Introduction
Work-related MSDs have consistently been reported to occur to a larger extent among women than men (Schneider and Irastorza
2010), and especially so for the neck and shoulder/arm regions (Nordander et al.
2016). Several risk factors pertaining to biomechanical exposures at work have been associated with neck–shoulder MSDs, such as heavy lifting, forceful exertions, awkward postures, vibrations and repetitive movements (National Research Council
2001). However, the specific mechanisms for why women would be more susceptible to neck–shoulder MSDs than men, even in the same jobs, remain largely unknown. Potential reasons include differences in task allocations to men and women within the job, differences in how men and women perform the same work tasks and even differences between men and women in physiological effects when performing the same tasks in the same way (Cote
2012; Lewis and Mathiassen
2013). Most likely, all of these factors contribute in a complex interaction to explain gender differences in MSD occurrences. The focus of the present study is to understand sex-specific differences in how men and women respond to performing the same fatiguing task, as a contribution to explaining possible sex differences in biomechanical and physiological responses of relevance to MSDs. We investigate a short-cycle, repetitive manual task as a model of repetitive work occurring, for instance, in manufacturing industries, meat cutting and cashier work, which have been shown to entail an increased risk for neck/shoulder MSDs (Kilbom
1994; Cote et al.
2008; da Costa and Vieira
2010).
According to a recent review of the literature on fatigability and associated physiological mechanisms (Hunter
2014), there is still a considerable lack of understanding of the reasons for sex differences in neuromuscular function and fatigability, and the consequences of these differences to functional performance of daily tasks. Sex differences have largely been studied using controlled isometric and isotonic contractions. As reviewed in Hunter
2014, these studies have shown that women have better muscular endurance than men in isometric, sub-maximal contractions of muscles in some parts of the body such as the lower back, thighs, arms and hands but not in others such as the ankle. Sex differences have also been reported to depend on the contraction intensity; for example, women had longer time-to-exhaustion than men in contractions of the biceps muscle at 20 % of their maximal voluntary capacity (MVC), while there was no difference at 80 % MVC (Yoon et al.
2007). The type of work task has also been shown to affect sex-based fatigue responses; for instance, Clark et al. (
2003) showed that women had longer time-to-exhaustion than men in a static exercise at 50 % MVC, but that women and men did not differ in the time-to-exhaustion when performing dynamic work.
However, whether these previously mentioned results are applicable, or even relevant, for understanding gender differences in performing real occupational work and the subsequent consequences on MSD risk is questionable. In realistic work involving dynamic tasks rather than isometric contractions, fatigability may be determined mainly by additional factors in motor control, such as motor coordination and the ability to vary or share effort between multiple muscles. The phenomenon of postures, movements and muscle activities varying between successive repetitions of a task (intended to be identical in performance) is referred to as ‘motor variability’ (Mathiassen
2006; Madeleine
2010; Srinivasan and Mathiassen
2012). Although motor variability has been proposed to be an important factor in determining individual differences in susceptibility to developing fatigue, pain and MSDs (Madeleine
2010; Srinivasan and Mathiassen
2012), whether systematic gender differences exist in motor variability has seldom been studied, except for a few studies (Cote
2012). For instance, Semmler et al. (
1999) showed that following training, women could increase endurance time in a low-force fatiguing contraction by altering the pattern of muscle activation and using rotating/alternating motor unit activity. On the other hand, in a study on experimental shoulder pain (Falla et al.
2008), women were not able to redistribute their shoulder muscle activity as much as men, and eventually, women also reported higher perceived pain than men. A study (Fedorowich et al.
2013) reported that women with initially high motor variability in the upper trapezius muscle activity showed longer endurance times to a repetitive pointing task than those with lower variability, and that this effect of initial variability on endurance time was not observed in men, suggesting gender specificity in the relationship between variability and fatigue.
Thus, although the recent literature shows that there may, indeed, be sex differences in both structural and functional aspects of motor control, including muscle coordination and movement strategies during upper limb tasks, there are not enough studies to explain what these specific differences may be. It is also not clear how these functional aspects can explain potential gender differences in the development of short-term responses to fatigue, which may be interpreted as signs of increased risk for developing MSDs in the long term. The aims of this study were to quantify gender differences that may exist in the performance, fatigability and muscle activity responses during a fatiguing, repetitive upper-extremity task.
This study aimed to determine, for a fatiguing, repetitive upper-extremity task:
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The extent to which women and men differ in variability in the activity of selected shoulder and elbow muscles while performing the task without fatigue;
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The extent to which women and men differ in time-to-task-termination, as a measure of fatigability;
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The extent to which women and men differ in the change in amplitude and variability of activities in selected shoulder and elbow muscles (cf. 1) from the start to termination of the task.
Discussion
To the best of our knowledge, this is the first study to compare motor responses accompanying fatigue between men and women in a standardized repetitive task involving the upper extremities. In contrast to earlier protocols utilized in controlled studies of gender differences in muscle activation and motor performance, this task involved muscles of both the shoulder and elbow joints at the same time and had a task performance requirement that could be satisfied using multiple coordination strategies (i.e., by utilizing the various biomechanical degrees of freedom about the shoulder and elbow joints).
In general, we found that there were no gender differences in EMG responses at baseline (Table
1), and that EMG RMS amplitude increased with fatigue in all muscles across the whole sample of individuals tested (Table
2). However, the time-to-task-termination, as defined by a rating of perceived exertion of 8 on the Borg CR-10 scale, did not differ between men and women. Similar results have been reported by earlier studies utilizing the same repetitive pointing task protocol and task termination criteria [e.g. (Fuller et al.
