences. However, SCM changes during upper limb fatigue may indicate overuse of neck muscles during repetitive tasks involving fatigue and upper limb elevation.

Electromyographic analysis of bilateral upper trapezius muscles at different levels of work-pace among sewing machine operators

Workers are driven to work faster in the industrial work environment to meet high productivity targets. An increased work pace leads to increased muscle activation. However, the effect of work pace on bilateral upper trapezius muscles during sewing machine operation in an industrial work environment has not been thoroughly investigated in experimental studies. Therefore, this research aims to conduct an experimental study to analyze the bilateral upper trapezius muscle activity of industrial sewing machine operators at different levels of work pace. Thirty subjects (15 males, and 15 females) continuously performed the sewing operation for two hours in an industrial work environment. Experiments were conducted for two levels of work pace i.e. low pace (100% of standard cycle time) and high pace (120% of standard cycle time). Electromyographic signals were recorded from the bilateral upper trapezius muscles. The EMG amplitude (RMS) among the muscles was computed. A statistically significant (p < 0.05) increase in muscle activity was observed with an increased work pace. In this study, right upper trapezius muscle activity increased by 30.4% during high work pace tasks compared to low pace, while the left upper trapezius showed a 24.12% increase. The right upper trapezius showed a mean difference of 0.696 (%MVC), and the left upper trapezius showed 0.399 (%MVC), both indicating greater activity during high-pace tasks. The increase in muscle activity with time indicated the presence of muscle fatigue among sewing machine operators. Furthermore, higher muscular activity was observed among females than males. This research highlights the critical need to balance productivity goals with the health and safety of workers, reducing the risk of muscle fatigue and associated work-related musculoskeletal disorders.

The effects of hand dominance, fatigue, and sex on muscle activation during a repetitive overhead fatiguing task

Previous studies have shown that the dominant arm is generally stronger and more resistant to fatigue. However, whether there are side differences in shoulder muscle activation during a fatiguing upper limb task, and whether this varies according to sex, is unknown. Thirty right-handed adults (15 females) were recruited to complete two sessions of an overhead repetitive fatiguing task (shoulder flexion between 90 and 135° at 1 Hz), performed in two separate sessions with their dominant arm (DA) and non-dominant arm (NDA) until exhaustion. Electromyographic (EMG) data was collected from 11 shoulder muscles of the moving arm, and their activation amplitude (RMS) and activation variability (SD) were assessed. Results show that time to exhaustion was not affected by arm or by sex. There were some main arm effects on EMG activity amplitude, with higher activity on the DA's pectoralis major (p < 0.001), and on the NDA's middle (p = 0.009) and posterior deltoid (p = 0.001) and infraspinatus (p < 0.001). The pectoralis major was affected by arm and fatigue mostly in males. Their DA's pectoralis major activity amplitude was higher, and the amplitude variability was lower, compared to the NDA, with both parameters showing fatigue-dependent decreases at the NDA only (arm x sex x fatigue: RMS: p = 0.007; SD: p = 0.001). As for females, the DA variability of their lower trapezius was smaller, and that of their subscapularis was higher, compared to the NDA (sex x arm, p = 0.028, p = 0.05). There was also more EMG variability on the supraspinatus' dominant side, and on the posterior deltoid and infraspinatus ND side. Results show an overhead shoulder flexion task dependency on pectoralis major control in males, and on lower trapezius and shoulder girdle stabilizers in females, which could be related to both sex- and gender-based factors. This knowledge can help identify side-specific injury risk factors due to overhead work in males and females, and help determine the appropriateness of implementing sex-specific workplace protocols, including alternating arms as fatigue compensatory and recovery strategies.

