Impact of acute hypoxic exposure on neuromuscular and hemodynamic responses during step intensity dynamic constant external resistance leg extension exercise

Objectives: This study examined the effects of acute normoxic and hypoxic exposure on neuromuscular and hemodynamic physiological responses performed during dynamic step muscle actions.

Methods: Thirteen recreationally active men (mean ± SD age: 21.2 ± 2.9 yrs) performed dynamic leg extensions unilaterally under Normoxic (FiO₂ = 21 %) and Hypoxic (FiO₂ = 13 %) conditions in a randomized order at 20 %, 40 %, 60 %, 80 %, and 100 % of their maximal strength. Electromyographic (EMG) amplitude, EMG frequency, (Oxygenated and Deoxygenated hemoglobin; Oxy_Hb, Deoxy_Hb), Total hemoglobin (Total_Hb), and skeletal muscle tissue oxygenation status (StO₂) were measured from the vastus lateralis during all contractions.

Results: There were no detectable differences in the neuromuscular responses between normoxia and hypoxia for EMG amplitude (p = 0.37-0.74) and frequency (p = 0.17-0.83). For EMG amplitude there were general increases with intensity (p < 0.01-0.03). EMG frequency remained similar from 20% to 80% and then increased at 100 % effort (p = 0.02). There was no significant difference in patterns of responses for Oxy_Hb (p = 0.870) and Total_Hb (p = 0.200) between normoxia and hypoxia. StO₂ (p = 0.028) decreased and Deoxy_Hb (p = 0.006) increased under hypoxia compared to normoxia during dynamic step muscle actions performed in a randomized order.

Conclusion: Unlike fatigue, acute hypoxemia in an unfatigued state does not impact the localized neuromuscular responses, but minimally impacts the hemodynamic responses.