Upper arm muscle activity is influenced by both forearm posture and wrist exertion direction during isometric wrist flexion and extension.

The purpose of this study was to determine how wrist exertion direction and forearm posture independently influence upper arm muscle activity during isometric wrist contractions. Surface electromyography was recorded from three muscles of the upper-limb: biceps brachii, triceps brachii, and brachioradialis. Participants were seated with their forearm supported in one of three postures (supinated/neutral/pronated) with an adjustable force transducer that could be placed either above, below, or to the right/left of the participant's hand. Participants performed randomized trials of isometric wrist flexion or extension at five relative intensities: 20, 40, 60, 80, or 100% of maximal force. Trials lasted 4.5 s and both wrist force and electromyography data were assessed. In general, the elbow flexors were more active during wrist flexion, while the triceps were more active in wrist extension, but this pattern reversed in certain forearm postures and wrist exertion directions. Both forearm posture and wrist exertion direction resulted in unique effects on upper arm muscle activity. These findings suggest that muscle activity of the upper arm muscles is influenced independently by both posture and force direction, which should be carefully considered by both motor control specialists and ergonomists.