Changes in normalized mutual information in response to strength training: An ancillary analysis of a quasi-randomized controlled trial

The aim of the present investigation was twofold. (1) to assess test-retest reliability of normalized mutual information (NMI) values extracted from the surface electromyography (sEMG) signal of muscles pairs of the upper body during dynamic bench press at a high load, and (2) to assess changes in NMI values from before to after a five-week quasi-randomized controlled bench press training intervention. For test-retest reliability, 20 strength trained males (age 25 ± 2 years, height 1.81 ± 0.07 m) performed two three-repetition maximum (3RM) tests in bench press, while sEMG was recorded from six upper body muscles. Tests were separated by 8.2 ± 2.9 days. For the training intervention, 17 male participants (age 26 ± 5 years, height 1.80 ± 0.07 m) trained bench press specific strength training for 5 weeks (TRA), while 13 male participants (age 23 ± 3 years, height 1.80 ± 0.08 m) constituted a control group (CON). 3RM bench press test and sEMG recordings were carried out before and after the intervention period. The NMI values ranged from poor to almost perfect reliability, with the majority displaying substantial reliability. TRA displayed a significant decrease in NMI values during the concentric phase for two agonist-agonist muscle pairs, while one agonist-agonist and two agonist-antagonist muscle pairs increased the NMI values during the eccentric phase. The observed changes did not exceed the minimal detectable threshold, and we therefore cannot surely ascertain that the changes observed in NMI values reflect genuine neural adaptations.

Electronic Design for Wearables Devices Addressed from a Gender Perspective: Cross-Influences and a Methodological Proposal

The design of wearable devices has been approached from many perspectives over the years, mainly from a functionality, electronics, mechanics, usability, wearability, or product design perspective. However, there is a missing point in these approaches: the gender perspective. Gender intersects with every approach and, considering the interrelationships and dependencies, can achieve a better adherence, reach a wider audience, and even change the conception of the wearables design paradigm. The electronics design addressed from a gender perspective must consider both the morphological and anatomical impacts and those emanating from socialization. This paper presents an analysis of the different factors to consider when designing the electronics of a wearable device, including the functionality to implement, sensors, communications, or the location, together with their interdependencies, and proposes a user-centered methodology that contemplates a gender perspective at every stage. Finally, we present a use case that validates the proposed methodology in a real design of a wearable device for the prevention of gender-based violence cases. For the application of the methodology, 59 experts have been interviewed, 300 verbatims have been extracted and analyzed, a dataset from the data of 100 women has been created and the wearable devices have been tested for a week by 15 users. The electronics design needs to be addressed from a multidisciplinary approach, by rethinking the decisions taken for granted and analyzing the implications and interrelationships from a gender perspective. We need to enroll more diverse people at every design stage and include gender as one of the variables to study.

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.

Analysis of the Hanging Actions and Operating Heights of Storage Furniture Suitable for the Elderly

The current functional scale design of storage furniture which the elderly use does not meet their needs, and unsuitable storage furniture may bring many physiological and psychological problems to their daily lives. The purpose of this study is to start with the hanging operation, to study the factors influencing the hanging operation heights of elderly people undergoing self-care in a standing posture and to determine the research methods to be used to study the appropriate hanging operation height of the elderly so as to provide data and theoretical support for the functional design scale of storage furniture suitable for the elderly. This study quantifies the situations of elderly people's hanging operation through an sEMG test performed on 18 elderly people at different hanging heights combined with a subjective evaluation before and after the operation and a curve fitting between the integrated sEMG indexes and the test heights. The test results show that the height of the elderly subjects had a significant effect on the hanging operation, and the main power muscles of the suspension operation were the anterior deltoid, upper trapezius and brachioradialis. Elderly people in different height groups had their own performance of the most comfortable hanging operation ranges. The suitable range for the hanging operation was 1536-1728 mm for seniors aged 60 or above whose heights were within the range of 1500-1799 mm, which could obtain a better action view and ensure the comfort of the operation. External hanging products, such as wardrobe hangers and hanging hooks, could also be determined according to this result.

