Effect of sojourn at 3200-m altitude on spinal reflexes in young adult males.

Studies of the H reflex and the Achilles tendon reflex (ATR) were performed in 12 healthy males under six conditions: 1) sea level control, 2) during the first 1-3 h at altitude while normoxic conditions were maintained, 3-5) at 6, 24 and 72 h of hypoxia, and 6) after Ve acclimatization at altitude in 5-14 d. After altitude acclimatization, the peak-to-peak amplitude of the H reflex and the ATR response was significantly elevated over control. A trend toward decreased amplitudes of both the ATR and the H reflex was observed at 6 h after the onset of hypoxia. There was no significant alteration in latency of the H reflex under any test condition. These findings suggest that sojourn at altitude by sea level residents induces changes in spinal motor reflexes. These changes are most probably mediated by altered high central modulation of the cord.