Unaltered Vo₂ kinetics despite greater muscle oxygenation during heavy-intensity two-legged knee extension versus cycle exercise in humans

Relative perfusion of active muscles is greater during knee extension ergometry (KE) than cycle ergometry (CE). This provides the opportunity to investigate the effects of increased O 2 delivery (Q̇o 2 ) on deoxygenation heterogeneity among quadriceps muscles and pulmonary oxygen uptake (V̇o 2 ) kinetics. Using time-resolved near-infrared spectroscopy, we hypothesized that compared with CE the superficial vastus lateralis (VL), superficial rectus femoris, and deep VL in KE would have 1) a smaller amplitude of the exercise-induced increase in deoxy[Hb + Mb] (related to the balance between V̇o 2 and Q̇o 2 ); 2) a greater amplitude of total[Hb + Mb] (related to the diffusive O 2 conductance); 3) a greater homogeneity of regional muscle deoxy[Hb + Mb]; and 4) no difference in pulmonary V̇o 2 kinetics. Eight participants performed square-wave KE and CE exercise from 20 W to heavy work rates. Deoxy[Hb + Mb] amplitude was less for all muscle regions in KE ( P < 0.05: superficial, KE 17–24 vs. CE 19–40; deep, KE 19 vs. CE 26 μM). Furthermore, the amplitude of total[Hb + Mb] was greater for KE than CE at all muscle sites ( P < 0.05: superficial, KE, 7–21 vs. CE, 1–16; deep, KE, 11 vs. CE, −3 μM). Although the amplitude and heterogeneity of deoxy[Hb + Mb] were significantly lower in KE than CE during the first minute of exercise, the pulmonary V̇o 2 kinetics was not different for KE and CE. These data show that the microvascular Q̇o 2 to V̇o 2 ratio, and thus tissue oxygenation, was greater in KE than CE. This suggests that pulmonary and muscle V̇o 2 kinetics in young healthy humans are not limited by Q̇o 2 during heavy-intensity cycling.