EFFECTS OF INITIAL KNEE ANGLES AND TYPES OF MUSCLE STREICHING ON VERTICAL JUMPS 1335

Abstract

This study investigated whether different initial knee starting angles and types of muscle stretching have significant effects on vertical jumps. Twelve subjects randomly performed maximum vertical jumps with a counter movement(CM) or with a static squatting (SS), from 30°, 60°, and 90° angles confined with a knee brace. A Kistler force platform, a Lafayette Electomyography (EMG) system and a video motion analysis system synchronized by a timing device, were used to record (200 Hz) the vertical jumping motion. The integrated EMG activities (EMG) from gastrocnemius (M1) and rectus femoris(M2), were collected and normalized with a maximum voluntary contraction. The maximum force was normalized to the body weight (FBW). The video film was digitized and kinematical variables were calculated. A repeated measure of 2 by 3 factors ANOVA with Scheffe test were employed to examine the differences across muscle stretching and knee angle conditions in the vertical jumps. The results showed that the jump height was significantly (p<.05) correlated with M1 and M2, but not with FBW and positive impulse (PI). Stretching type CM and SS had no significant effect on FBW, whereas the different knee angles had a significant (p<.05) effect on FBW. From SS to CM, the FBW for 30° decreased from 2.03 to 1.69, however, the FBWs for 60° and 90° increased. This result indicated that a counter movement does not always induce the stretch-shortening cycle if the range of motion is short and does not allow a sufficient stretching. The time of positive impulse is significantly shorter at 90° than at 30°, and this may conform with the literature which supports that the rate of increase of force is a more reliable predictor than the FBW