EFFECT OF RESIDUAL FORCE AND STRETCHING VELOCITY ON CONCENTRIC CONTRACTION VELOCITY

Introduction. The muscle tension enhances during muscle stretch resulting in an increased working capacity of the muscle during the subsequent muscle shortening using elastic energy stored in serial elastic elements (SEC)(Cavagna et al. 1968, Komi and Bosco 1978). Also, it is established that after stretch the muscle tension remained elevated for some seconds (Herzog and Leonard 2002). If the enhanced positive work is due to the elastic energy stored in SEC, then shortening velocity depends on the residual force rather than on the coupling time between eccentric (EC) and concentric contraction (CC). In our previous study we found that using high stretching velocity the torque production of the muscles sharply dropped then increased within 300-500 ms and remained constant for 2-3 s. We may assume that the decreasing or increasing or steady residual muscle torque would provide different conditions for producing work during concentric contraction. Material and methods. Six physically active male subjects were recruited in this study. The knee extensors of the non-dominant leg were used to test our hypothesis. The subjects were placed in a dynamometer (Muli-Cont II) in sitting position. Isometric torque (Mic) was performed at at joint angle of 60 and 90 degr. Muscle stretch was carried out at a pretension of 0.6Mic by applying 30 degr/s and 300 degr/s constant velocity flexing the knee between 60 and 90 degr. Having terminated the stretch the knee was allowed to extend immediately (0 s), after 0.2 s or 2 s delay. During concentric phase the subjects worked against the inertia of the lever arm and the lower leg, i.e. the velocity was not controlled. The subjects were instructed to exert force against the lever arm with the highest effort during the coupling time(CT). Maximum active torque enhancement (ATE), residual torque (RT) at the start of concentric contraction and maximum angular velocity (MAV) at concentric phase was determined. Results. Applying 3061616;/s ATE was 10.8%, 8.4% and 11.7% greater than Mic (90 degr) at CT 0, 0.2 and 2 s between EC and CC, respectively. RT was 9.9%, 3.8% greater and 3.7 less than Mic (90 degr) at CT 0, 0.2 and 2 s, respectively. MAV was almost the same at each CT condition (322.0±41.9; 318.6±36.7; 298.2±52.6). Applying 300 degr/s ATE was 2.6%, 7.6% and 4.0% greater than Mic (degr) at CT 0, 0.2 and 2 s. RT was 27.0%, 18.0% and 10.7% less than Mic (90 degr) at CT 0, 0.2 and 2 s, respectively. MAV was the lowest (197.8± 32.8 degr/s) at CT 0, and increased in the function of the increasing CT (0.2 s: 268.4± 27.3 degr/s; 2 s: 294.2± 43.9 degr/s). Discussion. The delay between eccentric and concentric contraction did not influence either residual torque or contraction velocity when low stretching velocity was applied. However, when the muscle tension dropped immediately after stretch due to the high stretching speed and the muscle shortening startted at this phase, the contraction velocity of the muscle was significantly less as compared to the situation at which the tension increased despite the muscle tension was similar. We may also assume that there is an optimum stretching velocity which may result greatest active and passive force enhancement and as a consequence greater positive work.