The impact of work configuration, target angle and hand force direction on upper extremity muscle activity during sub-maximal overhead work

Abstract

Overhead work has established links to upper extremity discomfort and disorders. As many jobs incorporate working overhead, this study aimed to identify working conditions requiring relatively lower muscular shoulder load. Eleven upper extremity muscles were monitored with electromyography during laboratory simulations of overhead work tasks. Tasks were defined with three criteria: work configuration (fixed, stature-specific); target angle (-15 degrees , 0 degrees , 15 degrees , 30 degrees from vertical); direction of applied hand force (pulling backwards, pushing forwards, downwards, sideways, upwards). Normalised electromyographic activity was greater for fixed configurations, particularly when pulling in a backward direction (total activity = 108.3% maximum voluntary exertion (MVE)) compared to pushing down or forward (total activity ranging from 10.5 to 17.3%MVE). Further, pulling backwards at angles of -15 degrees and 0 degrees showed the highest muscular demand (p < 0.05). These results suggest that, if possible, positioning overhead work in front of the body with exertions directed forwards will result in the lowest upper extremity muscle demand. STATEMENT OF RELEVANCE: Overhead work pervades occupational settings and is associated with risk of upper extremity musculoskeletal disorders. The muscular intensity associated with performing overhead work was assessed in several combinations of work placement and hand force direction. These findings should have utility for designing overhead work tasks that reduce muscular exposure.

The influence of cycle time on shoulder fatigue responses for a fixed total overhead workload

“…Indeed, many tools have been applied in the effort to understand the multifactorial nature of the risk associated with overhead work. These include medical imaging (Chopp et al, 2010a), kinematics Ebaugh et al, 2006;Ludewig and Cook, 2000), elapsed time (Wiker et al, 1989), intramuscular pressure (Jarvholm et al, 1988;Palmerud et al, 2000), electromyography (Anton et al, 2001;Chopp et al, 2010b;Nussbaum et al, 2001;Sood et al, 2007), mathematical simulation (Dickerson et al, 2007a, psychophysics (Dickerson et al, 2006(Dickerson et al, , 2007b, and discomfort ratings (Sood et al 2007;Wiker et al, 1989). The current study complements these previous efforts by fusing multiple measures and producing a novel data set for future probabilistic models through the isolation of a single aspect of workload exposure.…”

“…Hand forces in the vertical plane (lift/press) are associated with higher manual strength than in the horizontal plane (Haslegrave et al, 1997). For submaximal static forces, pushing backwards elicited the highest muscular demand compared to all other vertical and horizontal pushes (Chopp et al, 2010b). Perceived pain, muscular fatigue, and endurance time consequent to performing arm intensive tasks were sensitive to duty cycle and cycle times, but total work demand was not held constant (Garg et al, 2006;Iridiastadi and Nussbaum, 2006).…”

EMG frequency spectral characteristics of inspiratory accessory muscles during the shoulder lifting motion in throwing athletes

“…(15,(26)(27)(28) This study had limitations as the absence of conducting EMG assessments in the amplitude domain , evaluating and comparing the spectral frequency characteristics with RMS values and integrated EMG signal; and absence of a specific assessment of throwing gesture, even considering the EMG a recommended tool for evaluating isometric movements and not isotonic.…”

Simulation of fatigue-initiated subacromial impingement: clarifying mechanisms

“…We are in the midst of quantifying the shoulder rhythm across the widest range of arm postures ever described for a single population, and are doing so dynamically and statically. This will include the formerly unmeasured influence of axial humeral rotation, and in overhead postures, which are associated with negative physical outcomes [95][96]. Our working hypothesis is that the rhythm is highly dependent on all humeral rotations, and that this will emerge from our modeling procedures.…”

“…A stochastic model, by considering variability in the population, could help elucidate some of the mechanisms supposed to increase occupational shoulder risk. For instance, working with the arms above shoulder height is known to increase upper extremity muscle demand (including the rotator cuff) which may cause muscle fatigue, as well as pain or discomfort [95][96][105][106][107][108]. It is unclear, however, how intrinsic geometric variability alters the impact of these and many other exposures on health outcomes.…”