P35 Severe acute hypoxia reduces motor unit firing rate during isometric contractions
Mckeown DJ and Kavanagh JJ
Menzies Health Institute Queensland, Griffith University, Gold Coast, Queensland, Australia
Although reducing blood oxygen saturation (SpO2) can cause rapid changes in corticospinal-motoneuronal activity, few studies have examined the direct consequences that hypoxia has on muscle activation. The purpose of this study was to examine how severe acute hypoxia affects firing rate and recruitment characteristics of a population of motor units (MU) in the biceps brachii. Given that the amplitude of interference EMG is often reduced with hypoxia, it was hypothesised that (MU) firing rate would decrease, accompanied by an increase in MU recruitment threshold.
An altitude simulator was used to induce hypoxia, oxygen was titrated until 80% SpO2. Surface EMG was collected before, 1 hr and 2 hr into the hypoxia intervention. EMG was obtained from the biceps brachii during ramped 25% MVC isometric contractions, and MU action potential trains were identified from the EMG data using Delsys dEMG software.
Hypoxia did not affect MVC amplitude or force steadiness. However MU firing rate decreased 1 hr (p < 0.05) and 2 hr (p < 0.05) after hypoxic exposure, which was accompanied by a concomitant increase in MU recruitment threshold at 1 hr (p < 0.05) and 2 hr (p < 0.05).
While reducing SpO2 can cause rapid changes in cortical activity, the findings of this study indicate that the control of MUs are also affected by hypoxia. A potential consequence of reduced firing rates is that the control of higher intensity muscle contractions may be compromised, and the capacity to perform work under fatigue may be limited.