Fatigue-induced modulation of human soleus H-reflex in conditions of pair tibial nerve stimulation in untrained and trained people

Abstract

Monosynaptic reflex indices peculiarities, concerned with adaptative reactions to the long-term physical exercise, might add new data about mechanisms of human nervous system plasticity. The objective of the research was to investigate the influence of fatigue on human soleus H (Hoffmann) reflex in conditions of pair stimulation of tibial nerve with inter stimulus interval 500 m sin tenun trained people (age: M = 25,3, SE = 1,6 years) and ten trained athletes (age: M = 20,5, SE = 0,5 years). H-reflex study was performed using neurodiagnostic complex Nicolet Biomedical Viking Select (Viasys Health care, USA) at rest: before and after long-lasting isometric voluntary contraction of calf muscle, which caused the soleus muscle fatigue; the muscle force was equal to 75 % of maximal voluntary contraction. Test and conditioned responses (by means of stimulation with first and second impulses from pair) were registered. Homosynaptic postactivation depression (HPAD), associated with pair stimulation of tibial nerve, led to 56 % and 51 % inhibition of H-reflex in untrained and trained people at rest, respectively (p < 0,05). After fatiguing voluntary contraction the amplitudes of test and conditioned soleus H-reflex were both reduced approximately in half. Then both H-reflex amplitudes subsequently recovered, more rapidly in trained people. Soleus H-reflex inhibition might be due to the activation of the groups III and IV afferent nerves under the influence of mechanical and metabolic changes in the muscle. It was also found that HPAD H-reflex inhibition intensity increased by 20 % in untrained people and by 15 % in trained ones at 90 s after fatiguing voluntary contraction (p < 0,05). It is assumed that complex influence of fatigue and homosynaptic postactivation depression was more pronounced in untrained people in comparison with trained ones. It can be ascribed to athletes adaptation to the long-term physical exercise.