WMSDs are a major worldwide problem, and there is no one single reason for the development of these disorders, as these are multifactorial (Forde, Punnett et al. 2002). Ergonomists and scientists have been trying to understand the underlying mechanisms and possible causes for WMSDs. Out of many possible causations, it is proposed by many researchers that LMF can be a cause for the development of WMSDs, though it is not clear yet. Many theories, models, and mechanisms have been proposed indicating a potential link between LMF and WMSDs.
Out of several models, the dose-response model (Armstrong, Buckle et al. 1993) provides a better interpretation of the underlying mechanisms of WMSDs. The model is characterized by four variables: Exposure, dose, response, and capacity. Exposure is an external physical factor such as work requirement that causes an internal disturbance in the body called as dose, such as tissue loads and metabolic demands. Due to this dose, responses such as a change in the ionic concentration or temperature of the tissue are produced, and the capacity is the ability to resist the destabilization due to various doses.
According to this model, application of external load to the body (exposure) will induce loads on the muscle tissue (dose), which produces mechanical and physiological responses such as deformation and yielding of tissues, increase in intramuscular tissue pressure and change in the concentration of metabolites that leads to LMF. If there isn’t enough time for the body to regenerate the tissue due to multiple responses, it can reduce the capacity. When this cycle occurs repetitively, it can cause tissue deformation which produces pain and swelling. These effects are reversible if muscle rests sufficiently, as the muscle repair is faster than muscle damage. However, if the muscle does not receive enough time to recover, these “wear and tears” can accumulate, which may lead to long lasting impairments (i.e., WMSDs). This is a common scenario in daily work in occupational settings, which may result in gradual muscle disorders over a period of time, rather than acute disorders (Armstrong, Buckle et al. 1993).
In addition to this, mechanisms like induced muscular imbalance and reperfusion injury have been proposed that support the link between LMF and WMSDs (Forde, Punnett et al. 2002). To further strengthen the connection and to support these models and mechanisms, four theories regarding the muscle injury have been proposed by (Kumar 2001) namely the multivariate interaction theory, differential fatigue theory, cumulative load theory and overexertion theory. From these four theories, it is evident that LMF has a significant role in the causing an injury. Awkward and static postures, heavy work, repetitive work and insufficient rest that can cause soft tissue injuries, (Sommerich, McGlothlin et al. 1993) are also precursors of LMF, indicating the link between LMF and injuries. The tissues that are fatigued regularly get injured most of the time and in the long run, produces WMSDs. In summary, all these evidences support the role of LMF as a potential measure of injury risk, possibly in causal relationship with WMSDs.
Along with force generation impairment, MU recruitment also plays a role in the development of WMSDs (Armstrong, Buckle et al. 1993, Forde, Punnett et al. 2002). Even before discussing the relationship between MU recruitment and development of WMSDs, it is important to understand the concept of MU recruitment. MU can be considered as a functional unit of muscle and is defined as: “the collection of skeletal muscle fibers innervated by a single motor neuron”(Purves, Augustine et al. 2001). The MUs can be classified based on their recruitment thresholds into low and high threshold MUs, which require low and high amounts of force to activate them, respectively. Motor recruitment predominantly follows Cinderella hypothesis (Hagg 1991), according to which low threshold MUs are activated at low forces and these stay activated for most of the time. If the force output has to be increased, more MUs will be recruited to generate more force.If Cinderella hypothesis is to exist, the low threshold MUs will be activated for most of the time and will be maintaining higher firing rates for longer periods of time. These MUs are working close to their maximal capacity even if the overall load on the muscle is low. This continuous activation results in the damage of these MUs over the time and causes MSDs in the long run.
However, this problem of MU impairment can be dealt with MU substitution or rotation (Westgaard and De Luca 1999). During fatiguing contractions, low threshold MUs were observed to be inactive for smaller periods of time and were substituted by high threshold MUs during this period. When this substitution occurs, the EMG amplitude is found to decline and then increase, indicating a period of inactivity. This gives the low threshold MUs some time to rest and recover, thereby delaying fatigue. It is postulated that this substitution helps the MU from fatiguing quickly and helps the muscle to sustain the contraction for longer periods of time (Westgaard and De Luca 1999, Westad, Westgaard et al. 2003).
