The relationship of fatigue-induced changes of human soleus Hoffmann reflex and level of adaptation to physical exercise

Introduction . The effect of triceps muscle of calf (m.m. gastrocnemius-soleus) fatigue on the soleus muscle H (Hoffmann) reflex amplitude was investigated in both  heathy untrained people and  highly skilled athletes, 18-34 years of age. It is known that under the influence of permanent physical training the morphology and function of athlete’s organism change in order to extend  its functional possibilities. We supposed that neuromuscular system of trained and untrained persons would respond to fatigue differently. One of the informative methods for quantitative analysis of the functional state of neuromuscular system could be tests with use of stimulation electromyography in which the parameters of the H-reflex are measured. Purpose . The purpose of our work was to investigate in detail human soleus H-reflex changes after prolonged voluntary contraction of  triceps muscle of calf, which caused the fatigue of soleus muscle; and also to estimate features  of soleus H-reflex inhibition in two groups of people with the different level of adaptation to physical exercise. Methods. Fatigue was caused by sustained (6-9 min long) voluntary static contraction of triceps muscle of calf with effort egual to 75% of the maximal voluntary contraction. During the test, the person was sitting and pressing on fixed pedal with the foot, trying to realize sole flexion. The method of H-reflex of soleus muscle was used. Registration of electromyographic signals and tibial nerve stimulation were performed using neurodiagnostic complex (Nicolet Viking Select, USA-Germany). Results . It was found that in the group of untrained people an amplitude of soleus H-reflex was significantly reduced, egualing immediately after fatigue period of about 60% of initial value. Then H-reflex amplitude subsequently recovered and in 2-3 minutes attained approximately 90% of the initial amplitude. This was  followed by the period of slow recovery of the reflex, to  96-97%  of its initial value in 30 minutes after the fatigue period. At the same time, in a group of trained people (athletes) H-reflex inhibition was notably less evident. H-reflex amplitude was reduced after the fatigue period to 85% of initial value  with rapid recovery to  98% of initial  value in 1.5 minutes. Originality. In this paper for the first time features of human soleus H-reflex inhibition and recovering  after prolonged fatiguing voluntary contraction in two groups of persons with the different level of adaptation to physical exercise were investigated. Conclusion . It was suggested that the H-reflex inhibition occurred due to the activation of the groups III and IV afferent nerves under the influence of metabolic changes (such as accumulation of lactic acid) in the muscle during fatiguing contraction. Such activation could cause the increase of presynaptic inhibition intensity from Ia afferents to motoneurons. The more prolonged effects of the inhibition (for the period of tens of minutes) might be associated with the direct influence of the fatigue-induced biochemical shifts in the muscle. A clearly supressed amplitude reduction of the H-reflex in the group of trained persons might be the evidence of  high velocity of metabolites utilization, which is appropriate for athletes during the physical exercise.