EFFECTS OF REPETITIVE LIFTING ON KINEMATICS - INADEQUATE ANTICIPATORYCONTROL OR ADAPTIVE-CHANGES

In the present study, the effects of repetition on the kinematics in d iscrete lifting were studied in 10 subjects who lifted a barbell weigh ing 10% of body mass at a determined speed and along a determined traj ectory 630 times during about 40 min. Three-dimensional (3-D) kinemati cs of the feet, lower and upper legs, pelvis, and trunk were recorded in the first 3 and the final 3 lifting movements of each set of 70 lif ts. Over time, trunk extension velocity in the initial 250 ms of the l ifting movement decreased, reaching negative (increasing flexion) valu es in most subjects. In contrast, hip extension velocity increased. Th ose changes resulted in an increased phase lag between hip and trunk e xtension. Also, over time, subjects started the lifting movement with their legs more extended and their trunks further flexed. Finally, the motion of the trunk around its longitudinal axis (twisting) increased . The increase in phase lag between hip and trunk extension is interpr eted as a consequence of fatigue-more specifically, as the result of a decreased rate of force development of the back muscles. The change i n initial posture more likely is an adaptation that functions to retar d further fatigue development.