2009)]. However, this result is both in agreement with, and in contrast to results from the few other previous studies that have considered gender differences during fatiguing dynamic contractions. Women were less fatigable than men for a protocol of 30 maximal dynamic contractions with the knee extensor and knee flexor muscles at a relatively constant speed (Pincivero et al.
2003). Similarly, the time to task failure was longer for women than men in a dynamic task that required lifting and lowering of a load equivalent to 20 % MVC for as long as possible at the rate of 1 contraction every 3 s (Hunter
2014). However, these differences have been reported to diminish during repeated contractions at higher velocities (Senefeld et al.
2013). It is difficult to directly compare effects in men and women of tasks requiring maximal or sub-maximal isokinetic contractions or intermittent loads which are percentages of individual maximal strengths, simply because similar to the static isometric experiments, these differences may or may not exist if adjusted for strength as a covariate. Similarly, it is also difficult to directly compare results from the cited studies of endurance in sub-maximal dynamic tasks to fatigue responses in a dynamic task like ours, which was terminated prior to complete task failure. Thus, one should be careful in generalizing results from different studies as they may be highly task-specific.
Differences in performance of both static and dynamic tasks between genders have so far mainly been ascribed to: (a) differences in contractile properties associated with muscle fiber size and composition; women being reported to have a greater proportional area of type I muscle fibers than men and smaller type II fiber area in muscles such as the quadriceps, (Simoneau and Bouchard
1989), and (b) to differences in muscle perfusion and metabolism where women exhibit greater muscle perfusion than men and also show lower increase in metabolite build-up in some muscles such as the elbow flexors (Hunter and Enoka
2001). Specifically, in the trapezius muscle though, histological studies have shown that women and men have a similar fiber type composition, but women have a smaller fiber cross-sectional area than men, (Lindman et al.
1990,
1991), suggesting a lower force-generating capacity in women which may, in turn be associated with higher risks of developing neck–shoulder disorders (Meyland et al.
2014; Nordander et al.
2016).
Although the existing evidence on differences between genders in structural, morphological or physiological properties of individual muscles may, thus, offer some support to the hypothesis that women and men would differ in development of fatigue when performing a dynamic task, we did not find any gender differences in time-to-task-termination in our repetitive pointing task. This finding could be attributed to the task requiring multi-jointed movements with different possible coordination strategies: although women and men may differ in basic biological tissue properties, differences in motor control may compensate for those to the extent that there are, eventually, no differences in the ability to perform the task. For instance, women and men may utilize the available degrees of freedom and coordinate multiple muscles at the shoulder and elbow joints differently to perform the same repetitive task. The hypothesis that women and men may utilize different motor control/coordination strategies to preserve functional aspects of task performance has been previously proposed by (Cote
2012; Hunter
2014). Our findings of changes in variation of muscle activity with fatigue give some support to this hypothesis. We found that while both women and men exhibited the same variability in baseline muscle activity at the start of the task, and performed the task for similar durations, men exhibited a higher increase in trapezius muscle variability with fatigue, whereas women exhibited a greater increase in biceps muscle variability. Although only these two differences were statistically significant, the results shown in Fig.
2 and Table
3 illustrate that similar trends were also present in the anterior deltoid and triceps muscles; while men showed a greater increase in variability in the anterior deltoid activity, women showed a greater increase in variability in the triceps muscle activity with fatigue.
The results may be interpreted in consideration of the biomechanical requirements of the task. The pointing task was designed such that subjects extended their arm fully, to 100 % of their reach capacity to reach the distal target. The proximal target was positioned at 30 % of the participant’s full reach in front of them on the horizontal plane. To achieve the final reach posture, subjects were required to horizontally adduct their upper arm and extend their elbow. Consequently, the anterior deltoid and triceps muscles would be the prime movers in this task, while the trapezius muscle stabilized the shoulder and supported the weight of the arm, and the biceps would have a similar control and stabilization role for the forearm. The observed changes in variability combined with this understanding of the biomechanical demands of the task suggest that men showed a greater increase in the involvement of muscles stabilizing and moving the shoulder, while women showed a greater increase in the elbow stabilizing and prime mover muscles, thus implying a “shoulder-based” vs. an “elbow-based” control strategy in men and women, respectively, for this task.
An earlier study (Anders et al.
2004) reported that during isometric shoulder exercises (push-ups), men showed a tendency for a higher relative activation level in the shoulder muscles acting as primary movers compared to women who activated synergistic muscles that were less necessary for actual force production. The authors argued that a more goal-driven coordination pattern together with the much larger forces acting at the shoulder joint in men may lead to a higher risk for shoulder joint injuries because of a relatively less-stabilized functional configuration. In the present experimental task, since the trapezius muscle is the primary stabilizer, it may be that women implement a motor control strategy that defers the mechanical loading of the trapezius muscle down the kinetic chain (i.e., the biceps and triceps) to mitigate fatigue in the upper trapezius. These plausible alternative mechanisms (i.e., men use the shoulder more, or women use the shoulder less) highlight the need to better understand gender differences in motor control strategies: although women and men seem to have the same time-to-task-termination in this study, in prolonged performance of similar tasks in a working environment, different strategies used by women vs. men might lead to different risks of shoulder vs. elbow injuries in women and men.
Finally, while we focused on biological sex-specific mechanisms in this study of motor control and fatigue, we recognize that the occurrence and reporting of work-related MSDs in women and men will also be influenced, not only by additional factors at the individual level, such as differential reactions to pain symptoms, but also by numerous organizational, social and cultural factors. Consequently, although our study findings may provide some biological explanation for sex differences in upper-extremity occupational injuries, the work environment encountered by men and women, their interaction with that environment, and numerous factors outside work should also be considered when analyzing sex differences in upper-extremity MSDs.