Sex-specific myoelectric manifestations of localized fatigue during a multi-joint repetitive task

We have previously demonstrated that fatigue at different locations impacts joint angles, angular variability, and coordination variability differently. However, the neuromuscular control aspects underlying these kinematic changes have never been demonstrated. Seventeen young adults (8 males) were recruited. Electromyographic electrodes were placed on: upper trapezius, pectoralis major, anterior and middle deltoid, biceps and triceps brachii, and left and right erector spinae. Subjects performed the repetitive pointing task (RPT) at 1 Hz for 30 s before and after localized fatigue tasks, which consisted of one shoulder, one elbow and one lower back isometric fatiguing protocols until exhaustion in randomized order. Electromyographic amplitude (RMS), variability (SD) and mean power frequency (MnPF) were calculated for each of the pre-fatigue and post-fatigue RPT trials. There were sex × fatigue location interaction effects on upper trapezius RMS (p = 0.038) with males' values increasing the most after shoulder fatigue. Females' triceps brachii RMS was greater compared to males after shoulder, elbow, and trunk fatigue (p = 0.003, p = 0.001 and p = 0.007 respectively). There were sex × fatigue location effects on left erector spinae MnPF (p = 0.011) with males and females' values decreasing the most after trunk fatigue, but more so in males. Results demonstrate that males and females compensate differently during a repetitive pointing task when their elbows, shoulders and trunks are locally fatigued, which could have implications on sex-specific workplace injury risks. See Table 1 for acronyms.

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 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.

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

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

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

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

Sex Differences in Rotator Cuff Muscles' Response to Various Work-Related Factors

Occupational ApplicationsDespite the frequency and cost of rotator cuff injuries among male and female workers, very little is known about the strength, endurance, and perceived exertion and electromyographic response of rotator cuff muscles to different exertion levels. In this study, sex differences were studied using muscle-specific maximal and submaximal exertions of the supraspinatus, infraspinatus, and teres minor muscles. Females showed lower strength and endurance, but higher muscle activity and perceived exertion compared to males. The baseline data presented in this paper can assist ergonomic practitioners in determining the worker capacity to ensure that physically-demanding shoulder exertions can be performed without incurring injurious stress. Such data is also essential to establish population norms for the better design of workplace tasks.

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.

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.

Wrapping technique and wrapping height interact to modify physical exposures during manual pallet wrapping

Occupational tasks often involve musculoskeletal demands that contribute to injury risk. In pallet wrapping tasks, 36% of workplace claims involve over exertion and repetitive exposures (Workplace Safety and Prevention Services, 2012). Tools that modify how the wrap is handled by workers have been introduced to help mitigate over exertion and extreme postures wherever possible. A novel device has been introduced that places the required tension on the roll to mitigate these factors. However, the effect of reducing the necessary tension on the roll during pallet wrapping in limiting muscular demand and extreme postures is unknown. Fourteen healthy university aged participants completed 12 wrapping trials on a simulated pallet (2 repetitions of 2 wrapping techniques (device, hand wrapping) at 3 wrapping heights (low, medium, high)). Surface electromyography (sEMG) was measured on 6 shoulder and 2 low back muscles; anterior and middle deltoids, biceps brachii, infraspinatus, supraspinatus, upper trapezius, and erector spinae (T8 & L3). Kinematic data were collected for the torso and upper extremity and global to torso, and torso to upper arm angles were computed. Repeated measures ANOVAs were performed for the following experimental factors: 1) technique used (device or hand wrapping) and 2) the wrapping height (low, medium, high) for each muscle (8), angle (5), rating of perceived discomfort (1) and rating of perceived exertion (1). Pallet wrapping without a device required greater trunk flexion at lower heights and thoracohumeral elevation at higher wrapping heights as compared to using the device. Muscular activation increased when using the device, specifically at the higher and lower heights. Posture and muscular demands during pallet wrapping tasks are sensitive to both wrapping technique and wrapping height. Ergonomics interventions such as this device may mitigate postural risks associated with manual material handling tasks.

