The influence of posture variation on electromyographic signals in females obtained during maximum voluntary isometric contractions: A shoulder example.

Surface electromyography (sEMG) is commonly used to estimate muscle demands in occupational tasks. To allow for comparisons, sEMG amplitude is normalized to muscle specific maximum voluntary contractions (MVCs) performed in a standardized set of postures. However, maximal sEMG amplitude in shoulder muscles is highly dependent on arm posture and therefore, normalizing task related muscular activity to standard MVCs may lead to misinterpretation of task specific muscular demands. Therefore, the purpose of this study was to investigate differences in commonly monitored shoulder muscles using normalized sEMG amplitude between maximal exertions at different hand locations and across force exertion directions relative to standard MVCs. sEMG was recorded from the middle deltoid, pectoralis major sternal head, infraspinatus, latissimus dorsi, and upper trapezius. Participants completed standardized muscle-specific MVCs and two maximal exertions in 5 hand locations (low left, low right, high left, high right, and central) in each of the four force directions (push, pull, up, and down). Peak sEMG was analyzed in the direction(s) that elicited the highest signal for each muscle. All muscles differed by location (p < 0.05). Latissimus dorsi had the greatest activation during pulls (32-135% MVC); upper trapezius and middle deltoid while exerting upwards (73-103% and 42-78% MVC, respectively); infraspinatus while pushing (38-79% MVC); and pectoralis major activation was the highest during downwards exertions (48-84% MVC). Normalization of location specific maximal exertions to standard muscle specific MVCs underestimated maximal activity across 90% of the tasks in all shoulder muscles tested, except for latissimus dorsi where amplitudes were overestimated in low right hand location. Normalization of location specific muscle activity to standard muscle specific MVCs often underestimates muscle activity in task performance and is cautioned against if the goal is to accurately estimate muscle demands.

The effects of hand force variation on shoulder muscle activation during submaximal exertions.

Upper limb injuries are highly prevalent in the workplace and new tools are needed to proactively design workstations to reduce injury risk. The objective was to characterize spatial, load and direction dependency of muscle activity for hand exertions in the upper limb workspace. Electromyographic signals were collected from 14 upper limb muscles during exertions for all combinations of 4 submaximal hand forces (20/30/50/60 N) in 6 cardinal (up/down/left/right/forward/backward) directions at 5 hand locations. Linear muscle activity increases accompanied increased hand forces. Total muscle activity increases between 20 and 60 N hand forces ranged by direction from 92% (downward) to 189% (right). Prediction equations for all muscles depended on hand force, and linear, quadratic and interaction permutations of hand location. Muscle activity associated with manual tasks is load, direction and spatially dependent. Equations developed to describe these complex relationships can be used to better design future and evaluate current occupational activities.

The Influence of Hand Location and Force Direction on Shoulder Muscular Activity in Females During Nonsagittal Multidirectional Overhead Exertions.

OBJECTIVE: We examined interactions of overhead work location and direction of force application on shoulder muscular activity. BACKGROUND: Overhead work tasks are common occupational stressors. Previous research has quantified influences of overhead work spatial placement and different force application directions but typically separately or exclusively for tasks done in the median plane. METHOD: Twenty female participants exerted 40 N of force in six directions (forward/backward, upward/downward, left/right) 150 cm off the floor while seated. An asymmetric pattern of 14 work locations spaced 15 cm centered directly overhead were evaluated. RESULTS: Force direction and work location strongly influenced mean muscle activity (F = 559, p < .01). Interaction effects existed between force direction and hand location in the transverse plane (F = 21, p < .01), with increases as high as 49% in normalized mean muscle activity. CONCLUSION: Backward exertions produced the highest mean overall muscle activity across hand force directions, exceeding 30% maximum voluntary isometric exertion (MVE) across work locations, with higher activation of anterior deltoid, biceps, infraspinatus, supraspinatus, and upper and lower trapezius. Downward exertions had the lowest mean overall activity, with <10% MVE across work locations. Altered (up to 47%) muscular activity occurred as exertions moved laterally from the origin, and increasingly forward hand positions generally yielded decreased mean overall activity for most force directions. APPLICATION: This study provides previously unavailable submaximal shoulder muscular activity data for a wide range of overhead tasks. As such, it enables novel work design considerations that include modifying existing overhead elements to reduce or redistribute associated muscular demands.

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

The relationship between overhead work and musculoskeletal health depends on multiple task and individual factors. Knowledge gaps persist, despite examination of many of these factors individually and in combination. This investigation targeted task variation, as parameterized by cycle time within a fixed overall workload. Participants performed an intermittent overhead pressing task with four different cycle time conditions while overall workload and duty cycle was held constant. Several manifestations of fatigue were monitored during task performance. Endurance time was influenced by cycle time with shorter cycle times having endurance times up to 25% higher than longer cycle times. Surface electromyography (sEMG) results were mixed, with two muscles demonstrating amplitude increases (middle deltoid and upper trapezius) that varied with cycle time. sEMG frequency was not influenced by cycle time for any muscle monitored, despite decreases for several cycle times. Trends existed for the influence of cycle time on time-varying reported discomfort (p=0.056) and static strength (p=0.055); large effect sizes were present (ηp(2)=0.31 and 0.27, respectively). The equivocal association of fatigue indicators and cycle time is analogous to the influence of other factors implicated in overhead work musculoskeletal risk, and probabilistic modeling offers a compelling avenue for integration of the known variation in the many factors that combine to inform this risk.