Maximal strength and voluntary activation of adductor pollicis after a single session of acute intermittent hypercapnia or acute intermittent hypoxia

Acute intermittent hypoxia (AIH) can increase maximal strength of limb muscles in people with incomplete spinal cord injury (SCI), but it is mostly untested in people without SCI. Acute intermittent hypercapnia (AIC) may engage similar respiratory circuits to AIH, but the effects of AIC on human limb motor output are unknown. We examined whether single sessions of AIH or AIC improved motor output to a hand muscle in neurologically intact people. Twelve adults completed a single 30-min session of AIH (breathing alternate 1-min low oxygen air and 1-min normal air), AIC (alternate 1-min high carbon dioxide air and 1-min normal air), or SHAM (normal air). At baseline and for 80 min post-intervention, participants performed repeated isometric maximal voluntary thumb adductions. Transcranial magnetic stimulation elicited motor evoked potentials (MEPs) from first dorsal interosseous and adductor pollicis at each time point. Generalised linear mixed models were compared between conditions (AIH, AIC, SHAM). Normalised to baseline, voluntary activation was higher after AIC than SHAM (5.9%, P < 0.001) and AIH (5.5%, P < 0.001); MVC force was higher after AIC than SHAM (7.7%, P < 0.001), whereas maximal EMG was higher after AIH than SHAM (14.3%, P < 0.001). MEPs and maximal M-waves did not differ between conditions for either muscle (P > 0.25). Thus, single sessions of AIC induced small motor output improvements in people without SCI, but AIH did not. AIC increased maximal voluntary activation, but the mechanisms for this remain unclear because the MEPs provided no evidence for corticospinal facilitation.