Similar effects of fatigue induced by a repetitive pointing task on local and remote light touch and pain perception in men and women.

BACKGROUND: Women involved in repetitive, fatiguing, jobs develop more neck and/or shoulder musculoskeletal disorders than men. Sex differences in the pain response to exercise could contribute to the higher prevalence of neck/shoulder musculoskeletal disorders in women. The objective of this study was to assess sex differences in pain sensitivity following a fatiguing upper limb task. Relationships between measures of fatigue and of the sensitivity to nociceptive and to non-nociceptive stimulations were also explored. METHODS: Thirty healthy adults (15 women) performed a fatiguing repetitive pointing task with their dominant arm. Upper limb electromyography was recorded from the dominant upper trapezius, anterior deltoid and bicep brachii and from the contralateral tibialis anterior. Before and immediately after the repetitive pointing task, pressure pain and light touch sensitivity thresholds were measured over the same muscles. RESULTS: Electromyographic signs of fatigue were observed only in the anterior deltoid and biceps brachii muscles. Pressure pain thresholds over both muscles increased slightly (effect size ≤ 0.34), but no changes occurred over the upper trapezius and the tibialis anterior. Light touch thresholds increased moderately to importantly after the repetitive pointing task over all four muscles (effect sizes = 0.58 to 0.87). No sex differences were observed in any sensory variable. Moreover, no or weak correlations (r = -0.27 to 0.39) were observed between electromyographical signs of fatigue, light touch threshold and pressure pain threshold variables. CONCLUSIONS: We observed sex-independent effects of a repetitive upper limb task on the sensitivity to painful and to nonpainful stimuli. Moreover, the hypoalgesia induced by the repetitive pointing task was weak and localized, and did not directly correlate with the induced muscle fatigue. Results suggest that fatigue-related changes in the sensitivity to noxious and innocuous stimuli could not explain women's greater prevalence of neck/shoulder musculoskeletal disorders.

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.

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.

Differences in muscular and perceptual responses to a neck/shoulder fatiguing task between women and men.

Fatigue is a risk factor for the development of work-related musculoskeletal disorders (WMSDs) of the neck/shoulder, and exertion is a state that connects the sensory/affective and physical aspects of fatigue. Relationships between ratings of perceived exertion (RPE) and electromyogram (EMG) signal characteristics have been identified. However, the sex-specific effects on these relationships are unclear and could be affected by personal factors such as skinfold thickness. 28 healthy young adults completed a fatiguing task with the arm held at shoulder height. Neck/shoulder EMG and RPE were collected at the end of each minute. Males showed stronger correlations between RPE with anterior deltoid (AD, rs = 0.37) and upper trapezius (UT, rs = 0.48) EMG, whereas females showed a weak relationship between RPE and UT EMG (rs = 0.22), and no relationship with AD EMG. Significant correlations were observed between biceps EMG and skinfold thickness in both males and females. Findings suggest that neck/shoulder perceived exertion is strongly associated to the activity of shoulder mobilizers (anterior deltoid) in men, whereas in women, it is rather associated to the activity of shoulder stabilizers (upper trapezius), and to a lesser extent. It is possible that in turn, these sex differences affect fatigue adaptation strategies. Skinfold thickness may be an important variable to consider when studying sex-specific characteristics of muscle fatigue.

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.

Effects of additional external weight on posture and movement adaptations to fatigue induced by a repetitive pointing task.

Fatigue and additional weight are risk factors of injuries by falls. Posture and trunk movement changes occur with fatigue induced by a repetitive pointing task. These changes facilitate arm movement but they may also jeopardize postural stability. When equilibrium is challenged, e.g. with additional weight, strategies that represent less postural threat could develop with fatigue. Nineteen participants performed two sessions (without, with 20% body weight added load (Load)) of a repetitive pointing task until shoulder fatigue (8 on Borg CR-10). There was no difference in time to fatigue between the two sessions. Anterior deltoid, biceps and upper trapezius muscle activity significantly increased with fatigue. Peak medial-lateral center-of-pressure (CoP) velocity and the mean vertical position of the reaching shoulder were both significantly lower with fatigue, though these fatigue-induced decreases were smaller with the added load. Reach-to-reach variability in CoP displacement significantly increased with fatigue, and more so with the added load. With fatigue, significant contralateral shifts occurred at the reaching shoulder and elbow joints, and ranges of motion (RoM) significantly increased at most joints but not at the center-of-mass (CoM). Conversely, Load main effects were mostly seen in CoM dependent measures. Significantly increased variability in mean and range values was seen with fatigue and Load in most of our kinematic and CoP dependent measures, with the most notable effects on CoM dependent measures. Findings suggest that the postural control system adapts to combined perturbing factors of fatigue and added load, likely by using parallel control mechanisms.

