ABSTRACT
The purpose of the present study was to examine the importance of the agonist muscle activity of the post-impact 30 ms phase during drop jump (DJ) for effective rebound performance by comparing those of sprint runners and swimmers. The eight sprint runners (SPRINT) and twelve swimmers (SWIM) were participated in this study. They performed DJ from a 0.3-m height box with maximal rebound efforts. Electromyograms (EMG) of the lower leg muscles (medial gastrocnemius [MG], soleus [SOL] and tibialis anterior [TA]), and vertical ground reaction force together with kinematic data were measured simultaneously during DJ. In addition, the onsets of fascicle stretching of the MG and SOL muscles were measured by using high-speed ultrasonography (521Hz) during DJ. The onsets of the fascicle stretching of SOL during DJ were not significantly different between SPRINT and SWIM (15 ± 7 ms and 16 ± 6 ms, respectively). During DJ, SPRINT showed onset of the SOL EMG before the ground contact (-26 ± 19 ms). Meanwhile, SWIM showed the onset of the SOL EMG after the ground contact of DJ (16 ± 19 ms). These results suggest that the SOL muscles for SWIM cannot be fully-activated during the braking phase. Consequently, the rate of force development during the braking phase of DJ and subsequently rebound height could be reduced in SWIM.
ABSTRACT
In the context of energetics related to a pendular model, the mechanical power (<I>W</I>) and ‘pendular motion efficiency’ (<I>PME</I>) were determined during walking of the subjects who consist of 37 healthy elderly women (65-85 years) and 21 young women (18-25 years) . Using a force plate, the potential and kinetic energies of the body's centre of mass were measured at various constant speeds. Walking speeds were selected and controlled by a newly devised pace-maker. <I>PME</I>, which is equivalent to ‘% recovery’ by Cavagna (1976), indicates a sort of efficiency in transforming potential energy into kinetic energy and vice versa. The external power to accelerate the body (<I>Wext</I> ), which is thought to be supplied by muscles, increased with walking speed, and the rate of increase in <I>Wext</I> tended to be greater in the elderly than in the young subjects. It was noted that the maximal <I>PME</I> values at the optimum speed in both age groups were comparable, but <I>PME</I> values in the elderly decreased more markedly than in the young subjects as walking speed deviated from the optimum. This fact suggests that an adaptability to different walking speeds reduced in the elderly population.