• Pedro Mil-Homens Santos
  • P. Pezerat-Correia
  • A. Veloso


The combination of eccentric and concentric actions forms a natural type of muscle. function called a stretch-shortening cycle or SSC (Komi, 1984). Such SSC exercises are more efficient than pure concentre exercises (Aura and Komi, 1986). The prestretching increases the stiffness of the muscle-tendon complex and thus favors conditions that allow performance potentiation in the subsequent concentric phase. The fact that both the facilitator). and inhibitory sensory inputs from the muscle take part in the stiffness regulation implies that there is a great potential for adaptation. For training purposes, it is relevant that training could cause not only quantitative changes of the neuronal input to the muscle but also a qualitative shifting in the electromyogaphic (EMG) patterns (Schmidtbleicher et al. 1988). The present study was designed to investigate the changes in force-time characteristics in neuronal activation patterns before and after a training period using SSC exercises. Thirteen healthy males were involved in a 4 week training program. The subjects exercised 3 to 4 times per week. They performed reactive Dl from their best drop height. In each training unit they exercised 3 - 4 sets with 10-20 repetitions each and a rest interval of 5 minutes illbetwee11 sets. The testing procedures took place before and after the mining period. The subjects performed squat jumps (SJ), conter-movement jumps (CMJ) and reactive drop jump (Dl) exercises from the heights of 25, 40, 55 and 70 cm. The vertical ground reaction forces, the angular displacement of the knee and ankle joints as well as the surface electromyograms (EMG) of the triceps surae muscles (GAS and SOL), vastus medialis (VM) and biceps femoris (RF) were recorded. Each jump was performed six times. After normalization, force, angle displacement and EMG signals, were averaged for each jumping condition. The EMGs were fullwave rectified and integrated (iEMG) over different functional phases (Dietz et a].. 1979): Preactivation phase (PRE) (looms before ground contact); Reflex Induced Activation phase (i.e. the activation phase from 40 ms to 120 ms after impact); Late EMG Response phase (LER) (activity from 120 ms until the end of contact). The significance of the differences between means were tested with t-test for paired samples. The results of the height of rise of center of gravity (HRCG) showed non significant differences in SJ and CMJconditions. In DJ conditions the HRCG incresed from 38.1f3.9 cm ro 41.5f4.2 cm (p<0.001) inDJ25, from 39.2f1.5 cm to 42.0i4.8 cm (p<0.001) in DJ40, from 3X.5f4.7 cni to 40.9f5.7 cm (p