Muscle endurance and cervical electromyographic activity during submaximal efforts in women with and without migraine

BACKGROUND

Despite previous reports supporting cervical muscle weakness and altered motor control in migraine, the endurance under standardized submaximal loads has not been investigated. Therefore, this study aimed to assess the endurance and muscle activity of the cervical musculature during submaximal isometric contractions in women with migraine and those without headache.

METHODS

Cervical muscle endurance tests were performed for flexors and extensors at 25%, 50%, and 75% of the output force during maximal isometric contraction using the Multi-Cervical Rehabilitation Unit with customized biofeedback. Initial values and relative rates of changes in root mean square and median frequency were calculated using cervical muscle superficial electromyography.

FINDINGS

Women with chronic migraine presented significantly shorter flexor endurance time in all load tests than controls (25%, P = .001, 50%, P = .005; 75%, P = .013), while episodic migraine only differed from controls at 75% (P = .018). The frequency of neck pain and/or pain referred to the head after the endurance test was up 12% in the control group, 40% in the episodic migraine group and 68% of the chronic migraine group. Few differences between groups were observed in the electromyographic variables and none of them was related to a worse performance in the endurance tests.

INTERPRETATION

Cervical flexor endurance was reduced in women with chronic migraine when independent of the load, whereas it was reduced to 75% of the maximal force in those with episodic migraine. No difference in the electromyographic variables could be related to this reduced flexor endurance. Also, no differences were detected in extensors endurance.

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.

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.

Sex differences in postures of the upper body during a simulated work task performed above shoulder level

The aim of this study was to evaluate if there are sex differences in postures of upper arm, upper back, head and neck after muscle fatigue induced by a simulated work task, requiring upper arm movements performed above shoulder level. Nineteen females and 18 males were evaluated. Upper arm elevation, upper back, head and neck forward flexion postures were recorded using digital inclinometers. The first and the last cycles of the task were considered as pre- and post-fatigue periods, respectively. For both periods, Amplitude Probability Distribution Function (APDF) was calculated for 10th, 50th and 90th percentiles, as well as the angular ranges between the 5th and 95th percentiles (APDF_5-95 range). The APDF_5-95 range for upper arm elevation was higher with fatigue, but with a higher increase for males. After fatigue, there was also a decrease in the upper arm elevation, with a larger decrease in males for the 10th and 50th percentiles. For head and neck, females showed higher APDF_5-95 ranges than males, regardless of the task period evaluated. After muscle fatigue, males adopted more neutral postures of the upper arm than females. Furthermore, females showed more non-neutral postures for head and neck. These findings suggest that the kinematic strategies adopted by females and males during a simulated work task are different and may predispose females to a higher risk of developing work-related musculoskeletal disorders of the neck.

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.

Supraspinatus Tendons Have Different Mechanical Properties Across Sex

Sex differences in the mechanical properties of different musculoskeletal tissues and their impact on tendon function and disease are becoming increasingly recognized. Tendon mechanical properties are influenced by the presence or absence of sex hormones and these effects appear to be tendon- or ligament-specific. The objective of this study was to determine how sex and hormone differences in rats affect supraspinatus tendon and muscle properties. We hypothesized that male supraspinatus tendons would have increased cross-sectional area but no differences in tendon material properties or muscle composition when compared to supraspinatus tendons from female or ovariectomized (OVX) female rats. Uninjured supraspinatus tendons and muscles from male, female, and OVX female rats were collected and mechanical and histological properties were determined. Our analysis demonstrated decreased dynamic modulus and increased hysteresis and cross-sectional area in male tendons. We found that male tendons exhibited decreased dynamic modulus (during low strain frequency sweep and high strain fatigue loading), increased hysteresis, and increased cross-sectional area compared to female and OVX female tendons. Despite robust mechanical differences, tendon cell density and shape, and muscle composition remained unchanged between groups. Interestingly, these differences were unique compared to previously reported sex differences in rat Achilles tendons, which further supports the concept that the effect of sex on tendon varies anatomically. These differences may partially provide a mechanistic explanation for the increased rate of acute supraspinatus tendon ruptures seen in young males.

