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) over dot). We combined, on five well-trained subjects [mountain climbers; peak O-2 consumption ((V) over dot O-2peak), 51.0 +/- 4.2 (SD) ml.kg(-1).min(-1)] performing incremental exercise on a cycle ergometer (30 W 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 (Delta[O(2)Hb]) and deoxygenated Hb (Delta[HHb]) in the vastus lateralis muscle, by continuous-wave NIRS. A "point of inflection" of [La](b) vs. (w) over dot was arbitrarily identified at the lowest [La](b) value which was >0.5 mM lower than that obtained at the following (w) over dot. Total Hb volume (Delta[O(2)Hb + HHb]) in the muscle region of interest increased as a function of (w) over dot up to 60-65% of (V) over dot O-2peak, after which it remained unchanged. The oxygenation index (Delta[O(2)Hb - HHb]) showed an accelerated decrease from 60-65% of (V) over dot O-2peak. In the presence of a constant total Hb volume, the observed Delta[O(2)Hb - HHb] decrease indicates muscle deoxygenation (i.e., mainly capillary-venular Hb desaturation). The onset of muscle deoxygenation was significantly correlated (r(2) = 0.95; P < 0.01) with the point of inflection of [La](b) vs. (w) over dot, i.e., with the onset of blood lactate accumulation. Previous studies showed relatively constant femoral venous P-O2 levels at (w) over dot higher than similar to 60% of maximal O-2 consumption. Thus muscle deoxygenation observed in the present study from 60-65% of (V) over dot O-2peak could be attributed to capillary-venular Hb desaturation in the presence of relatively constant capillary-venular P-O2 levels, as a consequence of a rightward shift of the O(2)Hb dissociation curve determined by the onset of lactic acidosis.