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

Abstract

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.

Ergonomics and Work-Related Musculoskeletal Disorders: Characteristics Among Female Interventionists

“…These findings correspond with the reports by Armijo et al described previously [33]. Slopecki et al found that women had a significantly higher percentage change in the anterior deltoid when measuring muscular activity with surface electrodes when the anthropometric load was included as a covariate [35]. These results suggested that women often experience work-related upper extremity pain than the lumbar spine and cervical spine pain reported by men when performing the same work.…”

“…Further reviews analyzing WMSDs in manufacturing workers have found that women are more likely to develop upper extremity pain and fatigue while performing the same industrial work as men [34,35]. Nordander et al showed that female workers in rubber manufacturing and mechanical assembly plants had higher maximal voluntary contractions of the trapezius and forearm extensors when compared to men [34].…”

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

“…As stated by Fuller et al, the literature suggests that muscle fatigue "leads to within movement alteration at both the inter-segmental and inter-muscular levels" but "relatively little is known about how muscle fatigue may affect timedependent characteristics of posture and movement control during multijoint tasks" [23]. In fact, many studies rely on the hypothesis that adaptations in postures and movement during a repetitive pointing task are induced by muscle fatigue [11,[18][19][20][21][22][23].…”

“…Such a fundamental activity with muscle fatigue has already been described in the literature, for instance by Côté et al [11,[18][19][20][21][22]. These papers deal with "binary" comparisons of the subjects' postures and movements (before or after fatigue, male or female, with or without pain, with or without additional mass attached to the waist, etc.).…”

Untitled

“…The muscles acting out of phase or with increased activity in female patients with hyperlaxity are a target for physical therapy to restore normal mechanics 30 . Moreover, studies have demonstrated sex differences in the fatigue response of shoulder musculature, as females must work at higher intensities to generate similar workloads to men 31,32 . Physical therapy can yield satisfactory results, especially in the young, female population with hypermobility who will experience a decrease in instability symptoms as they age 24,27,33,34 .…”