Sex differences in muscle activity and motor variability in response to a non-fatiguing repetitive screwing task

Background

Musculoskeletal disorders are more prevalent among women than among men, which may be explained by aspects of motor control, including neuromuscular requirements and motor variability. Using an exploratory approach, this study aimed to evaluate sex differences in neuromuscular responses and motor variability during a repetitive task performed on 3 days.

Methods

Thirty women and 27 men performed the non-fatiguing, repetitive, 1-h screwing task. For neuromuscular responses, the mean and difference values of static, median, and peak percentile muscle activity levels (normalized to a reference voluntary contraction force) and, for motor variability, the mean and difference values of relative and absolute cycle-to-cycle variability across days were compared between both sexes for each muscle. A mixed-design analysis of variance was used to assess differences between both sexes.

Results

The non-fatiguing character of the screwing task was confirmed by the absence of decreased force levels in maximal voluntary contractions performed before and after the task and by absence of electromyographic signs of muscle fatigue. The static and median muscle activity levels tended to be higher among women (on average 7.86 and 27.23 %RVE) than men (on average 6.04 and 26.66 %RVE). Relative motor variability of the flexor and biceps muscles and absolute motor variability of both upper arm muscles were lower in women (on average 0.79 and 29.70 %RVE) than in men (on average 0.89 and 37.55 %RVE). The median activity level of both upper arms muscles tended to decrease within days among women (on average - 2.63 %RVE) but increase among men (on average + 1.19 %RVE). Absolute motor variability decreased within days among women (on average - 5.32 to - 0.34%RVE), whereas it tended to decrease less or increase within days among men (on average - 1.21 to + 0.25 %RVE).

Conclusion

Women showed higher levels of muscle activity and lower initial relative and absolute motor variability than males when performing the same occupational task, implying women may have a higher risk for developing disorders and point to both sexes using different intrinsic motor control strategies in task performance. Clearly, biological aspects alone cannot explain why women would be at higher risk for developing disorders than men. Therefore, a wider range of individual and environmental factors should be taken into account for optimizing work station designs and organizations by taking into account sex differences.

Sex-specific effects of sitting vs standing on upper body muscle activity during text typing

Standing computer work is increasingly popular. However, despite the higher rates of computer work-related disorders in women, no studies have compared how standing work affects men and women. Twelve males and 12 females completed 90-min typing tasks in each posture while electromyography (EMG) data was recorded from eight muscles of the upper body. Results show that females had significantly higher EMG root-mean-squared (RMS) values in the anterior deltoid than males when seated, but higher EMG RMS in the medial trapezius than males when standing (SBC ≤ 0.05). In standing, they also had lower values than males in the erector spinae. Overall, standing elicited less activity in the upper trapezius, wrist extensors and erector spinae than sitting. Results suggest that the standing posture is generally less muscularly demanding than the seated one, although men and women's neck/shoulder musculature responds differently to the same task performed while seated or standing.

Effects of cycling while typing on upper limb and performance characteristics

Active computer workstations may help reduce workplace sedentarism. However, their impact on the upper limb musculoskeletal system is unknown. Subjects participated in two 60-min computer laptop-based Bike-and-Type sessions at different cycling intensities (LOW, HIGH). Upper trapezius and wrist extensor muscle blood flow, sensitivity and pain thresholds, and typing performance were measured intermittently. Neck/shoulder discomfort increased over time (p < 0.001), and was higher in the HIGH intensity (p = 0.036). Blood flow to the trapezius (p = 0.041) and wrist (p = 0.021) muscles were higher during HIGH, and wrist blood flow increased over time (p = 0.01). Trapezius sensitivity threshold significantly decreased over time (p = 0.003). There were no effects on pressure pain thresholds (interaction p = 0.091). Average typing speed was greater during HIGH (p = 0.046) and increased over time (p < 0.001). Time spent biking while typing effectively improved performance, which may facilitate muscle regeneration, although effects depend on biking intensity.

