Energy metabolism and muscle activation heterogeneity explain slow component and muscle fatigue of cycling at different intensities.

NEW FINDINGS

What is the central question of this study? What are the physiological mechanisms underlying muscle fatigue and the increase in the O 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 slow component ( ), but they share the same origin. Muscle activation heterogeneity is associated with muscle fatigue and . Knowing this may improve training prescriptions for healthy people leading to improved public health outcomes.

ABSTRACT

This study aimed to explain the slow component ( ) 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 , near-infrared spectroscopy signals and EMG activity were analysed (thigh muscles). was higher during VHI exercise (∼70% vs. ∼28% of 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 and muscle force production was observed. The τ of muscle was significantly related (R  = 0.47) with torque decrease for VHI. Type I and II muscle fibre recruitment mainly in the rectus femoris moderately explained the muscle fatigue (R  = 0.30 and 0.31, respectively) and the (R  = 0.39 and 0.27, respectively). The is also partially explained by blood lactate accumulation (R  = 0.42). In conclusion muscle fatigue and O cost seem to share the same physiological cause linked with a decrease in the muscle and a change in lactate accumulation. Muscle fatigue and are associated with muscle activation heterogeneity and metabolism of different muscles activated during cycling.