Effects of pedal frequency on estimated muscle microvascular O₂ extraction

Abstract

An increase in muscle contraction frequency could limit muscle blood flow ${\left( {\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}_{{\text{M}}} } \right)},$ compromising the matching of $\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}_{{\text{M}}}$ and muscle oxygen uptake ${\left( {\ifmmode\expandafter\dot\else\expandafter\.\fi{V}_{{{\text{O}}_{{{\text{2M}}}} }} } \right)}.$ This study examined the effects of pedal cadence on skeletal muscle oxygenation at low, moderate and peak exercise. Nine healthy subjects [24.7±6.3 years (SD)] performed incremental cycling exercise at 60 and 100 rpm. Pulmonary $\ifmmode\expandafter\dot\else\expandafter\.\fi{V}_{{{\text{O}}_{{\text{2}}} }} {\left( {\ifmmode\expandafter\dot\else\expandafter\.\fi{V}_{{{\text{O}}_{{{\text{2P}}}} }} } \right)}$ was measured breath-by-breath and vastus lateralis oxygenation was determined by near-infrared spectroscopy (NIRS). The deoxyhemoglobin signal ([HHb]) from NIRS was used to estimate microvascular O₂ extraction (i.e., [HHb] ∝ $\ifmmode\expandafter\dot\else\expandafter\.\fi{V}_{{{\text{O}}_{{{\text{2M}}}} }} {\text{/}}\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}_{{\text{M}}}$). The $\ifmmode\expandafter\dot\else\expandafter\.\fi{V}_{{{\text{O}}_{{{\text{2P}}}} }}$ and [HHb] for low, moderate and at peak exercise were determined. The $\ifmmode\expandafter\dot\else\expandafter\.\fi{V}_{{{\text{O}}_{{{\text{2P}}}} }}$ at 60 rpm (low=0.64±0.13, moderate=2.03±0.38 and peak=3.39±0.84 l/min) were lower (P<0.01) than at 100 rpm (1.29±0.23, 2.14±0.39 and 3.54±0.88 l/min, respectively). There was a progressive increase in [HHb] from low to peak exercise. However, there was no significant difference (ANOVA, P=0.94) for the 60 (in μM, low=24.0±9.5, moderate=30.5±13.8 and peak=36.7±16.5) and 100 contractions/min (in μM, low=25.7±11.6, moderate=32.1±14.0 and peak=35.4±16.5). We conclude that vastus lateralis O₂ extraction was similar at 60 and 100 cpm, suggesting that the $\ifmmode\expandafter\dot\else\expandafter\.\fi{V}_{{{\text{O}}_{{{\text{2M}}}} }} {\text{/}}\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}_{{\text{M}}}$ in the microcirculation was not altered and, presumably, no impairment of $\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}_{{\text{M}}}$ occurred with the increase in pedal frequency.