Posture and hand load alter muscular response to sudden elbow perturbations

Joint stiffness and stability are reliant on coordinated muscle activity which may differ depending on initial posture and loading during sudden perturbations. This study investigated the effects of arm posture and hand load on muscle activity during perturbations of the arm. Fifteen male participants experienced perturbations to the wrist causing elbow extension using a combination of three body postures (standing, supine, sitting) and three hand load conditions (no, solid, and fluid loads), with known and unknown timing. Surface EMG was collected from eight muscles of the right upper extremity. The response to sudden loading was examined using muscle activities pre (baseline) and post (reflex) perturbation. During the baseline period, known perturbation timing resulted in greater muscular activity than for unknown timing, while the opposite was found for the reflex period. During the reflex period with fluid load, biceps brachii and brachioradialis demonstrated increased activity of 2.4% and 4.0% of maximum respectively, from supine to standing. During the reflex period, the fluid load resulted in forearm co-contraction 23% and 47% greater than the solid and no load conditions. Body orientation and hand loading influenced muscular response to elbow perturbations. Muscle co-contraction at the elbow during known timing suggests a contribution to elbow joint stability that may reduce injury risk caused by sudden elbow loading.