Energy metabolism and muscle activation heterogeneity explain V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ 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 trueV̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ slow component (trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$), but they share the same origin. Muscle activation heterogeneity is associated with muscle fatigue and trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$. Knowing this may improve training prescriptions for healthy people leading to improved public health outcomes. Abstract This study aimed to explain the trueV̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ slow component (trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$) 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 trueV̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$, near‐infrared spectroscopy signals and EMG activity were analysed (thigh muscles). trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$ was higher during VHI exercise (∼70% vs. ∼28% of trueV̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ 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 trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$ and muscle force production was observed. The τ of muscle trueV̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ 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 trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$ (R2 = 0.39 and 0.27, respectively). The trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$ 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 trueV̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ and a change in lactate accumulation. Muscle fatigue and trueV̇normalO2sc${\dot{V}}_{{{\rm{O}}}_{\rm{2}}{\rm{sc}}}$ are associated with muscle activation heterogeneity and metabolism of different muscles activated during cycling.

‘Fatigue makes cowards of us all’

“…In conclusion, the temporal (but not quantitative) dissociation of fatigue from the $${\dot V_{{{\rm{O}}_2}}}$$ slow component by do Nascimento Salvador et al. (2023) reinforces previous observations valuably. Moreover, their new associations of muscle oxidative capacity, tissue O 2 saturation index, recruitment of additional type I and II motor units (rectus femoris) with muscle fatigue, although only correlative, are exciting and will hopefully provide imperative and direction for prospective mechanistic investigations.…”

“…As noted previously by others (e.g., Cannon et al., 2011 ), do Nascimento Salvador et al. ( 2023 ) found a temporal dissociation between muscle fatigue (early) and the slow component (later), concluding that fatigue is a prerequisite for the slow component. This conclusion embodies the principle that ‘ kinetics sow the seeds of exercise intolerance’, acknowledging that exercise performance is influenced not only by the fundamental fast kinetics but also by the slow component, which itself might originate close to exercise onset (i.e., within 2–3 min of exercise onset).…”

“…Specifically, using nonexhaustive cycle ergometry bouts in the moderate, heavy and very heavy/severe exercise intensity domains, their data support an association between vastus lateralis muscle oxidative capacity, tissue O 2 saturation index, recruitment of additional type I and II motor units (rectus femoris) and muscle fatigue. As noted previously by others (e.g.,Cannon et al, 2011), do Nascimento Salvador et al (2023 found a temporal dissociation between muscle fatigue (early) and the VO 2 slow component (later), concluding that fatigue is a prerequisite for the VO 2 slow component. This conclusion embodies the principle that ' VO 2 kinetics sow the seeds of exercise intolerance' , acknowledging that exercise performance is influenced not only by the fundamental fast VO 2 kinetics but also by the VO 2 slow component, which itself might originate close to exercise onset (i.e., within 2-3 min of exercise onset).…”

“…Experimental manipulation of the $${\dot V_{{{\rm{O}}_2}}}$$ slow component by altered work output or O 2 transport markedly impacts exercise tolerance, demonstrating that the two are inextricably intertwined, and it is therefore appropriate that this relationship provided the foundation for the recent paper by do Nascimento Salvador et al. (2023) investigating the mechanisms of fatigue.…”

“…Notwithstanding these challenges, the paper by do Nascimento Salvador et al. ( 2023 ) provides some pertinent, new and intriguing insights. Specifically, using non‐exhaustive cycle ergometry bouts in the moderate, heavy and very heavy/severe exercise intensity domains, their data support an association between vastus lateralis muscle oxidative capacity, tissue O 2 saturation index, recruitment of additional type I and II motor units (rectus femoris) and muscle fatigue.…”