From the above discussions, LMF is a measure that can be considered to reduce the development of WMSDs and MU substitution can be an efficient mechanism that can be useful in the reduction of LMF. So, it is required to modify the working patterns in the industry such that they induce less LMF, to reduce the development of WMSDs and how it has been done so far is discussed briefly in the next section.
Current trends in the industry are inclined towards introducing standardized production principles, which forces workers to perform low intense repetitive jobs. Even though the initial solution to WMSDs was to reduce the peak loads in the industry to acceptable levels (Mathiassen 1993), this reduction may not be enough to overcome the risk of developing disorders. However, variation in the exposure (Physical load) can be helpful in reducing these disorders and interventions based on increasing the variation are always recommended (Fallentin, Kilbom et al.). Variation in the biomechanical domain is defined as the change in the exposure across time (Mathiassen 2006) and is associated with the reduction of similarity in load and postures during the task execution. Variation can be attained by modifying the job, by adding additional tasks or breaks or any other interventions such as increased force periodically or periods with zero rest, to reduce similarity in the load patterns (Mathiassen 2006).
Variation-related interventions that potentially can reduce the development of WMSDs can be classified into extrinsic and intrinsic, based on the type of intervention. If the interventions are focused on changing the working conditions external to the worker, such as additional breaks, it is called as an extrinsic variation. On the other hand, exertion variations that are conducted based on how a worker performs a task, such as a change in the posture and movement patterns are known as intrinsic variation (Srinivasan and Mathiassen 2012).
B.5.1 Extrinsic variation:
Introducing periods of brief rest during the work was considered as a principal way to introduce load variation. It has been demonstrated by many research groups in experimental studies (Rohmert 1973, Björkstén and Jonsson 1977) and office work (McLean, Tingley et al. 2001) that introducing rest breaks reduces the muscle fatigue. In an experimental study, (Yung, Mathiassen et al. 2012) demonstrated that working conditions with more gaps in the muscle activity (when EMG dropped to less than 1%), indicated that fatigue induced in such conditions was lower than a sustained isometric contraction as shown by measures such as increased endurance time (Yung, Mathiassen et al. 2012). This may be because breaks provide rest to continuously activated muscle fibers and thus increase the endurance time (Mathiassen 1993). However, care must be taken while introducing breaks, if not scheduled properly, the work flow of the workers can be disturbed.
As discussed, breaks can reduce the rate of fatigue in muscles, but increasing the length of the breaks will not further reduce fatigue development (Blangsted, Søgaard et al. 2004), implying that longer rest breaks between work, won’t provide complete relaxation of muscles. Active breaks are another source of intervention to reduce the development of WMSDs, where workers will perform some low-intensity job instead of taking rest. Comparing to complete rest, (Samani, Holtermann et al. 2009) indicated that active pauses provide higher variability, which would induce lower level of fatigue in the muscle. The effects of active pauses during the computer work were investigated by (Crenshaw, Djupsjöbacka et al. 2006) and concluded that oxygenation and blood flow increased during active breaks, but there was no evidence for reduced fatigue. In addition to this, pauses with gymnastic work (active pause) had beneficial effect in reduction of the fatigue during a computer task(Sundelin and Hagberg 1989). The reduction in the fatigue could be due to better removal of metabolites during the active pauses (Crenshaw, Djupsjöbacka et al. 2006). The removal of metabolites occurs during any kind of break but the rate of removal would be higher during active breaks when compared to the normal breaks, indicating that active breaks can help in alleviating the fatigue development.
Even though breaks reduce the fatigue, sometimes it is not feasible to provide a complete rest with no muscle contraction, muscles may not be reaching to zero force levels for prolonged periods of time, which doesn’t provide proper rest for muscles. Instead, to induce variability in the pool of active muscle fibers, loads can be increased periodically. (Falla and Farina 2007) demonstrated that these periodic increases in force level during an isometric contraction reduced the fatigue development. Moreover, brief increases in force have indicated reduced fatigue in trapezoidal muscles in an experimental study conducted by (Westad, Westgaard et al. 2003). Increased forces can help in MU substitution, thus avoiding the over usage of fatigued MU and reducing the risk of developing WMSDs. Depending on how you perceive, breaks can be considered as periods of no work or working periods with low exposure (Mathiassen 2006). So, the force variation, either low (breaks) or high (working period) exposure can be helpful in reducing the fatigue (Westad, Westgaard et al. 2003).
