Maximal O2 Uptake Is Higher During Whole-Body Compared to Lower-Body Exercise due to Higher Convective, but not Diffusive, O2 Transport

PURPOSE: Maximal O2 uptake (V̇O2max) is higher during whole-body compared to lower-body large muscle mass exercise. However, whether this is explained by differences in convective or diffusive O2 transport is unclear. We examined differences in components of the O2 pathway and muscle tissue (de)oxygenation between whole-body (uphill walking/running with ski poles) and lower-body (cycling) exercise in 14 Tier 3 male triathletes. METHODS: Participants completed ramp-incremental tests to exhaustion in both modalities. We assessed systemic O2 transport via ergospirometry, pulse oximetry, impedance cardiography and hemoglobin concentration. Muscle oxygenation (vastus lateralis) was monitored using near-infrared spectroscopy. We modeled convective (Q̇aO2max) and diffusive (ḊO2max) O2 transport at maximal exercise using the Helsinki O2 Pathway Tool. The slopes for convective O2 flow (Q̇aO2slope) and arterial mixed-venous O2 content (C(a - v)O2slope) were determined against O2 uptake (V̇O2). RESULTS: V̇O2max and Q̇aO2max were higher during whole-body than lower-body exercise (mean [95%CI]: 5.07 [4.83-5.31] versus 4.70 [4.4-4.95] L/min, q = 0.0002, and; 6.02 [5.61-6.43] versus 5.39 [4.96-5.82] L/min, q = 0.0004, respectively). ḊO2max did not differ between whole-body (147 [113-181] ml/min/mmHg) and lower-body (169 [130-208] ml/min/mmHg) exercise (q = 0.184). Q̇aO2 was higher and C(a - v)O2 lower as a function of V̇O2 during whole-body than lower-body exercise. Peak amplitudes of total- and deoxygenated hemo- and myoglobin were lower during whole-body compared to lower-body exercise (q = 0.002 and q = 0.0005, respectively), but tissue saturation index was unchanged (q = 0.453). CONCLUSIONS: V̇O2max was higher during whole-body compared to lower-body exercise due to higher convective, but not diffusive O2 transport. Throughout incremental exercise, V̇O2 relied more on Q̇aO2 during whole-body compared to lower-body exercise.