Abstract
Acute exposure to hypoxia increases postural sway, but the underlying neurophysiological factors are unclear. Golgi tendon organs (GTOs), located within the musculotendinous junction (MTJ), provide inhibitory signals to plantar flexor muscles that are important for balance control; however, it is uncertain if GTO function is influenced by hypoxia. The aim of this study was to determine how normobaric hypoxia influences lower limb tendon-evoked inhibitory reflexes during upright stance. We hypothesized that tendon-evoked reflex area and duration would decrease during hypoxia, indicating less inhibition of postural muscles compared to normoxia. At baseline (BL; 0.21 fraction of inspired oxygen, FIO2) and at ~2 (H2) and 4 (H4) hours of normobaric hypoxia (0.11 FIO2) in a normobaric hypoxic chamber, sixteen healthy participants received electrical musculotendinous stimulation (MTstim) to the MTJ of the left Achilles tendon. The MTstim was delivered as two sets of 50 stimuli while the participant stood on a force plate with their feet together. Tendon-evoked inhibitory reflexes were recorded from the surface electromyogram of the ipsilateral medial gastrocnemius, and center of pressure (CoP) variables were recorded from the force plate. Normobaric hypoxia increased CoP velocity (p ≤ 0.002) but not CoP standard deviation (p ≥0.12). Compared to BL, normobaric hypoxia reduced tendon-evoked inhibitory reflex area by 45% at H2 and 53% at H4 (p ≤ 0.002). In contrast, reflex duration was unchanged during hypoxia. The reduced inhibitory feedback from the GTO pathway could likely play a role in the increased postural sway observed during acute exposure to hypoxia.
Keywords:
Electromyography; Golgi tendon organ stimulation; Low oxygen; Postural control; Standing balance.