Abstract
A link between muscle fatigue, decreased efficiency and the slow component of oxygen uptake (VO2sc) has been suggested. However, a cause-effect relationship remains to be elucidated. Although alterations in VO2 kinetics after elevated baseline work rate have previously been reported, to date no study has observed the effect on muscle force production (MFP) behavior considering physiological differences between male and female subjects. This study investigated the effect of elevated baseline work rate on the VO2 kinetics and MFP in 10 male and 10 female healthy subjects. Subjects performed 4 transitions of very-heavy (VH) intensity cycling in a randomized order after unloaded (U-VH) or moderate (M-VH) exercise. Maximal isokinetic efforts (MIE) were performed before and after each condition at two different cadences (60 or 120 rpm). Whereas baseline VO2 and time constant (τ) were significantly higher in M-VH compared to U-VH, the fundamental amplitude and the VO2 slow component (VO2sc) were significantly lower in M-VH (p < 0.05) in both sexes. Blood lactate concentration ([La]) and rate of perceived exertion (RPE) were not influenced by condition or sex (p > 0.05). The MFP post-exercise was not significantly influenced by condition in both sexes and cadences (Δtorque for males: at 60 rpm in U-VH = 13 ± 10 Nm, in M-VH = 13 ± 9 Nm; at 120 rpm in U-VH = 22 ± 14 Nm, in M-VH = 21 ± 12 Nm; for females: at 120 rpm in U-VH = 10 ± 9 Nm, in M-VH = 12 ± 8 Nm; p > 0.05), with the exception that female subjects presented smaller decreases in M-UH at 60 rpm compared to U-VH (11 ± 13 vs. 18 ± 14 Nm, respectively, p < 0.05). There was no correlation between the decrease in torque production and VO2 kinetics parameters (p > 0.05). The alterations in VO2 kinetics which have been suggested to be linked to changes in motor unit recruitment after elevated baseline work rate did not reflect alterations in MFP and fatigue in both sexes.
Highlights
Over the last decades, numerous studies have aimed to understand the physiological mechanisms underlying a loss of work efficiency or an increase in the O2 cost per unit of work during constant-load exercise above the gas exchange threshold (GET), i.e., the slow component of O2 uptake kinetics (VO2SC)
This study investigated the effect of elevated baseline work rate on the VO2 kinetics and muscle force production (MFP) in 10 male and 10 female healthy subjects
Numerous studies have aimed to understand the physiological mechanisms underlying a loss of work efficiency or an increase in the O2 cost per unit of work during constant-load exercise above the gas exchange threshold (GET), i.e., the slow component of O2 uptake kinetics (VO2SC)
Summary
Numerous studies have aimed to understand the physiological mechanisms underlying a loss of work efficiency or an increase in the O2 cost per unit of work during constant-load exercise above the gas exchange threshold (GET), i.e., the slow component of O2 uptake kinetics (VO2SC). Cannon et al (2011) described muscle fatigue as an increased ATP cost of already recruited motor units, instead of the recruitment of less efficient muscle fibers, as the primary mechanisms explaining the VO2SC. Notwithstanding, Keir et al (2016) showed a relationship between the VO2SC and the time course of peripheral muscle fatigue (muscle torque production in response to electrically stimulated contractions) during high-intensity exercise. Temesi et al (2017) suggested that subjects with slower VO2 kinetics (i.e., higher τ-values) experienced more peripheral fatigue during very-heavy (VH) cycling exercise. This relationship between VO2 kinetics and muscle fatigue remains to be elucidated. De Souza et al (2016) showed differences in the VO2SC and τ despite a similar magnitude of muscle fatigue during VH cycling exercise. do Nascimento Salvador et al (2018) demonstrated that prior cycling exercise decreased the VO2SC behavior, but did not modify the time-course of muscle torque production in a subsequent VH cycling bout
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