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

INTRODUCTION

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Sex differences in perceptual responses to experimental pain before and after an experimental fatiguing arm task

BACKGROUND

The incidence and prevalence of musculoskeletal disorders (MSDs) is about twice as high in women compared to men, and those of the neck/shoulder region are particularly high among women. Fatigue and responses towards pain are known risk factor for MSDs. However, women have been shown to be less fatigable than men, but more sensitive to experimental pain. From a general standpoint, sex differences in the relationships between the fatigue and pain pathways are poorly understood. This may be due to differences in how men and women conceptually define the sensations of fatigue and pain. The objective of this study was to compare physical and verbal descriptors of fatigue and pain between men and women undergoing an experimental protocol where fatigue and pain were manipulated.

METHODS

Healthy adult volunteers (14 men and 14 women) underwent experimental pain tests to identify pressure pain threshold (PPT) at biceps brachii (BIC), anterior deltoid (AD), and upper trapezius (UT) followed by the Short-form McGill Pain Questionnaire (SF-MPQ) and Pain Catastrophizing Scale (PCS) before and after a repetitive arm task performed at shoulder height until reaching a rating of neck/shoulder perceived exertion, using the Borg Category Ratio 10 (CR10), greater than 8/10. PPT and MPQ data were analyzed using repeated measures analyses of variance (ANOVA) (time × sex). Correlational analyses were used to investigate relationships between pain measures with time and fatigue.

RESULTS

UT PPT was reduced following the fatiguing task (p ≤ 0.01). Men overall reported higher AD PPT levels compared to women (p ≤ 0.05). MPQ and PCS magnification scores were significantly higher after the fatiguing task (p ≤ 0.05), with no sex differences. Time to fatigue correlated with changes in AD PPT in men and with PCS scores in women.

CONCLUSIONS

Findings suggest that mechanisms underlying the sensation of acute pain following a repetitive shoulder height task are closely linked with PPT changes in shoulder stabilizers (UT) irrespective of sex, and more so with physical pain responses in men and in attitudes towards pain in women. Sex differences in pain perception may contribute to a better understanding of sex-specific mechanisms underlying neck/shoulder MSDs.

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.

Arm posture influences on regional supraspinatus and infraspinatus activation in isometric arm elevation efforts

This study aimed to evaluate the effect of arm posture on activation of the anterior and posterior regions of supraspinatus and the superior and middle regions of infraspinatus during resisted isometric arm elevations. Thirty-one healthy participants performed 18 isometric resistance exertions against a force cube in three elevation planes (flexion, scaption, abduction) and three elevation angles (30°, 90°, 150°) in maximal and sub-maximal resistance conditions. EMG data were obtained using four pairs of fine wire electrodes. The mean activation of each region and the activation ratios were compared across postures using ANOVAs. Supraspinatus anterior was significantly more active during abduction and scaption, and in higher elevation angles, while the posterior region showed similar activation levels across postures. Infraspinatus regions were more active during flexion with more relative activation of the infraspinatus superior at 90° flexion. The results suggest that regional activation of supraspinatus and infraspinatus should be considered for assessment and rehabilitation purposes. In any clinical setting where it is important to reduce the stress on the supraspinatus anterior, isometric flexion exercises performed with arm in low elevation angles could provide the opportunity to strengthen the posterior region of supraspinatus with limited stress on the anterior region. Beside external rotation exertions, resisted flexion tests may be useful for evaluation of infraspinatus regions.

