Stretch shortening cycle fatigue: Interactions among joint stiffness, reflex, and muscle mechanical performance in the drop jump

The purpose of the present study was to investigate the effect of strenuous stretch-shortening cycle exercise on the relationship between reflex and stiffness regulation during the drop jump. Ten healthy male subjects performed submaximal stretch-shortening cycle exercise on a special sledge apparatus. Exhaustion occurred on average within 3 min. A drop jump test from a 50-cm height was performed immediately before and after the sledge exercise, as well as 2 h, 2 days and 4 days later. The fatigue exercise showed relatively high blood lactate concentrations 12.5 (SD 2.6) mmol . l(-1) and a 2-day delayed increase of serum cretaine kinase concentration. In drop jumps, the short Latency M1 component of the vastus lateralis muscle electromyogram (EMG) response showed a continuous decline throughout the entire follow-up period after fatigue (NS), whereas the medium latency EMG component increased 2 days after the postfatigue sessions (P < 0.05). Immediately after the fatigue exercise a positive correlation (P < 0.05) was found between the changes in the short latency EMG response and in the amount of knee joint stiffness during the early post-landing phase of the drop jump. This suggests that the M1 response was closely related to the stiffness changes during the initial braking phase of the drop jump. Increase of creatine kinase concentration on the 2nd day correlated negatively with the changes in the drop jump performance (P < 0.05). Since the short latency EMG component has almost recovered on the 2nd day, impairment of the mechanical function of the muscle might have taken place. In conclusion, exhausting stretch-shortening cycle exercise induced local muscle impairment, which resulted in modulation of the reflex and stiffness interaction in the drop jump as well as compensation by central motor command.