Fatigue during stretch-shortening cycle exercises. II. Changes in neuromuscular activation patterns of human skeletal muscle.

Detailed electromyographic (EMG) analysis of primarily triceps brachii muscle was carried out on subjects who performed 100 repeated and exhaustive stretch-shortening cycle (SSC) exercises on a special sledge apparatus incorporating a force plate. The fatigue contractions were performed on submaximal levels but the before-after comparison included also maximal "drop jumps" on the sledge as well as falls on to the floor. SSC contractions caused clear reduction in the mechanical performance capacity of the muscles. EMG analysis revealed that integrated EMG (IEMG) increased during the course of fatigue both during eccentric and concentric phases of the cycle. However, when IEMG was time-normalized, the changes were nonsignificant. The IEMG/force ratio increased especially during the eccentric phase both in submaximal and maximal drops. In maximal drops, there was also a great reduction in concentric IEMG. Rectified EMG curves were also averaged in blocks of ten successive fatigue contractions. This analysis revealed clear appearance of short (SLC), medium (MLC), and long (LLC) latency reflex components in most of the subjects during the early contact on the platform. The fatigue effects on these components were individual and also condition dependent so that in maximal drops there was a trend for reflex potentiation. In maximal drops on to the floor, which represented the highest stretch loads on the muscles, there was a tendency for reduction of amplitudes of the reflex components. The results suggest that in exhaustive SSC exercise muscular activation processes are reduced and primarily in the eccentric phase. Muscle spindles are suggested to become differentially affected by fatigue substances depending on the load intensity during eccentric contraction.