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

Keywords: Contraction impedance; Critical power; Limb blood flow; Vascular conductance.

Figure 1.. Cardiovascular responses to exercise.

The mean absolute values of limb blood flow (LBF; A), mean arterial pressure (MAP; B), and limb vascular conductance (LVC; C) at baseline and end-exercise below (▽) and above (▲) CP. Individual responses are presented in the background. * Significantly greater than baseline (p < 0.001). † Significant difference between below- and above-CP tests (p < 0.05).

Figure 2.. Cardiovascular responses during early recovery from exercise.

The mean absolute change in limb blood flow (LBF; A), mean arterial pressure (MAP; B), and limb vascular conductance (LVC; C) from end-exercise to early recovery (cardiac cycles 1–3, 4–6, and 7–9; CC_1–3, CC_4–6, and CC_7–9, respectively) following exercise below (▽) and above (▲) CP. Individual responses are presented in the background. * Significantly different from end-exercise (p < 0.001). † Significant difference between below- and above-CP tests (p < 0.001).

Figure 3.. Percent impedance of limb blood flow and percent change in limb vascular conductance during immediate recovery as a function of power-output.

The mean percent impedance of limb blood flow (%IMP_LBF; A) and percent change in limb vascular conductance (ΔLVC; B) from end-exercise to immediate recovery (cardiac cycles 1–3; CC_1–3,) following exercise below (▽) and above (▲) CP. Individual responses are presented in the background. * Significantly greater than below CP (p < 0.001). † Significant difference in variance between below- and above-CP tests (p < 0.01).

Figure 4.. Limb blood flow and changes in limb vascular conductance during immediate recovery as a function of power-output.

End-exercise limb blood flow (LBF; A) and the absolute change in limb vascular conductance (ΔLVC; B) during immediate recovery (cardiac cycles 1–3; CC_1–3) from exercise below (▽) and above (▲) CP as a function of test power-output for each subject. Note 1) the >30% reduction in slope of the LBF/W relationship above, compared to below, CP and 2) the significant relationship between ΔLVC and power-output above, but not below, CP (p < 0.001; r = 0.85).

Figure 5.. Relationship between critical power and below-versus-above critical power differences in limb vascular conductance changes during immediate recovery.

Individual critical powers (CPs) and changes in limb vascular conductance (ΔLVC) during immediate recovery (CC_1–3) above CP as a percent greater than the below-CP response. Note no significant relationship suggesting individual differences in CP (and therefore below-to-above CP power-output differences) do not explain the relationship between absolute ΔLVC and power-output above-CP in Fig. 4B.