Effects of grip type and wrist posture on forearm EMG activity, endurance time and movement accuracy

Abstract

Musculoskeletal Disorders (MSDs), especially those of the upper limb are a common concern in modern industry, and physical risk factors such as force and posture are linked to their causation. The effects of the combination of forceful gripping or grasping (especially pinch grips) and awkward postures should be considered in the causation of MSDs. Current guidelines recommend that wherever possible a power grip should be used instead of a pinch grip. However, in an industrial context these grip types are not easily interchangeable. There is a force-precision trade-off; more powerful grip types are less precise, so the selection of an incorrect grip type will affect quality and performance of a task. There are further negative implications for quality and performance where task factors such as precision, speed and repetition are present. The purpose of this study was to investigate the effects of wrist posture, force and grip type on operator muscle activity, endurance and task performance.A laboratory study was devised to test three hypothesises: 1. grip type (2 precision & 1 power grip) and wrist posture (neutral & flexed) affect forearm muscle (ECU, ECR, FCU, FCR) activity; 2. grip type and posture deviations affect grip endurance; 3. grip type, wrist posture and grip exertion level affect task performance.ANOVA revealed grip type had a significant effect on muscle activity (p = 0.0001) for all 4 muscles studied. For 50% MVC exertions, muscle activity was highest for power grip, followed by chuck, followed by pulp pinch. Posture and the posture × grip type two way interaction both had significant effects on endurance time (p = 0.01 and p = 0.021 respectively) with higher times for the neutral wrist. Grip type and posture had significant effects on precision performance (p < 0.05) but force level did not. Task performance was higher for the neutral wrist and for chuck and pulp grips. Relevance to industryIn many industrial tasks, precision and productivity/performance are of critical importance, but are often contradictory. Higher precision requirements often slow tasks and reduce output. In repetitive industrial tasks grip type has important effects on precision but also maximum strength and hence risk of injury. The results from this study highlight the effects of grip type in combination with deviated wrist postures, on forearm muscle loading and precision task performance.