Acute and delayed neuromuscular adjustments of the triceps surae muscle group to exhaustive stretch-shortening cycle fatigue

Abstract

Stretch-shortening cycle (SSC)-type fatigue is associated with acute and delayed functional defects, and appears to be a useful model to reveal the flexibility of both central and reflex adjustments to the contractile failure. SSC fatigue was induced in an experimental (EXP) group (n=6) on a sledge ergometer with an exhaustive rebound exercise with submaximal effort. The acute (POST) and 2-day delayed (2D) neuromuscular changes with fatigue were examined in a short submaximal rebound task (REBOUND) and in a maximal isometric plantarflexion test (ISOM). The EXP group results were compared to those of a control group (n=6) who did not perform the exhaustive SSC exercise and did not present any change in the tests. In the EXP group, the ISOM test revealed mostly a large decrease in maximal plantarflexion force at 2D that was correlated with the reduced mean soleus muscle (SOL) activation. Indicating "task-dependent" fatigue effects on the neural changes, the REBOUND test revealed both acute and delayed increases in SOL activation. Supporting central neural changes, SOL preactivation increased in POST and 2D. The neural flexibility along time and across muscles was demonstrated by the shifted increase in SOL activation from the braking phase in POST to the push-off phase in 2D, and associated increased gastrocnemius medialis preactivation in 2D. In contrast, activation during the stretch-reflex period was constant in POST, and decreased in 2D. These results would support the influence of musculotendinous afferents on the flexible neural adjustments to the SSC-induced contractile failure.