This manuscript quantified spleen volume changes and examined the relationship between those changes and V̇O2 kinetics during supine cycling. Ten volunteers (age=22±3), completed three step-transitions from 20 W to their power output at 90% gas exchange threshold. Ultrasonic measurements of the spleen were performed each minute. The largest spleen volume reduction was 105 mL (p=.001). No associations existed between: i) spleen volumes at rest ii) spleen volume changes (%) and τV̇O2p. Larger resting spleen volume and greater emptying do not correlate with a faster τV̇O2p. Novelty: • Greater splenic contractions do not augment τV̇O2p, irrespective of spleen emptying and subsequent erythrocyte release.

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Purpose The role of splenic emptying in O 2 transport during aerobic exercise still remains a matter of debate. Our study compared the differences in spleen volume changes between aerobically trained and untrained individuals during step-transition supine cycling exercise at moderate-intensity. We also examined the relationship between spleen volume changes, erythrocyte release, and O 2 uptake parameters. Methods Fourteen healthy men completed all study procedures, including a detailed medical examination, supine maximal O 2 uptake ( max.) test, and three step-transitions from 20 W to a moderate-intensity power output, equivalent to uptake at 90% gas exchange threshold. During these step-transitions pulmonary , near-infrared spectroscopy of the vastus lateralis , and cardiovascular responses were continuously measured. In parallel, minute-by-minute ultrasonic measurements of the spleen were performed. Blood samples were taken before and immediately after step-transition cycling. Results On average, max. was 10 mL kg min −1 ( p = 0.001) higher in trained compared to their aerobically untrained peers. In response to supine step-transition cycling, the splenic volume was significantly reduced, and the largest reduction (~ 106 to 115 mL, ~ 38%, p = 0.001) was similar in both aerobically trained and untrained individuals. Erythrocyte concentration and platelet count transiently increased after exercise cessation, with no differences observed between groups. However, the vastus lateralis deoxygenation amplitude was 30% ( p = 0.001) greater in trained compared to untrained individuals. No associations existed between: (i) spleen volumes at rest (ii) spleen volume changes (%), (iii) resting hematocrit and oxygen uptake parameters. Conclusion Greater splenic emptying and subsequent erythrocyte release do not lead to a slower , regardless of individual max. readings.

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There is an ongoing debate in the literature as to whether an exercise-induced reduction in spleen volume, and a subsequent increase in erythrocyte concentration contribute to superior V̇O2 uptake during exercise and recovery, and thus improve endurance performance. In the present study, we investigated whether larger splenic emptying contributes to faster excess post-exercise O2 consumption (EPOC) after supine cycling. We hypothesized that a larger reduction in splenic volume would be associated with faster EPOC (including absolute values and recovery time constant, τ), since a larger splenic emptying has been reported in the literature as an advantage of aerobically trained individuals over healthy controls (Holmström et al., 2021). Here, fifteen healthy participants (age 24±4, 47% women) completed three laboratory visits at least 48 hours apart. After obtaining medical clearance and familiarizing themselves with the test, they performed a ramp V̇O2 peak test in the supine position until task failure. Next, they completed three step-transition test from 20 W to a moderate-intensity power output (PO), equivalent to V̇O2 uptake at 90% GET, where data on metabolic, cardiovascular, and splenic responses were recorded simultaneously. After step-transition test cessation, O2 debt (EPOCfast) was recorded, and the first 10 minutes of the recovery period were used for analysis. Blood samples were collected before and immediately after the end of exercise. In response to moderate-intensity supine cycling (V̇O2 uptake ~2 L·min-1), we observed a ~35% (p=0.001) reduction in splenic volume resulting in a transient ~3-4% (p=0.001) increase in erythrocyte concentration in mixed venous blood. During recovery, mean τV̇O2 was 45±18 sec., the amplitude was 2.4±0.5 L·min-1, and EPOCfast was 1.69 L·min-1. Significant correlations were observed between the percent change in spleen volume and i) EPOCfast (r=-0.657, p=0.008) and ii) τV̇O2 (r=-0.619, p=0.008), but not for iii) V̇O2 peak (p=.435, p=0.105) or iv) the amplitude of V̇O2 recovery kinetics (r=0.447, p=0.941). Our work suggests that individuals with larger splenic emptying tend to have slower O2 recovery kinetics and a greater O2 debt. Apparently, the transient, exercise-induced increase in circulating erythrocytes observed here is rather insufficient to increase O2 storage capacity in blood, and thereby improve endurance performance. References: Zubac D, Obad A, Šupe-Domić D, et al. (2022). No differences in splenic emptying during on-transient supine cycling between aerobically trained and untrained participants. Eur J Appl Physiol, 11;1-15. doi: 10.1007/s00421-021-04843-w ; Holmström PK, Karlsson Ö, Lindblom H, et al. (2021). Enhanced splenic volume and contraction in elite endurance athletes. J Appl Physiol, 131(2), 474-486, doi: 10.1152/japplphysiol.01066.2020 . All authors declare no conflict of interest. There was no external funding for this work. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.