Muscle and Joint Stiffness Regulation during Normal and Fatiguing Stretch-Shortening Cycle Exercise

Kuitunen, Sami Muscle and joint stiffness regulation during normal and fatiguing stretchshortening cycle exercise Jyväskylä: University of Jyväskylä, 2010, 76 p. (Studies in Sport, Physical Education and Health, ISSN 0356-1070; 149) ISBN 978-951-39-3969-4 (PDF), 978-951-39-3954-0 (nid.) Finnish summary Diss. The present series of studies were designed to investigate the regulation of leg stiffness during stretch-shortening cycle (SSC) exercise. More specifically, this thesis examined the neuromuscular mechanisms affecting stiffness regulation at joint and muscle level, both in non-fatiguing and fatiguing SSC conditions. The results suggest that ankle joint, particularly the gastrocnemius (Ga) muscle, plays a key role in leg stiffness modulation both in non-fatiguing and fatiguing SSC conditions. It was also observed that the interaction between muscle fascicles and tendinous structures during short-contact SSC muscle actions differ between the soleus (Sol) and Ga muscles, indicating muscleand intensity-specific dependency. Centrally pre-programmed motor commands are suggested to play major roles in leg stiffness adjustments during SSC exercise with short ground contact time. Exhaustive SSC exercise induced acute and long-term impairments in stiffness regulation that did not recover in one week in some cases. The results indicate that both central and peripheral fatigue mechanisms contribute to the decreased muscle performance and consequently, to stiffness modulation during the SSC exercise. The long-lasting impairments in muscle performance and stiffness regulation are likely related to exercise-induced muscle damage and the subsequent inflammation process. High muscle activation prior to and during the early phase of the ground contact seems to be a pre-requisite for efficient stiffness regulation that may also delay the exhaustion during intensive SSC exercise.