Influence of muscular contraction on vascular conductance during exercise above versus below critical power

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Highlights

  • Increases in limb vascular conductance were greater during immediate recovery from exercise above, compared to below, critical power.

  • The influence of muscular contraction on limb vascular conductance was intensity dependent above, but not below, critical power.

  • Our data suggest critical power represents a contraction intensity threshold above which a critical level of blood flow impedance is elicited.

  • These findings may help to explain the progressive metabolic and neuromuscular responses that occur during exercise above critical power.

Abstract

We tested the hypothesis that limb vascular conductance (LVC) would increase during the immediate recovery phase of dynamic exercise above, but not below, critical power (CP) indicating a threshold for muscular contraction-induced impedance of limb blood flow (LBF). CP (115 ± 26 W) was determined in 7 men and 7 women who subsequently performed ∼5 min of near-supine cycling exercise both below and above CP. LVC demonstrated a greater increase during immediate recovery and remained significantly higher following exercise above, compared to below, CP (all p < 0.001). Power output was associated with the immediate increases in LVC following exercise above, but not below, CP (p < 0.001; r = 0.85). Additionally, variance in percent LBF impedance was significantly lower above (CV: 10.7 %), compared to below (CV: 53.2 %), CP (p < 0.01). CP appears to represent a threshold above which the characteristics of LBF impedance by muscular contraction become intensity-dependent. These data suggest a critical level of LBF impedance relative to contraction intensity exists and, once attained, may promote the progressive metabolic and neuromuscular responses known to occur above CP.

Introduction

Skeletal muscle contractions are accompanied by increases in intramuscular pressure and therefore mechanical compression of blood vessels and increased vascular resistance (Barcroft and Dornhorst, 1949; Gaffney et al., 1990; Radegran and Saltin, 1998; Sadamoto et al., 1983; Sjogaard et al., 1986). During rhythmic exercise, blood flow is highest during the relaxation phase of the contraction-relaxation cycle when intramuscular pressure is low (Barcroft and Dornhorst, 1949; Lutjemeier et al., 2005; Radegran and Saltin, 1998; Robergs et al., 1997; Walloe and Wesche, 1988). Thus, altering relaxation time between contractions (i.e., duty-cycle or contraction-frequency) has a profound effect on muscle blood flow (Bentley et al., 2017; Broxterman et al., 2014; Caldwell et al., 2018; Hoelting et al., 2001). Indeed, Broxterman et al. demonstrated that an increase in duty cycle (i.e., reduced relaxation time) attenuated the blood flow response to exhaustive rhythmic handgrip exercise (Broxterman et al., 2014). Additionally, the time to task failure (Tlim), and therefore the asymptote of the hyperbolic power-duration relationship (i.e., critical power; CP), was significantly reduced (Broxterman et al., 2014). While the oxygen delivery dependency of CP is well documented (Dekerle et al., 2012; Moritani et al., 1981; Vanhatalo et al., 2010), the influence of muscular contraction on limb vascular conductance (LVC) during exercise above versus below CP remains unknown.

CP is an important physiological threshold (Poole et al., 2016) that differentiates between steady-state and progressive metabolic (Jones et al., 2008; Poole et al., 1988) and neuromuscular (Burnley et al., 2012) responses to exercise. Recently, our laboratory has demonstrated that limb blood flow (LBF) and microvascular oxygen extraction responses to isometric handgrip exercise above critical force (CF; the isometric analog of CP) are distinct from those below CF. Specifically, while steady-state responses were demonstrated below CF, LBF and microvascular oxygen delivery appeared to reach task-specific physiological maximums above CF suggesting intensity-dependent limitations (Hammer et al., 2020b). Additionally, threshold-like (i.e., biphasic) responses in microvascular oxygen delivery have been observed during incremental exercise with apparent limitations (i.e., submaximal plateaus) existing at higher intensities (Boushel et al., 2002; Chin et al., 2011; Habazettl et al., 2010; Hammer et al., 2018; Koga et al., 2014). These findings are consistent with those of Lutjemeier et al. who experimentally described the effect of muscular contraction on LBF as being positive and neutral at low and moderate work rates, respectively, but negative (i.e., blood flow impedance) at higher intensities (Lutjemeier et al., 2005). These observations have wide-reaching implications for muscular performance and fatigue development yet remain to be contextualized within the CP framework of exercise tolerance.

