Effect of hypoxia on muscle activation at equivalent absolute and relative intensity during incremental and constant load exercise to task failure

We examined the impact of moderate hypoxia (HYPO) on muscle activation during incremental exercise matched for both absolute and equivalent relative intensity. Fifteen active subjects (10 males, 5 females) completed two ramp incremental test and two step tests in normoxia (NORM; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>F</mml:mi> <mml:mrow><mml:mi>i</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> <mml:annotation>${F_{{\mathrm{i}}{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> = 0.209) and HYPO ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>F</mml:mi> <mml:mrow><mml:mi>i</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> <mml:annotation>${F_{{\mathrm{i}}{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> ≈ 0.135) in counterbalanced order. The respiratory compensation point (RCP) determined from ramp testing was used to normalize relative intensity during step testing, which included a final stage to task failure (TF) above RCP. Electromyography (EMG) was recorded for rectus femoris (RF), vastus lateralis (VL) and vastus medialis (VM), and normalized to a pre-test maximal sprint effort. Linear mixed modelling was used to examine fixed effects of condition (NORM, HYPO) and intensity (absolute, relative) on EMG activity. During the ramp test, HYPO significantly reduced <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>peak</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${\dot V_{{{\mathrm{O}}_2}{\mathrm{peak}}}}$</mml:annotation></mml:semantics> </mml:math> (∼13%), PPO (∼15%), and power at RCP (∼16%). EMG breakpoints occurred at lower absolute intensity in HYPO for RF and VL. When matched for relative intensity, muscle activity was lower in HYPO for VM and VL, but not RF. EMG activity at TF revealed a similar pattern whereby a strong association to absolute power was present regardless of test protocol or <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>F</mml:mi> <mml:mrow><mml:mi>i</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> <mml:annotation>${F_{{\mathrm{i}}{{\mathrm{O}}_2}}}$</mml:annotation></mml:semantics> </mml:math> . These results suggest that altered relative metabolic stress has a negligible impact on muscle activation at work rates below the RCP. For exercise in the severe domain our data aligns with the theory that muscle activation is not critically regulated to a given level at TF, but appears to be task-specific and independent of oxygen availability.