Along with the load variation, task variation can also be considered as an alternative method to tackle the development of WMSDs. This task variation can be regarded as a type of extrinsic variation because the conditions external to the worker are being changed. The first type of task variation is temporal variation where the activity type and the amount of work done are constant but the working pattern over time changes, i.e., cycle time (CT) changes and duty cycle (DC) is kept constant (Luger, Bosch et al. 2014). CT is the sum of the exercise period and the rest period where as DC is the ratio of exercise period to the CT (MathiassEn and Winkel 1991). In the review conducted by (Luger, Bosch et al. 2014), lower CTs, which is temporal variation, indicated positive effects of reduced fatigue but most of these studies are based on subjective measures such a ratings of perceived discomfort (RPD). In addition to this, (Rashedi and Nussbaum 2016) used diverse measures such as RPD, performance (force fluctuations) and muscle capacity (MVC and low-frequency twitch responses), and indicated that lower CT (i.e., 30 sec) resulted to lower levels of fatigue, comparing to CT = 60 sec, which may be helpful in reducing the WMSDs.
The other kind of task variation is activity variation where the force patterns and movement patterns are changed in an activity, commonly known as job rotation. Job rotation is the activity of changing the tasks the worker is performing every day and the evidence that job rotation can reduce the development of WMSD is limited (Srinivasan and Mathiassen 2012). Few research groups such as (Rissén, Melin et al. 2002, Kuijer, De Vries et al. 2004, Yung, Mathiassen et al. 2012) indicated in their works that, introducing job rotation reduced LMF and pain due to variation. On the other side, (Keir, Sanei et al. 2011, Horton, Nussbaum et al. 2012) indicated that the effects of job rotation were dependent on the intensity of the work as it reduced the fatigue development only for high-intensity tasks but not for low-intensity tasks. So the effects of job rotation on the reduction of fatigue are ambiguous (Luger, Bosch et al. 2014) and if the job rotation reduces variability, it can even be detrimental.
B.5.2. Intrinsic variation:
As defined earlier, intrinsic variation is performing interventions to change the way a worker is performing a job by for example modifying his postures and movement patterns. This can cause invariable task performance and would be difficult to implement in real life. Despite its difficulty in implementation, researchers are still focusing on methods of increasing motor variability due to its role in reduction of WMSDs. Motor variability is the type of intrinsic variation in which variability is present in MU recruitment due to changes in movement and postures. MU variation, in a general sense, is giving the overloaded MUs some time to relax by shifting the muscle activity. This shift can be obtained by activating different muscles in the same synergy, different regions of the same muscle or different MUs in the same muscle (Mathiassen 2006). As discussed in the WMSDs section earlier, activation of different MUs in the same muscle is called as MU substitution, and this can delay the occurrence of LMF (Srinivasan and Mathiassen 2012).
Motor variability also helps in preserving the performance during fatiguing tasks by modifying the MU recruitment patterns. Even though few studies have indicated reduction in LMF due to the presence of motor variability (Westad, Westgaard et al. 2003, Falla and Farina 2007, Yung, Mathiassen et al. 2012), few studies indicated a reduction in performance and increased LMF, due to motor variability (Huysmans, Hoozemans et al. 2008, Missenard, Mottet et al. 2008). These contrasting results regarding the effects of LMF on the performance may be due to a difference in the individual capacities and effects of fatigue being task specific (Srinivasan and Mathiassen 2012).
In conclusion, it can be understood that both load variation and motor variability helps in the reduction of LMF. As discussed above in the WMSD section, LMF can be a precursor for the WMSDs and interventions in the occupational setting can be focused on increasing the variability which can reduce LMF, thereby reduces the chance of WMSDs.