"There is a possibility that muscle fiber composition has some influence on jump performance. Bosco et al. (5) found that there were significant correlations between jump height and muscle fiber composition. They argued that fast-twitch fibers in their knee extensor muscles could use more elastic energy storage in vertical jumps. However, the present results would be strongly associated with the tendon structures, not with muscle fiber composition. The reason for this idea was that there were no significant differences between the two groups in jump height and relative peak reaction force at the push-off phase. Furthermore, both groups included some sprinters (possibly having higher %fast-twitch fibers) and distance runners (possibly having higher %slow-twitch fibers). Therefore, although the possibility of the influence of muscle fiber composition on jump performance cannot be excluded, it seems reasonable to suppose that the elastic properties of tendon structures considerably affect the differences in vertical jump height with and without countermovement."
J Appl Physiol 87: 2090-2096, 1999