Upper limb mechanical changes following short duration repetitive eccentric exertions

Abstract

Background. Power hand tool use is considered a risk factor for upper extremity musculoskeletal disorders. It is unclear if submaximal eccentric activity inherent to power tool use adversely affects the mechanical properties of muscle. Methods. This study investigated in vivo changes in human upper limb dynamic mechanical properties following exposure to short-term repetitive submaximal eccentric exertions that are similar to operating an industrial power hand tool. Eighteen subjects (12 males and 6 females) were assigned to one of three exercise groups (isometric, eccentric or control) and exercised 10 min for 60 repetitions at 50% of isometric forearm supination maximum voluntary contraction. Supination strength and dynamic mechanical properties (stiffness, effective mass, and damping) of forearm rotation, modeled as a single-degree-of-freedom system during maximal exertion, were ascertained prior to exercise, immediately following exercise and 24 h later. Findings. Strength decreased for the isometric (17%) ( P < 0.05) and eccentric (34%) ( P < 0.01) groups following exercise. Only the eccentric exercise group had a reduction in mechanical stiffness (53%) ( P < 0.01) and effective mass (58%) ( P < 0.05). The other groups had no changes in mechanical properties. Interpretation. The change in mechanical properties following repetitive submaximal eccentric activity could negatively impact the ability of the arm to react to rapid forceful loading during repetitive industrial work activities and may indicate mechanical strain on the upper limb.