The aim of this study was to determine if CP represents a threshold for muscular contraction-induced impedance of LBF during dynamic locomotor-muscle exercise. Specifically, we compared LVC during near-supine cycling to changes in LVC immediately following exercise termination. Under the assumption that blood vessel diameter during immediate recovery reflects the level of vasodilation during exercise but without the influence of muscular contractions, changes in LVC subsequent to the near-instantaneous cessation of muscular contractions have been previously used to inversely reflect the influence of contraction on LBF during exercise (Barcroft and Dornhorst, 1949; Broxterman et al., 2014; Lutjemeier et al., 2005; Tschakovsky et al., 2004). We hypothesized that LVC would increase during the immediate recovery phase of dynamic exercise above, but not below, CP. If confirmed, we would interpret these findings to indicate that CP represents a threshold for muscular contraction-induced impedance of LBF during dynamic locomotor-muscle exercise.

Section snippets

Ethical approval

All experimental procedures were approved by the Institutional Review Board for Research Involving Human Subjects at Kansas State University (#9954) and conformed to the standards set forth by the Declaration of Helsinki apart from database registration. Subjects were informed of all testing procedures and potential risks of participation before providing written, informed consent.

Subjects

Seven men and seven women (mean ± SD; 23 ± 3 yr, 172 ± 8 cm, 70.7 ± 16.3 kg) volunteered to participate in this

Incremental ramp data and determination of CP

Ppeak and V̇O2peak were 146 ± 30 W and 33.1 ± 7.2 mL/kg/min, respectively. CP was 115 ± 26 W (79.0 ± 8.4 %Ppeak). Standard error of the power-output intercept used to determine CP ranged from 0.39 to 6.27 %CP. Power-outputs below and above CP were 71.1 ± 7.6 and 86.9 ± 9.2 %Ppeak, respectively.

Hemodynamic responses to exercise

Hemodynamic values at baseline and end-exercise during the below- and above-CP tests are presented in Table 1. No significant differences were detected in any hemodynamic measurements at baseline between

Major findings

This study demonstrated that CP represents a threshold for intensity-dependent characteristics of muscular contraction-induced impedance of LBF during dynamic locomotor-muscle exercise. Consistent with our hypothesis, LVC increased significantly during the immediate recovery phase of dynamic exercise above CP (Fig. 2C) suggesting impedance of LBF by muscular contractions. Indeed, LVC was significantly lower during exercise above, compared to below, CP (Fig. 1C). However, in contrast to our

Conclusions

This is the first study that provides LBF measurements during locomotor-muscle exercise below and above the CP threshold in humans. This study demonstrated that muscular contraction-induced impedance to LBF was significantly greater above, compared to below, CP. Additionally, CP appears to represent a threshold above which the characteristics of LBF impedance by muscular contraction become intensity-dependent. Remarkably, while it remains to be elucidated if greater LBF impedance occurs with

Data availability

The data that support the findings of this study are available on request from the corresponding author.

Author contributions

This work was completed at Kansas State University. SMH, STH, AMA, KDD, JRS, TJB, and CJA conceived and designed this study. SMH, STH, SKP, AMA, VGT, ZJW, KDD, JRS, TJB, and CJA acquired, analyzed, and interpreted the data. SMH prepared the first draft of the manuscript. STH, SKP, AMA, VGT, KDD, JRS, TJB, and CJA revised it critically for important intellectual content. All authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the

Funding

This work was supported by National Institutes of Health awards: T32HL07111 to SMH and JRS; K12HD065987 to JRS.

Declaration of Competing Interest

The authors report no competing interests for this work.

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