Effects of repetitive dynamic contractions upon electromechanical delay

The effect of repeated maximal effort isotonic contractions on electromecha nical delay was studied. Over 4 days, 17 male subjects performed 400 rapid elbow flexion trials. The kinematics and surface electromyographic (EMG) ac tivity of the biceps brachii of these subjects were recorded. The period fr om the onset of the EMG until the beginning of movement was defined as the electromechanical delay. The period from the beginning of movement until th e end of the EMG was defined as the second component of the contraction. Ov er the 4 day period there was an increase in the speed of limb movement. Th e mean power frequency and the duration of the EMG during the electromechan ical delay did not change, while the root-mean-square amplitude increased. The duration of the EMG during the second component of the contraction rema ined stable. The mean power frequency and the root-mean-square amplitude of the EMG during the second component of the contraction increased with the speed of limb movement. We conclude that the faster contractions were a res ult of changes in motor unit recruitment during the second component of the contraction, rather than in the electromechanical delay.