The effect of acute intermittent hypoxia on human limb motoneurone output

New Findings What is the central question of this study? Does a single session of repeated bouts of acute intermittent hypoxic breathing enhance the motoneuronal output of the limb muscles of healthy able‐bodied participants? What is the main finding and its importance? Compared to breathing room air, there were some increases in motoneuronal output following acute intermittent hypoxia, but the increases were variable across participants and in time after the intervention and depended on which neurophysiological measure was checked. Abstract Acute intermittent hypoxia (AIH) induces persistent increases in output from rat phrenic motoneurones. Studies in people with spinal cord injury (SCI) suggest that AIH improves limb performance, perhaps via postsynaptic changes at cortico‐motoneuronal synapses. We assessed whether limb motoneurone output in response to reflex and descending synaptic activation is facilitated after one session of AIH in healthy able‐bodied volunteers. Fourteen participants completed two experimental days, with either AIH or a sham intervention (randomised crossover design). We measured H‐reflex recruitment curves and homosynaptic post‐activation depression (HPAD) of the H‐reflex in soleus, and motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation (TMS) and their recruitment curves in first dorsal interosseous. All measurements were performed at rest and occurred at baseline, 0, 20, 40 and 60 min post‐intervention. The intervention was 30 min of either normoxia (sham, ≈ 0.21) or AIH (alternate 1‐min hypoxia [ ≈ 0.09], 1‐min normoxia). After AIH, the H‐reflex recruitment curve shifted leftward. Lower stimulation intensities were needed to evoke 5%, 50% and 99% of the maximal H‐reflex at 40 and 60 min after AIH ( P < 0.04). The maximal H‐reflex, recruitment slope and HPAD were unchanged after AIH. MEPs evoked by constant intensity TMS were larger 40 min after AIH ( P = 0.027). There was no change in MEP recruitment or the maximal MEP. In conclusion, some measures of the evoked responses from limb motoneurones increased after a single AIH session, but only at discrete time points. It is unclear to what extent these changes alter functional performance.