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

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

Effects of standing on typing task performance and upper limb discomfort, vascular and muscular indicators

Standing is a popular alternative to traditionally seated computer work. However, no studies have described how standing impacts both upper body muscular and vascular outcomes during a computer typing task. Twenty healthy adults completed two 90-min simulated work sessions, seated or standing. Upper limb discomfort, electromyography (EMG) from eight upper body muscles, typing performance and neck/shoulder and forearm blood flow were collected. Results showed significantly less upper body discomfort and higher typing speed during standing. Lower Trapezius EMG amplitude was higher during standing, but this postural difference decreased with time (interaction effect), and its variability was 68% higher during standing compared to sitting. There were no effects on blood flow. Results suggest that standing computer work may engage shoulder girdle stabilizers while reducing discomfort and improving performance. Studies are needed to identify how standing affects more complex computer tasks over longer work bouts in symptomatic workers.

Experimental validation of a subject-specific maximum endurance time model

This study aimed at experimentally validating a subject-specific maximum endurance time (MET) model. Thirty health participants (15 males and 15 females; Age: mean = 21.5 years, SD = 1.6 years) volunteered to conduct an isometric elbow flexion task until exhaustion. The endurance times of each participant were measured under relative exertion levels ranging from 30% MVC (Maximum Voluntary Contraction) to 70% MVC at 10% intervals. Assessment of the model showed that the intensity-endurance time relationship for each studied individual could be well fitted by the subject-specific MET model (R² > 0.89). The fatigue rates identified from the model fitting were normally distributed (Mean = 0.96 min^-1, SD = 0.29 min^-1). In addition, the fatigue rates of the male group were significantly higher than the female group. The subject-specific MET model can be used to predict the MET for individual workers, and further support physical task design, based on the fatigability data of a targeted worker population. Practitioner Summary: Ergonomists have extensively used MET models in physical fatigue assessment and physical task design. A subject-specific MET model could be used to predict the MET at individual levels, and also to support work design for a target worker population, based on the fatigability data distribution obtained from sampled workers.

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.

Changes in shoulder girdle strength in 3 consecutive years in elite adolescent swimmers: a longitudinal cohort study

BACKGROUND

The shoulder complex is highly loaded during swimming. No studies were found analyzing the changes in shoulder girdle strength in young swimmers, through the years.

OBJECTIVE

To analyze the changes in strength of internal rotators and external rotators of the arm, lower trapezius and supraspinatus and in the external rotators/internal rotators ratio in young elite swimmers through 3 years.

METHODS

31 adolescent elite swimmers (14-18 years, of both sexes) participated in the study. Isometric strength of the shoulder girdle muscles was performed using a handheld dynamometer during 3 years.

RESULTS

For boys absolute data, internal rotators increased from the first to the second (p=0.0001; mean difference 45.6N; 95%CI 26.7-65.0) and third years (p=0.01; mean difference: 32.4; 95%CI: 9.3-55.5). Considering the weight-normalized data, internal rotators increased from the first to the second year (p<0.0001; mean difference: 0.52; 95%CI: 0.26-0.78), external rotators decreased from the first to the third year (p=0.003; mean difference: -0.33; 95%CI: -0.53 to -0.13) and from the second to the third year (p=0.0004; mean difference: -0.29; 95%CI: -0.46 to -0.12) and supraspinatus decreased from the second to the third year (p=0.006; mean difference: -0.17; 95%CI: -0.28 to -0.06). For girls, there were no significant differences in the absolute strength. Considering the weight-normalized data, lower trapezius decreased from the first to the third year (p=0.02; mean difference: -0.15; 95%CI: -0.27 to 0.03). Considering both sexes, the external rotators/internal rotators ratio decreased from the first to the second (p<0.0001; mean difference -0.12N; 95%CI -0.13 to -0.11) and third years (p<0.0001; mean difference -0.15N; 95%CI -0.16 to -0.14).

CONCLUSION

Muscle imbalance can occur in the shoulder girdle in young swimmers in 3 years, with increased internal rotators and decreased external rotators and supraspinatus strength in boys, and decreased strength of the lower trapezius in girls. Attention should be given in young swimmers' shoulder girdle muscle balance.