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

Previous studies have associated amplitude and frequency characteristics of the electromyogram (EMG) to the risk of developing musculoskeletal disorders (MSDs) with repetitive tasks. However, few studies have investigated whether EMG variability and between-muscle activity characteristics may be associated with MSD risk. Twenty-six healthy volunteers (13 men, 13 women) performed a repetitive pointing task at shoulder height until scoring 8 on a Borg CR-10 scale. Electromyographic (EMG) signals were recorded from six neck/shoulder muscle sites. EMG amplitude (RMS), variability and mutual information (MI) among muscle pairs were computed. Muscle fatigue was evidenced by increased EMG RMS of four muscles (Upper Trapezius (UT): +17%; supraspinatus (SUPRA): +28%; middle deltoid: +13%; biceps brachii: +38%) and increased SUPRA variability. Correlations between minute 1 patterns and endurance time indicated that in women, initially high variability in UTR (r=0.79) and SUPRA (r=0.71) predicted higher endurance, whereas in men, initially low MI in LT-UT (-0.69) and in LT-SUPRA (-0.77) pairs predicted high endurance. Significant correlations suggest that variability and between-muscle patterns may be associated with fatigue and injury mechanisms, in a gender-specific way. Differing fatigue mechanisms between genders could help explain gender differences in injury mechanisms.

Repetitive arm motion-induced fatigue affects shoulder but not endpoint position sense.

Neck/shoulder pain has previously been linked to repetitive work and muscle fatigue. We have shown that asymptomatic people performing repetitive upper limb tasks display signs of shoulder fatigue and of whole-body compensatory strategies. However, the role played by the proprioceptive system in the production of these compensatory strategies has not been studied. A group of asymptomatic adults (n = 18) performed a repetitive pointing task at shoulder height to fatigue. Before and after fatigue, they performed two position sense tasks, eyes closed: a single-joint task where they abducted their shoulder to the perceived horizontal and a multi-joint task, where they stood and placed their finger at the perceived location of a target in front of them at shoulder height. After fatigue, subjects made larger shoulder errors by raising their elbow higher above the horizontal (~ +1.3 cm) than before fatigue; however, their finger position accuracy was not changed, despite all subjects performing the movement in less time (~ -0.18 s) while fatigued. There were no gender differences in shoulder or finger position accuracy before or after fatigue; however, there were gender differences in the perceived finger-target location and in the temporal characteristics of the finger movement toward the target. Results suggest that healthy individuals are able to develop strategies to compensate for fatigue-induced deficits at one joint to maintain the endpoint accuracy of a multi-joint task constant. Gender differences in movement strategies and perception of endpoint location may play parts in the previously reported gender differences in work-related neck/shoulder symptoms.

The effects of a Pilates training program on arm-trunk posture and movement.

BACKGROUND: Shoulder biomechanics and spine alignment have been found to be related to occasional and/or chronic neck-shoulder pain. Pilates is a physical training approach that focuses on posture, flexibility, segmental alignment and core control, through posture and movement exercises. The objectives of this study were to determine the effect of a Pilates training program on arm-trunk posture, strength, flexibility and biomechanical patterns during a functional shoulder flexion task. METHODS: Nineteen subjects (9 controls, 10 experimental) were assessed twice, 12 weeks apart, during which the experimental group was submitted to a Pilates training program (two 1-h sessions per week). The assessment consisted of trials of seated posture, abdominal strength, shoulder range of motion, and maximal shoulder flexion, during which neck, shoulder and trunk kinematics and the activity of 16 muscles were recorded. FINDINGS: After training, subjects showed smaller static thoracic kyphosis during quiet sitting and greater abdominal strength. The experimental group also showed reduced posterior and mediolateral scapular displacements, upper thoracic extension and lumbar lateral flexion, as well as higher activity of the ipsilateral cervical erector spinae, contralateral rhomboid muscles and lower activity of the ipsilateral lumbar erector spinae during the shoulder flexion task. INTERPRETATION: The Pilates training program was effective in improving abdominal strength and upper spine posture as well as in stabilizing core posture as shoulder flexion movements were performed. Since deficits in these functional aspects have previously been associated with symptoms in the neck-shoulder region, our results support the use of Pilates in the prevention of neck-shoulder disorders.