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.

FIGURE 1

Experimental design of the study. GET, gas exchange threshold; HHb, deoxygenated haemoglobin value; MIE, maximal isokinetic effort; HVY, heavy exercise; VHI, very heavy intensity exercise; ${\dot{V}}_{{\mathrm{O}}_2\mathrm{peak}}$, the highest ${\dot{V}}_{{\mathrm{O}}_2}$ value obtained in a 15‐s interval during ramp test.

FIGURE 2

Mean values of ${\dot{V}}_{{\mathrm{O}}_2}$ kinetics during moderate (triangles), heavy (HVY, rhombus) and very heavy intensity (VHI, circles) exercise. Vertical dotted lines indicate the start of ${\dot{V}}_{{\mathrm{O}}_2}$ slow component (${\dot{V}}_{{\mathrm{O}}_2\mathrm{sc}}$) and horizontal dashed lines indicate the ${\dot{V}}_{{\mathrm{O}}_2}$ asymptote projection. Bold lines show the exponential curves and grey line shows the residual mean values. The inside figure shows the individual values of ${\dot{V}}_{{\mathrm{O}}_2\mathrm{sc}}$ during HVY and VHI conditions (${\dot{V}}_{{\mathrm{O}}_2\mathrm{sc}}$, t = 3.83; P = 0.002). Note that the peak values of ${\dot{V}}_{{\mathrm{O}}_2}$ are not achieved during any condition; n = 16 participants.

FIGURE 3

Individual values in each variable for HHb kinetics during heavy (HVY) and very‐heavy intensity (VHI) exercise. Different letters denote statistical differences, P < 0.05. There were significant differences between intensities for HHb_base, higher for VHI vs. MOD, P = 0.026; HHb_A, lower at MOD vs. HVY P < 0.0001 and vs. VHI, P < 0.0001; HHb_A _total, higher for VHI vs. MOD, P < 0.0001 and vs. HVY, P = 0.025 and HVY vs. MOD, P < 0.0001; HHb_SC, lower at MOD vs. HVY, P = 0.001 and vs. VHI, P = 0.001; HHb_end, higher for VHI vs. MOD, P < 0.0001 and vs. HVY, P = 0.029 and HVY vs. MOD, P < 0.0001; HHb_TD, lower at MOD vs. HVY, P < 0.0001 and vs. VHI, P < 0.0001. HHb_τ (F = 2.98, P = 0.093) and MRT (F = 3.13, P = 0.088) were not significantly different between intensities; n = 16 participants. HHb, deoxygenated haemoglobin; HHb_base, mean of the last minute of baseline cycling; HHb_A, fundamental amplitude; HHb_A _total, HHb_A + HHb_base; HHb_end, mean of the last 20 s of exercise; HHb_SC, slow component of HHb (subtraction of HHb_end ‐ HHb_A _total); HHb_τ, time constant; HHb_TD, time delay; MRT, mean response time (TD + τ); PN, physiological normalization.

FIGURE 4

Individual values in each moment for torque production during moderate (dashed bars), heavy (grey bars) and very‐heavy intensity (white bars) exercise in the third part of this study. Bars indicate mean values and error bars indicate standard deviation. Different letters denote statistical differences, P < 0.05. There was a significant condition vs. time interaction (F = 8.29; P < 0.0001). There was also a significant main effect between conditions (F = 22.43; P < 0.0001) and time (F = 15.47; P < 0.0001); n = 16 participants.

FIGURE 5

Left upper and lower panels: mean power frequency (MPF) of electromyogram activity of rectus femoris during heavy (black circles) and very heavy intensity (open circles) exercise. In both panels there was a significant effect of time (B1 + B2, F = 2.21, P = 0.011; B4 + B5, F = 6.63, P < 0.0001) and of condition (B1 + B2, F = 29.4, P < 0.0001; B4 + B5, F = 66.7, P < 0.0001) but there was no interaction (B1 + B2, F = 0.39, P = 0.966; B4 + B5, F = 0.89, P = 0.556). Right panels: relationship between ${\dot{V}}_{{\mathrm{O}}_2}$ slow component (${\dot{V}}_{{\mathrm{O}}_2\mathrm{sc}}$) and different MPF bands in all muscles grouped. ∆MPF, difference between 12 min and zero). Upper panel shows lower bands (B1 + B2) and lower panel shows higher bands (B4 + B5); n = 11 participants.

FIGURE 6

Relationship between ${\dot{V}}_{{\mathrm{O}}_2}$ slow component (${\dot{V}}_{{\mathrm{O}}_2\mathrm{sc}}$) and different mean power frequency (MPF) bands in the vastus lateralis (VL, upper panels; B1 + B2, R ² = 0.224, F = 5.8, P = 0.026; B4 + B5, R ² = 0.130, F = 3.0, P = 0.099) and rectus femoris (RF, lower panels; B1 + B2, R ² = 0.386, F = 12.6, P = 0.002; B4 + B5, R ² = 0.266, F = 7.2, P = 0.014) muscles when analysed separately. ∆MPF, difference between 12 min and zero. Left panels show lower bands (B1 + B2) and right panels show higher bands (B4 + B5); n = 11 participants.