Stiffness regulation during stretch-shortening cycle exercise

The present series of studies were designed to investigate the motor control mechanism during and after stretch-shortening cycle (SSC) exercise. Special emphasis was placed on stiffness regulation during SSC exercise. Neuromuscular mechanisms of the SSC action were studied in non-fatigued situations as well as in exhaustive, SSC exercise-induced fatigue conditions. In non-fatigue situations SSC performance can be characterized by a combination of pre-landing joint kinematics and post-landing muscle-tendon stiffness during the eccentric phase of the cycle. The centrally mediated pre-landing activity and eccentric action of the cycle seem therefore to play a role in regulating SSC. In the fatigue experiment, exhaustive SSC exercise induced delayed and slow recovery of performance and stiffness accompanied with delayed muscle damage, especially in the eccentric part of the cycle. Reduced and delayed stiffness was related to the damage-induced movement alteration of the SSC action. This may suggest that a pain-induced mechanism can have an inhibitory influence on the movement strategy. During SSC exercise, a great amount of alteration was observed in the prelanding kinematics, which indicates greater sensitivity on the part of the pre-landing motor control strategy for the functional requirement of the peripheral system. Prelanding ankle joint kinematics influenced post-landing stiffness and performance. In addition, alteration in post-landing stiffness was related to post-exercise delayed muscle damage. These interactions emphasize the chain reaction between pre-landing movement strategy, post-landing muscle mechanics and post-exercise performance. During the course of the SSC exercise, the neuromuscular adaptation process flexibly adapts across the joints and action phases of SSC. Consequently, the present series of studies indicates the possibility that the central nervous system maintains an optimum balance between the facilitation and inhibition of the peripheral system in terms of stiffness regulation during SSC exercise