Effect Of Stretch-shortening Cycle Exercises On The Mechanical And Neuromuscular Properties

Abstract

PURPOSE To compare the effects of Stretch-Shortening Cycle (SSC) exercises on the mechanical and neuromuscular properties during the vertical jump for non-experienced jumpers and experienced jumpers. METHODS Sixteen subjects were recruited for the experiments. The First group (EJ), had at least 10 years of experience in jump training and therefore constituted experienced jumpers. The second group (NEJ) had at least five years of experience in the college sports but no prior experience in jump training. For the fatigue exercise, subjects were instructed to perform a long lasting SSC exercises inducing 50% of decraese in Maximal Isometric Force (MVC). For the testing protocols a leg press was used to measure isometric force and dynamic concentric movements at 40%, 50% and 60% of each subject's 1RM. A force plate was used to measure force, power, and height during the squat jump (SJ) and the countermovement jump (CMJ) on both feet. A 3-D system was used to analyze kinematic data for the most prominent protuberance of the hip greater trochanter, lateral femoral epicondyle of the knee, and the ankle lateral maleolus during each jumping exercises. Joint moment and stiffness of the knee was calculated using an inverse dynamics approach given segmental displacements, velocities, and accelerations obtained from the kinematic record and the force plate data. Myoelectric signals were recorded for the following muscles: vastus medialis, vastus lateralis, and biceps femoris for the jumping and leg press exercises. RESULTS For both groups, the SSC exercise suppressed voluntary isometric force and maximal concentric power (p<0.0006) and SJ performance (p<0.008) while only the experienced jumpers maintained their performance during the CMJ after fatigue exercise. EMG analysis (as interpreted through the root mean square and mean power frequency values) showed that the maximal concentric power, SJ and CMJ were performed similarly before and after the fatigue exercise for both the EJ and NEJ groups. CONCLUSIONS The major finding in our study was that non-experienced and experienced jumpers differ in their patterns of fatigue (as interpreted through the jump performance findings and the kinematic data). It appears that for the experienced jumpers, fatiguing the knee extensor did not affect the CMJ height. The experienced jumpers re-optimize knee extensor muscle recruitment by altering their jump strategy during the CMJ (as interprted through the increase in the eccentric and transition phases). Exhausting SSC exercise induced local muscle impairment and a decrease in muscle performance, which is not simply a direct effect of central command fatigue.