The coordination of shoulder girdle muscles during repetitive arm movements at either slow or fast pace among women with or without neck-shoulder pain

PURPOSE

The aim of this study was to evaluate the coordination of the shoulder girdle muscles among subjects with or without neck-shoulder pain performing repetitive arm movement at either a slow or fast pace.

METHODS

Thirty female adults were allocated to one of two groups-healthy controls or cases with neck-shoulder pain. Surface electromyography (sEMG) signals from the clavicular, acromial, middle and lower trapezius portions and the serratus anterior muscles were recorded during a task performed for 20min at a slow pace and 20min at a fast pace. The root mean square (RMS), relative rest time (RRT) and normalised mutual information (NMI, an index of functional connectivity between two muscles in a pair) were computed.

RESULTS

No significant differences on RMS, RRT and NMI were found between groups. For both groups, the fast movement pace resulted in increased levels of RMS, lower degrees of RRT and higher NMI compared to the slow pace. No interaction between group and movement pace was found.

CONCLUSIONS

This study highlights the change in sEMG activity of muscles to meet the demands of performing a task at fast movement pace. The fast pace imposed a higher muscle demand evidenced by increased sEMG amplitude, low degree of muscle rest and increased functional connectivity for subjects in both the case and control groups. No indication of impaired sEMG activity was found in individuals with neck-shoulder pain.

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.

The spatial dependency of shoulder muscular demands during upward and downward exertions

Lifting and lowering are common occupational tasks contributing to shoulder injury risk. Quantifying task interaction with physical demand can precipitate better workstation designs. Nineteen university-aged males performed one-handed, submaximal upward/downward manual force exertions at 70 hand locations; unilateral electromyography (EMG) of 14 muscles was recorded. EMG across planes was evaluated with ANOVA. Predictive equations for muscle activity throughout the reach envelope were developed with stepwise regression. Total muscle activity (sum of individual muscle activity) was most sensitive to vertical hand location for upward exertions, where activation at superior locations was 192% of values for inferior locations. For upward exertions, activation differences for hand location occurred along all anatomical axes, and along anterior/posterior and superior/inferior axes for downward exertions. Predictive equations were non-linear, reflecting complex muscular demand with three-dimensional hand location. This work details foundational exposure data for lifting/lowering exertions. Results are applicable to workstation design to minimise occupational shoulder muscular demands. Practitioner Summary: Lifting and lowering in the workplace contribute to shoulder injury risk. Shoulder muscle activity magnitudes revealed a dependence on three-dimensional hand location in the reach envelope for a defined hand force. This information can inform evidence-based workstation designs that reduce shoulder muscular demands for numerous materials handling scenarios.

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.

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.

Ergonomic task reduction prevents bone osteopenia in a rat model of upper extremity overuse

We evaluated the effectiveness of ergonomic workload reduction of switching rats from a high repetition high force (HRHF) lever pulling task to a reduced force and reach rate task for preventing task-induced osteopenic changes in distal forelimb bones. Distal radius and ulna trabecular structure was examined in young adult rats performing one of three handle-pulling tasks for 12 wk: (1) HRHF, (2) low repetition low force (LRLF); or (3) HRHF for 4 wk and than LRLF thereafter (HRHF-to-LRLF). Results were compared to age-matched controls rats. Distal forelimb bones of 12-wk HRHF rats showed increased trabecular resorption and decreased volume, as control rats. HRHF-to-LRLF rats had similar trabecular bone quality as control rats; and decreased bone resorption (decreased trabecular bone volume and serum CTX1), increased bone formation (increased mineral apposition, bone formation rate, and serum osteocalcin), and decreased osteoclasts and inflammatory cytokines, than HRHF rats. Thus, an ergonomic intervention of HRHF-to-LRLF prevented loss of trabecular bone volume occurring with prolonged performance of a repetitive upper extremity task. These findings support the idea of reduced workload as an effective approach to management of work-related musculoskeletal disorders, and begin to define reach rate and load level boundaries for such interventions.