The Influence of Hand Posture on Upper Arm Muscle Activity During Isometric Wrist Exertions Is Dependent on Exertion Direction and Wrist Action.

Recent studies have demonstrated that isometric wrist actions performed with an open hand have a direction-dependent influence on upper arm muscle activity. It is presently unclear if these findings are translatable to motor tasks requiring a closed hand, as it common in most hand-object interactions. Thus, the purpose of the present study was to determine the influence of hand posture on upper arm muscle activity during isometric wrist exertions in different directions. Participants were seated with their forearm unsupported within a custom-built apparatus that could position an adjustable force transducer either above, below, or to the right/left of the participant's hand. Muscle activity was recorded from the biceps and triceps brachii while participants exerted 50% of their maximal force against the transducer in one of four randomized wrist actions: flexion, extension, or radial or ulnar deviation. Results demonstrated that a closed-hand posture produced greater biceps (open: 4.92 ± 4.17% maximal voluntary excitation [MVE], closed: 8.18 ± 6.44% MVE, p < .001) and triceps brachii (open: 16.91 ± 8.0% MVE, closed: 19.09 ± 8.54% MVE, p = .002) muscle activity when exerting wrist forces downwards but less activity when forces were exerted to the right. The influence of wrist exertion direction on upper arm muscle activity also depended on hand posture. These findings add valuable new evidence that a neural coupling likely exists between the distal and proximal regions of the upper extremity. They also indicate that research into open-hand postures may not fully translate to motor tasks requiring a closed hand.