Effects of previous muscle contractions on cyclic movement dynamics.

In addition to muscle elastic energy, enhancement of movement performance in a stretch-shortening cycle could also be due to an increase in initial muscle force during the stretching phase. This hypothesis was tested by examining 9 male physical education students during maximum voluntary knee extensions performed with and without previous knee flexion. In both conditions movements were performed with various external loads. In addition, the force-velocity curve (FVC) parameters of the knee extensor muscles were also determined. As simple model of a muscle impulse was constructed in order to select independent biomechanical variables relevant to movement dynamics. The experimental results demonstrated that previous knee flexion enhanced the maximum angular velocity of knee extension. This effect decreased with increasing movement duration (i.e. increased external load), as well as giving positive correlation coefficients between the magnitude of this effect and the rate of development of knee extensor tension. These results are discussed in relation to a model of the dynamics. It is shown that previous muscle contractions performed during braking in the negative movement phase might play an important role in enhancing performance in cyclic movements. This role would be especially important in transient contractions of primarily slow twitch fiber muscles.