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.

The purpose of the present study was to examine characteristics of muscle anatomical cross-sectional area (CSA) for different regions from proximal to distal parts of each muscle of the hamstring muscles in high-level sprinters, and to examine the relationship with those and the sprint performance. The CSA of the semitendinosus (ST), semimembranosus (SM), biceps femoris long head (BFL) and biceps femoris short head (BFS) at the four different region of hamstring muscles for twenty sprinters (SPRINT) and twenty healthy male control subjects (CTRL) were measured by using B-mode ultrasonography. The measured regions were divided into four parts from proximal to distal parts (PRO1, PRO2, DIS2, DIS1). The results clearly showed that absolute CSA values in distal parts for all muscles together with PRO2 in ST were greater in SPRINT than in CTRL. When relative CSA values to the entire hamstrings muscles in each region were compared, only relative CSA at PRO1 in ST was greater in SPRINT than in CTRL, conversely, that at proximal regions in BFL and distal regions in BFS were smaller in SPRINT. In the relationships with sprint performance, the CSAs at PRO1 and PRO2 in ST and at PRO1 in SM were only related negatively. These results suggest that distal parts of hamstring muscles for SPRINT may be characteristics for sprint runners. However, the movements related to the specific hypertrophy (PRO1 and PRO2 in ST, PRO1in SM) may play important roles of the improvement of their sprint performance.