Effect of exercise intensity and apnea on splenic contraction and hemoglobin increase in well-trained cross-country skiers

The human spleen acts as a reservoir for red blood cells, which is mobilized into the systemic circulation during various conditions such as hypoxia and physical exertion. Cross-country (XC) skiers, renowned for their exceptional aerobic capacity, are regularly exposed to high-intensity exercise and local oxygen deficits. We investigated a putative dose-dependent relationship between splenic contraction and concomitant hemoglobin concentration ([Hb]) elevation across four exercise intensities in well-trained XC skiers. Fourteen male XC skiers voluntarily participated in a 2-day protocol, encompassing a serial apnea test and a VO2max test (day 1), followed by three submaximal exercise intensities on a roller skiing treadmill corresponding to 55, 70, and 85% of VO2max (day 2). Spleen volume was measured via ultrasonic imaging, and venous blood samples were used to determine [Hb] levels. Baseline spleen volume was similar (266(35) mL) for all conditions (NS). Notably, all conditions induced significant splenic contractions and transient [Hb] elevations. The VO2max test exhibited the most pronounced splenic contraction (35.8%, p < 0.001) and a [Hb] increase of 8.1%, while the 85% exercise intensity led to 27.1% contraction and the greatest [Hb] increase (8.3%, < 0.001) compared to baseline. The apnea test induced relatively smaller responses (splenic contraction: 20.4%, [Hb] = 3.3%, p < 0.001), akin to the response observed at the 70% exercise intensity (splenic contraction = 23%, [Hb] = 6.4%, p < 0,001) and 55% (splenic contraction = 20.0%, [Hb] = 4.8%, p < 0.001). This study shows a discernible dose-dependent relationship between splenic contraction and [Hb] increase with levels of exercise, effectively distinguishing between submaximal and maximal exercise intensity.