Blood lactate accumulation and muscle deoxygenation during incremental exercise

Abstract

Near-infrared spectroscopy (NIRS) could allow insights into controversial issues related to blood lactate concentration ([La](b)) increases at submaximal workloads (ẇ). We combined, on five well-trained subjects [mountain climbers; peak O2 consumption (V̇O(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 (Δ[O2Hb]) and deoxygenated Hb (Δ[HHb]) in the vastus lateralis muscle, by continuous-wave NIRS. A 'point of inflection' of [La](b) vs. ẇ was arbitrarily identified at the lowest [La](b) value which was >0.5 mM lower than that obtained at the following ẇ. Total Hb volume (A[O2Hb + HHb]) in the muscle region of interest increased as a function of ẇ up to 60-65% of V̇O(2peak), after which it remained unchanged. The oxygenation index (A[O2Hb - HHb]) showed an accelerated decrease from 60-65% of V̇O(2peak). In the presence of a constant total Hb volume, the observed Δ[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. ẇ i.e., with the onset of blood lactate accumulation. Previous studies showed relatively constant femoral venous PO2 levels at ẇ higher than ~60% of maximal O2 consumption. Thus muscle deoxygenation observed in the present study from 60- 65% of V̇O(2peak) could be attributed to capillary-venular 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.