Pre- and post-synaptic control of motoneuron excitability in athletes

The purpose of this study was to examine the efficacy of two spinal mechanisms in gating motoneuron excitability in power-trained athletes (N = 9), endurance-trained athletes (N = 9), and untrained subjects (N = 9). The dependent variable for each protocol was the peak-to-peak amplitude of the conditioned soleus Hoffmann reflex (H-reflex). Modulations of the test reflex amplitude were evaluated for each subject by using two experimental conditioning protocols: recurrent inhibition (RI) and paired-reflex depression (PRD). Also, to assess the effects of different levels of input on motoneuron excitability, two H-reflex stimulus intensities were used (10% and 30% of maximal motor response (M-max)). For each protocol, seven conditioned H-reflex trials were obtained from each subject during quiet stance. The RI protocol consisted of two reflex responses that were separated by 10 ms. The first was of the same intensity as the unconditioned trials. The second stimulus was of an intensity great enough to elicit a supramaximal motor response. The PRD protocol utilized two reflex stimuli of the same intensity separated by 80 ms. A group by intensity (3 x 2) analysis of variance was performed to determine group differences within each condition. For both protocols, increases in stimulus intensity resulted in significantly greater inhibition. Significant differences were observed between the trained groups for both the RI and the PRD protocols. For the RI protocol, the endurance-trained athletes demonstrated significantly less RI than either the power-trained athletes or the untrained subjects. For the PRD protocol, the endurance-trained athletes demonstrated significantly greater PRD than either the power-trained athletes or the untrained subjects. These observations indicate differential control of motoneuron excitability as a result of segmental reflex pathways among differently trained athletes.