Non-invasive motor unit analysis reveals specific responses during maximal muscle contraction under normobaric hypoxia

Hypoxia has been extensively studied as a stressor which pushes human bodily systems to responses and adaptations. Nevertheless, a few evidence exist onto constituent trains of motor unit action potential, despite recent advancements which allow to decompose surface electromyographic signals. This study aimed to investigate motor unit properties from noninvasive approaches during maximal isometric exercise in normobaric hypoxia. Applying a cross-over design, 18 participants (gender-matched, on average age 22.6 y, BMI 23.6 kg/m², and bioimpedance phase angle 6.4) were exposed twice to hypoxia (FiO₂ ≊ 15.0% and FiO₂ ≊ 13.4%, separately, by using a tent connected with a hypoxic generator) and once to normobaric normoxia. After ≊ 30 min inside the tent, participants performed a series of 9 unilateral isometric contractions of the right knee extensors at maximum intensity for 5 s, interspersed with 15 s of passive recovery, while acquiring high-density surface EMG signals through a 64-electrodes grid and cardiorespiratory variables, and registering symptoms; then, a post-processing motor unit decomposition technique was applied. We found an increase in MU discharge rate as a response to acute normobaric hypoxia, although to a little extent and differently across sexes. Moreover, males experienced a more prominent increase of MU conduction velocity due to hypoxia. MUs responses to normobaric hypoxia were only slightly and non-homogeneously associated with hypoxic cardiorespiratory responses. Normobaric hypoxia affects the neuromuscular system with a relatively greater effect on peripheral rather than central features.