Anatomical and mechanical changes following repetitive eccentric exertions.

BACKGROUND Submaximal eccentric exertions occur occupationally when rapidly rising tool-generated forces exceed the operator's capacity to react against them. The purpose of this study was to investigate the effects of short duration repetitive submaximal eccentric forearm exertions at levels comparable to industrial power hand tool use on dynamic mechanical properties (stiffness, effective mass and damping) and on forearm edema. METHODS This study investigated changes following short term repetitive submaximal eccentric exertions comparable to occupational levels. Eight male participants exercised eccentrically for 30 min at 50% of isometric maximum voluntary contraction forearm supination in a posture and loading similar to power hand tool use in the workplace. Dynamic mechanical properties (stiffness, effective mass and damping) of the upper limb were measured before, immediately following, and daily for three days after the activity. An MRI scan to assess edema was also performed for five of the participants before, on day one and day three following the activity. FINDINGS Mechanical stiffness decreased 51% (P < 0.05) and effective mass decreased 43% (P = 0.052) immediately following eccentric exercise. Average isometric strength also decreased 42% immediately following exercise (P < 0.01) and pain persisted for two days. The recovery of static strength however was not correlated with changes in mechanical stiffness (r = 0.56) or effective mass (r = 0.30). The exercised arms had a 360% increase (P < 0.01) in supinator-extensor T(2) relaxation time difference, a quantifiable measure of edema, one day after exercise while the non-exercised arms had no significant changes. INTERPRETATION Changes in both T(2) relaxation time, indicative of edema, and forearm mechanical properties, were observed following short duration submaximal repetitive exercise. If similar changes in dynamic mechanical properties of the upper extremity occur following repetitive submaximal eccentric activity in the workplace, they could negatively impact the ability of the arm to react to rapid forceful loading during repetitive industrial work activities and increase mechanical loading of the upper limb.

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