Effects of eccentric phase velocity of plyometric training on the vertical jump.

The aim of this study was to compare the effects of plyometric training performed with rapid or slow stretch contraction on jump performance and muscle properties. Thirty males between the ages of 19 and 22 volunteered for the 8-week experiment. Subjects were divided into the following three groups: training group 1 (TG1), training group 2 (TG2), and control group (CG). Each of the two experimental groups underwent a unique training regimen. For the first group (TG1, n = 12): from a standing position the subject flexed his knees to a 90 degrees angle with velocity standardized and controlled at 0.4 m/s and immediately performed a leg extension as quickly as possible. For the second group (TG2, n = 12): from a standing position, the subject flexed his knees to a 90 degrees angle with velocity standardized at 0.2 m/s and then performed a leg extension as quickly as possible. Each exercise consisted of six sets of ten repetitions with a barbell on the shoulders at 70 % of the maximal isometric force (1 RM). The 70 % load was modified at two-week intervals by evaluating a new 1 RM. Exercises were performed four times a week over the eight-week period. The third group (CG, n = 6), served as the control group. Maximal isometric force (MVC), maximal concentric force, squat jump (SJ) and counter movement jump (CMJ) exercises were performed before and after the training program. Subjects were filmed (100 Hz) and each jump was divided into three phases: eccentric phase (ECC), transition phase (TR) and concentric phase (CON). Surface EMG was used to determine the changes in the electromyographic (EMG) activity before and after the training program. There was an increase in leg extension force, velocity and electrical activity for SJ and CMJ for the two training groups (p < 0.05). However, TG1 showed a significant advantage in CMJ performance as well as a significant decrease in TR compared to the TG2 (p < 0.05). The results of this study show that when plyometric training is performed with rapid stretch contraction the CMJ jump height increases and the TR decreases.