Energy metabolism and muscle activation heterogeneity explain V ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ slow component and muscle fatigue of cycling at different intensities

Abstract

New findings: What is the central question of this study? What are the physiological mechanisms underlying muscle fatigue and the increase in the O₂ cost per unit of work during high-intensity exercise? What is the main finding and its importance? Muscle fatigue happens before, and does not explain, the ${\dot{V}}_{O_2}$ slow component ( ${\dot{V}}_{O_2\mathrm{sc}}$ ), but they share the same origin. Muscle activation heterogeneity is associated with muscle fatigue and ${\dot{V}}_{O_2\mathrm{sc}}$ . Knowing this may improve training prescriptions for healthy people leading to improved public health outcomes.

Abstract: This study aimed to explain the ${\dot{V}}_{O_2}$ slow component ( ${\dot{V}}_{O_2\mathrm{sc}}$ ) and muscle fatigue during cycling at different intensities. The muscle fatigue of 16 participants was determined through maximal isokinetic effort lasting 3 s during constant work rate bouts of moderate (MOD), heavy (HVY) and very heavy intensity (VHI) exercise. Breath-by-breath ${\dot{V}}_{O_2}$ , near-infrared spectroscopy signals and EMG activity were analysed (thigh muscles). ${\dot{V}}_{O_2\mathrm{sc}}$ was higher during VHI exercise (∼70% vs. ∼28% of ${\dot{V}}_{O_2}$ reserve in HVY). The deoxygenated haemoglobin final value during VHI exercise was higher than during HVY and MOD exercise (∼90% of HHb physiological normalization, vs. ∼82% HVY and ∼45% MOD). The muscle fatigue was greater after VHI exercise (∼22% vs. HVY ∼5%). There was no muscle fatigue after MOD exercise. The greatest magnitude of muscle fatigue occurred within 2 min (VHI ∼17%; HVY ∼9%), after which it stabilized. No significant relationship between ${\dot{V}}_{O_2\mathrm{sc}}$ and muscle force production was observed. The τ of muscle ${\dot{V}}_{O_2}$ was significantly related (R² = 0.47) with torque decrease for VHI. Type I and II muscle fibre recruitment mainly in the rectus femoris moderately explained the muscle fatigue (R² = 0.30 and 0.31, respectively) and the ${\dot{V}}_{O_2\mathrm{sc}}$ (R² = 0.39 and 0.27, respectively). The ${\dot{V}}_{O_2\mathrm{sc}}$ is also partially explained by blood lactate accumulation (R² = 0.42). In conclusion muscle fatigue and O₂ cost seem to share the same physiological cause linked with a decrease in the muscle ${\dot{V}}_{O_2}$ and a change in lactate accumulation. Muscle fatigue and ${\dot{V}}_{O_2\mathrm{sc}}$ are associated with muscle activation heterogeneity and metabolism of different muscles activated during cycling.

Keywords: efficiency; muscle fatigue; oxidative metabolism; oxygen extraction; oxygen uptake slow component.