Blood lactate accumulation and muscle deoxygenation during incremental exercise

Near-infrared spectroscopy (NIRS) could allow insights into controversial issues related to blood lactate concentration ([La]b) increases at submaximal workloads (w˙). We combined, on five welltrained subjects [mountain climbers; peak O2 consumption (V˙O2peak), 51.0 ± 4.2 (SD) ml•kgˉ1•minˉ1] performing incremental exercise on a cycle ergometer (30W added every 4 min up to voluntary exhaustion), measurements of pulmonary gas exchange and earlobe [La]b with determinations of concentration changes of oxygenated Hb (∆[O2Hb]) and deoxygenated Hb (∆[HHb]) in the vastus lateralis muscle, by continuouswave NIRS. A ‘‘point of inflection’’ of [La]b vs. w˙ was arbitrarily identified at the lowest [La]b value which was >0.5 mM lower than that obtained at the following w˙. Total Hb volume (∆[O2Hb + HHb]) in the muscle region of interest increased as a function of w˙ up to 60–65% of VO2peak, after which it remained unchanged. The oxygenation index (∆ [O2Hb - HHb]) showed an accelerated decrease from 60–65% of VO2peak. In the presence of a constant total Hb volume, the observed ∆[O2Hb - HHb] decrease indicates muscle deoxygenation (i.e., mainly capillary-venular Hb desaturation). The onset of muscle deoxygenation was significantly correlated (r2 = 0.95; P < 0.01) with the point of inflection of [La]b vs. w˙, i.e., with the onset of blood lactate accumulation. Previous studies showed relatively constant femoral venous PO2 levels at w˙ higher than~60% of maximal O2 consumption. Thus muscle deoxygenation observed in the present study from 60–65% of VO2peak could be attributed to capillaryvenular Hb desaturation in the presence of relatively constant capillary-venular PO2 levels, as a consequence of a rightward shift of the O2Hb dissociation curve determined by the onset of lactic acidosis.