Fatigue during stretch-shortening cycle exercises: changes in mechanical performance of human skeletal muscle.

Stretch-shortening cycle (SSC), which is a normal contraction behavior of muscle, was used as a model to investigate muscular fatigue. Nine male volunteers were subjected to 100 repeated and exhaustive SSC contractions of the forearm extensors using a special sledge apparatus incorporating a force plate system. The fatigue contractions were performed on submaximal levels but the before-after comparison also included maximal drop-jump condition on the sledge as well as falls on to the floor. The results indicated that in the 100 submaximal SSCs the fatigue was characterized by increases in the contact times for both the eccentric and concentric phases of SSC, but the influence was more pronounced on the concentric part. The force-time curves during contact on the platform were influenced by fatigue so that the initial force peak became higher and the subsequent initial drop of force more pronounced. During submaximal and maximal drops, the angular velocities changed in the two phases of SSC. With progressing fatigue, the eccentric maximal angular velocity increased and the corresponding concentric velocities decreased. These changes were accompanied by slight changes in the elbow joint mechanism with respect to the contact, release, and maximal flexion angles. The results suggest that repeated SSC induces fatigue and the fatigue effects on the mechanical behavior of the muscle are very much similar to those induced by either isometric or concentric fatigue contractions. However, the transfer of the energy between eccentric and concentric phases was drastically reduced and this implies that SSCs can be used effectively to examine the fatiguability of the system regulating muscle stiffness during exercise.