Skeletal Muscle Deoxygenation Amplitude, Rather Than Splenic Emptying Contributes to Higher V̇O 2 max. During Supine Cycling in Healthy Active Men.

The aim of the present study was to investigate whether greater splenic emptying or higher skeletal muscle deoxygenation amplitude (determined by near-infrared spectroscopy, NIRS) contributes to higher V̇O max. during supine cycling. We hypothesized that the reduction in splenic volume, and subsequent manipulations of O delivery by blood ejected from the spleen, would not contribute to higher V̇O max. during supine cycling, because this transient increase in circulating red blood cells (RBC's) would be insufficient to increase the O storage capacity in the blood. Rather, O uptake during supine cycling in healthy subjects is expected to be limited, at least in part, by skeletal muscle bioenergetics. Here, fourteen healthy young individuals completed all study procedures. On three separate days they underwent a medical examination, supine V̇O max. test, and three step-transitions from 20 W to a moderate-intensity power output equivalent to V̇O uptake at 90% gas exchange threshold. During these step-transitions, pulmonary V̇O , NIRS of the vastus lateralis (VL), and cardiovascular responses (via Finapres) were measured continuously. In parallel, minute-by-minute ultrasound measurements of the spleen were performed. Blood samples were taken before and immediately after the step-transition cycling. Mean V̇O max. was 46.5±6.5 mL·kg·min . In response to supine step-transition cycling, spleen volume was significantly reduced (by ~38%, p=.001). Both hematocrit and hemoglobin concentration increased by ~4%, after exercise cessation. There was no correlation between: i)spleen volume at rest, ii) changes in spleen volume (%), iii) hematocrit at rest and V̇O max. However, a linear, near-significant correlation was observed between VL deoxygenation amplitude and V̇O max. Apparently, individuals with a higher V̇O max. are capable of greater O extraction from the superficial knee extensors, whereas in parallel, greater splenic emptying does not correlate with higher V̇O max, regardless of the subsequent release of erythrocytes. Therefore, it is reasonable to suggest that maximal O utilization during supine cycling in healthy young men is limited, in part, by skeletal muscle bioenergetics. REFERENCES: Holmström, PK, Karlsson Ö, Lindblom H, McGawley K, & Schagatay, EK. (2021). Enhanced splenic volume and contraction in elite endurance athletes. J Appl Physiol, 131(2), 474-486, DOI: 10.1152/japplphysiol.01066.2020; Zubac, D, Obad, A, Bosnjak, A, Zec, M, Ivancev, V, Valic, Z. (2021). Spleen emptying does not correlate with faster oxygen kinetics during a step-transition supine cycling. Appl Physiol Nutr Metab. Accepted for publication in Jun 24. doi: 10.1139/apnm-2021-0294.2021.