Behaviour of vastus lateralis muscle–tendon during high intensity SSC exercises in vivo

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Abstract
The interaction between fascicle and tendinous tissue of human vastus lateralis muscle was investigated during varying intensity stretch-shortening cycle (SSC) jumps performed on a sledge apparatus. Eight subjects performed single leg squat (SJ) and drop jumps (DJ) from a constant dropping height but to different rebound heights. The fascicle length of the vastus lateralis muscle (VL) was determined from real-time ultrasonography during the movement. Tendon length changes were calculated by subtracting the horizontal part of the fascicle length from the muscle-tendon unit (MTU) length. Simultaneously, kinematic, kinetic and electromyographic data were recorded from leg muscles. In addition, the in vivo patella tendon force was measured from one subject during the trials. In all DJs, where MTU was stretched prior to shortening, the fascicle and tendinous tissue of the VL also underwent a SSC. The fascicle lengths decreased and the recoil of tendinous tissue increased with increased rebound intensities (P<0.05). The force-velocity curves obtained from the MTU showed the expected force-velocity relationship for SSC activities, demonstrating performance enhancement. However, the increased MTU power during the shortening phase of the movement was due primarily to the enhancement of the tendon compartment. The results of this study show that, at higher rebound intensities, the fascicle is controlled during the braking phase in a distinct manner so that the effective recoil of the tendon is possible during the final push-off phase. In addition, the results suggest that the behaviour of fascicle length change depends on the muscle in question in addition to the movement intensity.