Forearm support is associated with reduced upper arm muscle activity during direction-specific isometric wrist exertions in young healthy adults

Upper arm muscles exhibit high muscle activity during distal upper limb motor tasks, which may contribute to musculoskeletal disorders of the upper arm. More specifically, elbow flexor muscle activity is elevated during upwards and leftwards wrist exertions, while downwards exertions produce high elbow extensor activity. While some evidence suggests that forearm supports may reduce the activity of these upper arm muscles, this has not been extensively investigated, nor is it clear how support might interact with wrist exertion direction. Thus, the purpose of the present study was to determine whether forearm support is associated with a reduction of upper arm muscle activity during isometric wrist actions in various exertion directions (up/down/left/right), the findings of which will help inform ergonomists in the assessment, design, and interventions of upper limb workplace tasks. Participants were seated with their forearm either supported or unsupported within a custom-built apparatus that could position an adjustable force transducer above, below, or to the right/left of the participant's hand. Surface electromyography was recorded from the biceps brachii, triceps brachii, and brachioradialis of the right arm while participants performed randomized trials of isometric wrist flexion, extension, and radial/ulnar deviation against the transducer in different directions. Across nearly all conditions, forearm support was associated with a reduction of upper arm muscle activity, but this reduction was dependent on wrist exertion direction. The elbow flexors experienced the largest reductions during upwards and leftwards wrist exertions, while the triceps brachii demonstrated the largest reduction during downwards exertions. These findings highlight the need for ergonomists to consider support placement when attempting to reduce upper arm muscle loading in workplaces or tasks where upper arm musculoskeletal disorders are common.