Mechanical behaviour of rat skeletal muscle during fatiguing stretch-shortening cycles

The effects of muscle fatigue on the mechanical efficiency of muscle work performance were studied in situ, in stretch-shortening cycles that resembled physiological conditions. The experiments were performed on the medial gastrocnemius muscles of seven rats. To calculate efficiency, a simple Hill-type model was used, consisting of the contractile component (CC) and the series elastic component (SEC). Mechanical efficiency was defined as the external work done by the muscle-tendon complex divided by the external work done upon the muscle-tendon complex plus work done by the CC. The fatiguing protocol consisted of a l Hz stretch-shortening cycle with a 0.3 duty factor (i.e. the muscle was stimulated for 300 ms in each cycle). In total, 240 cycles were imposed. The mechanical performance (work, force and mechanical efficiency) was determined throughout the fatiguing process. Fatigue reduced muscle power and force production to approximately 17 and 30%, respectively. SEC stiffness increased, but did not have any effect on muscle performance and probably reflects altered cross-bridge characteristics. Mechanical efficiency was enhanced during the imposed stretch-shortening cycles from 0.74 to 0.83. As a result of the lower peak forces, proportionally less series elastic energy was wasted during the relaxation period of each strength-shortening cycle in the fully fatigued muscles. These results indicate that, in vivo, the timing of muscle contractions in the fresh and the fatigued state may be different in order to perform muscle work efficiently. After a period of 30 min recovery, the condition of the muscles had (partially) returned to the fresh state in all aspects.