Effect of Hypoxia on Muscle Activation During Incremental Exercise at Equivalent Relative Intensity

Introduction

The objective of this study was to investigate the impact of hypoxia on the connection between metabolic stress, adjusted for relative intensity, absolute power production, and muscle activation as indicated by EMG activity in the context of step and ramp incremental exercise.

Methods

Seventeen healthy active adults, male and female, with a background in cycling, completed a ramp and step incremental exercise testing in normoxia (NORM; FiO₂≈21%) and hypoxia (HYPO; FiO₂≈13.5%) on four occasions. Both ramp tests determined the respiratory compensation point (RCP) and gas exchange threshold (GET). The RCP was the benchmark for determining the relative intensity in the step tests among two conditions. Electromyography (EMG) was used to assess muscle activation based on the relative intensity of both conditions. 2-way ANOVA, Paired t-tests, and effect sizes were used to examine differences between NORM and HYPO.

Results

There was a significant main effect of condition in muscle activation rmsRF% (P < 0.001; ηp2 = 0.80), rmsVM% (P < 0.001; ηp2 = 0.66), and rmsVL% (P= 0.002; ηp2 = 0.56) during ramp test. In the ramp test, there was a significant decrease in muscle activation for rmsRF% (P= 0.049; d = 0.61) and rmsVM% (P = 0.01; d = 0.84) in peak power output (PPO). For the step test, there was a main effect of conditions in rmsVM% (P = 0.001; ηp2 = 0.57) and rmsVL% (P = 0.04; ηp2 = 0.32), whereas there was no significant difference in rmsRF% (P = 0.74; ηp2 = < 0.009). There was a significant reduction in the rmsVM% (P = 0.01; d = 0.76) and rmsVL% (P = 0.03; d = 0.69) in HYPO compared to NORM in the task failure stage (TF). There was a strong relationship between muscle activation and power output in the ramp and step tests (PO).

Conclusion

During non-steady-state exercise at workrates below RCP, there was little difference in muscle activation between NORM and HYPO, however during steady-state exercise, rmsEMG activity was lower in HYPO. At task failure, there was a positive association between the external workload and muscle activation irrespective of condition or test protocol. These results suggest that relative intensity and metabolic stress do not play a major role in motor unit recruitment, but rather the force required to produce a given mechanical workload is the dominant factor. However, this might be modulated by steady-state versus non-steady-state metabolism.