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 O₂ 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[O₂Hb]) 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[O₂Hb + 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[O₂Hb − HHb]) showed an accelerated decrease from 60– 65% ofV˙o_{2 peak}. 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]_bvs. w˙, i.e., with the onset of blood lactate accumulation. Previous studies showed relatively constant femoral venous$P{O}_2$ levels at w˙ higher than ∼60% of maximal O₂consumption. 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$P{O}_2$ levels, as a consequence of a rightward shift of the O₂Hb dissociation curve determined by the onset of lactic acidosis.