Effects of pedal frequency on estimated muscle microvascular O2 extraction

An increase in muscle contraction frequency could limit muscle blood flow ((Q)over dot(M)), compromising the matching of (Q)over dot(M) and muscle oxygen uptake ((V)over dot(O2M)). This study examined the effects of pedal cadence on skeletal muscle oxygenation at low, moderate and peak exercise. Nine healthy subject [24.7 +/- 6.3 years (SD)] performed incremental cycling exercise at 60 and 100 rpm. Pulmonary (V)over dot(O2) ((V)over dot(O2P)) 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 O-2 extraction (i.e., [HHB] proportional to (V)over dot(O2M)/(Q)over dot(M)). The (V)over dot(O2P) and [HHb] for low, moderate and at peak excecise were determined. The (V)over dot(O2P) at 60 rpm (low = 0.64 +/- 0.13, moderate = 2.03 +/- 0.38 and peak= 3.39 +/- 0.84 1/min) were lower (P < 0.01) than at 100 rpm ( 1.29 +/- 0.23, 2.14 +/- 0.39 and 3.54 +/- 0.88 1/min, respectively). There was a progressive increase in [HHb] from low to peak excercise. However, there was no significant difference ( ANOVA, P = 0.94) for the 60 (in mu M, low= 24.0 +/- 9.5, moderate = 30.5 +/- 13.8 and peak = 36.7 +/- 16.5) and 100 contractions/min (in mu M, low = 25.7 +/- 11.6, moderate = 32.1 +/- 14.0 and peak= 35.4 +/- 16.5). We conclude that vastus lateralis O-2 extraction was similar at 60 and 100 cpm, suggesting that the (V)over dot(2M)/(Q)over dot(M) in the microcirculation was not altered and, presumably, no impairment of (Q)over dot(M) occurred with the increase in pedal frequency.