Stretch-shortening cycle: a powerful model to study normal and fatigued muscle

Stretch-shortening cycle (SSC) in human skeletal muscle gives unique possib ilities to study normal and fatigued muscle function. The in vivo force mea surement systems, buckle transducer technique and optic fiber technique, ha ve revealed that, as compared to a pure concentric action, a non-fatiguing SSC exercise demonstrates considerable performance enhancement with increas ed force at a given shortening velocity. Characteristic to this phenomenon is very low EMG-activity in the concentric phase of the cycle, but a very p ronounced contribution of the short-latency stretch-reflex component. This reflex contributes significantly to force generation during the transition (stretch-shortening) phase in SSC action such as hopping and running. The a mplitude of the stretch reflex component - and the subsequent force enhance ment - may vary according to the increased stretch-load but also to the lev el of fatigue. While moderate SSC fatigue may result in slight potentiation , the exhaustive SSC fatigue can dramatically reduce the same reflex contri bution. SSC fatigue is a useful model to study the processes of reversible muscle damage and how they interact with muscle mechanics, joint and muscle stiffness. All these parameters and their reduction during SSC fatigue cha nges stiffness regulation through direct influences on muscle spindle (disf acilitation), and by activating III and IV afferent nerve endings (proprios eptic inhibition). The resulting reduced stretch reflex sensitivity and mus cle stiffness deteriorate the force potentiation mechanisms. Recovery of th ese processes is long lasting and follows the bimodal trend of recovery. Di rect mechanical disturbances in the sarcomere structural proteins, such as titin, may also occur as a result of an exhaustive SSC exercise bout. (C) 2 000 Elsevier Science Ltd. All rights reserved.