Mechanical output about the ankle joint in isokinetic plantar flexion and jumping

BOBBERT, M. F. and G. J. VAN INGEN SCHENAU. Mechanical output about the ankle joint in isokinetic plantar flexion and jumping. Med. Sci. Sports Exerc., Vol. 22, No. 5, pp. 660–668, 1990. The purpose of this study was to compare for a group of ten subjects the mechanical output about the ankle during isokinetic plantar flexion with that during one-legged vertical jumps. For evaluation of the mechanical output the plantar flexion moment of force was related to the angular velocity of plantar flexion. The relationship for isokinetic plantar flexion was obtained using an isokinetic dynamometer; that for plantar flexion in jumping was obtained by combining kinematics and ground reaction forces. It was found that, at any given angular velocity of plantar flexion above 1 rad·s−1, the subjects produced much larger moments during jumping than during isokinetic plantar flexion. In order to explain the observed differences in mechanical output about the ankle, a model was used to simulate isokinetic plantar flexion and plantar flexion during jumping. The model represented both m. soleus and m. gastrocnemius as a complex composed of elastic tissue in series with muscle fibers. The force of the muscle fibers depended on fiber length, shortening velocity (Vfibers), and active state. The input variables of the model were histories of shortening velocities of the complexes, determined from kinematics, and active state. Among the output variables were Vfibers and plantar flexion moment. The simulation results were very similar to the experimental findings. According to the simulation results there are two reasons why at the same angular velocity of plantar flexion larger moments were produced during jumping than during isokinetic plantar flexion. One reason is that, in simulated isokinetic plantar flexion at high angular velocities, the duration of the movement is so short that the active state cannot rise to its maximum, and the moment therefore remains submaximal. The second reason is that in jumping Vfibers is lower than in isokinetic plantar flexion, so the muscle fibers are operating in a more favorable region of their force-velocity relationship. It was concluded that the mechanical output about joints during explosive movements cannot be explained from the results of isokinetic experiments without the help of an appropriate model.