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 well-trained subjects [mountain climbers; peak O 2 consumption (V˙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 (Δ[O 2 Hb]) and deoxygenated Hb (Δ[HHb]) in the vastus lateralis muscle, by continuous-wave 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 (Δ[O 2 Hb + HHb]) in the muscle region of interest increased as a function ofw˙ up to 60–65% ofV˙o 2 peak , after which it remained unchanged. The oxygenation index (Δ[O 2 Hb − HHb]) showed an accelerated decrease from 60– 65% ofV˙o 2 peak . In the presence of a constant total Hb volume, the observed Δ[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˙, i.e., with the onset of blood lactate accumulation. Previous studies showed relatively constant femoral venous[Formula: see text] levels at w˙ higher than ∼60% of maximal O 2 consumption. Thus muscle deoxygenation observed in the present study from 60–65% ofV˙o 2 peak could be attributed to capillary-venular Hb desaturation in the presence of relatively constant capillary-venular[Formula: see text] levels, as a consequence of a rightward shift of the O 2 Hb dissociation curve determined by the onset of lactic acidosis.