maximal shortening velocity during plantar flexion effects of preactivity and initial stretching state we investigated the effects of the initial length of the muscletendon unit mtu and muscle preactivation on muscletendon interactions during plantarflexion performed at maximal velocity ultrasound images of gastrocnemius medialis were obtained on 11 participants in three conditions a active plantarflexion performed at maximal velocity from three increasingly stretched positions 10 20 and 30 dorsiflexion b passive plantarflexion induced by a quick release of the ankle joint from the same three positions and c preactivation which consisted of a maximal isometric contraction of the plantarflexors at 10 of dorsiflexion followed by a quick release of ankle joint during the active condition at maximal velocity initial mtu stretch positively influenced ankle joint velocity 153 and tendinous tissues shortening velocity 376 but not the shortening velocity peak value reached by muscle fascicle the muscle fascicle was shortened during the passive condition however its shortening velocity never exceeded peak velocity measured in the active condition muscle preactivation resulted in a considerable increase in ankle joint 1147 and tendinous tissues velocities 2391 although we observed a decrease in muscle fascicle shortening velocity during active plantarflexion at maximal velocity initial mtu length positively influences ankle joint velocity by increasing the contribution of tendinous tissues although greater initial stretch of the plantarflexors ie 30 dorsiflexion increased the passive velocity of the fascicle during initial movement its peak velocity was not affected as muscle preactivation prevented reaching the maximal muscle fascicle shortening velocity this condition should be used to characterize tendinous tissues rather than muscle contractile properties