Alpha-motoneuron excitability at high altitude.

It has been hypothesized that chronic hypobaric hypoxia could lead to inhibition of the alpha-motoneuron pool, thus limiting the maximal activation of working skeletal muscles. To test this hypothesis six subjects [32 (SEM 2) years] were evaluated in resting conditions, at sea level and after acclimatization at 5,050 m. The recruitment curves of the Hofmann-reflex (H-) and the direct muscle-response. (M-) of the right soleus muscle were obtained by stimulating the posterior tibial nerve with different intensities while recording the electromyogram of the soleus muscle. From the recorded data the net alpha-motoneuron excitability (ratio of maximal H-reflex to M-response Hmax:Mmax ratio), the threshold and gain for both responses, obtained from linear regressions through the rising phase of the recruitment curves of both responses, as well as the latency times of both responses were determined. The latency times and the Hmax:Mmax ratio were unchanged at altitude. The thresholds of both responses and the gain of the M-response were unaltered. The gain of the H-response was significantly higher at altitude when compared to sea level.(ABSTRACT TRUNCATED AT 250 WORDS)