The Magnitude Of Exercise-induced Hypervolemia Following High-intensity Interval Exercise Depends On Session Duration

The mechanisms of blood volume expansion during endurance training are poorly understood, but the exercise-induced increase in plasma volume ~ 24 hours post-exercise may act as an erythropoietic signal. PURPOSE: To determine the impact of high-intensity interval exercise duration on plasma volume responses. METHODS: On separate days, 16 healthy, recreationally active participants (n = 8 males; n = 8 females) performed four (4x4) or eight intervals (8x4) consisting of 4 min at 105% critical power interspersed with 3 min rest. We collected venous blood samples before, immediately after, and 24 hours after each HIIT session. Changes in plasma volume from baseline were calculated using hemoglobin concentration and hematocrit. Blood levels of renin, aldosterone, and NTpro-ANP (corrected for plasma volume changes) were measured using ELISA kits to assess plasma volume regulation. Data were analyzed using two-way, repeated measures ANOVAs and followed with Tukey’s multiple comparison tests. Results were considered statistically significant at p < 0.05. RESULTS: Plasma volume (Figure 1A) decreased from baseline immediately after both protocols (4x4: -4.4 ± 3.5%, p < 0.05; 8x4: -4.4 ± 3.6%, p < 0.05) but was only significantly elevated above baseline 24 hours after the 8x4 protocol (4x4: 1.0 ± 7.1%, p > 0.05; 8x4: 5.6 ± 4.6%, p < 0.05). Renin (1B) was higher than baseline immediately post-exercise after the 8x4 protocol. NTpro-ANP (1C) and aldosterone (1D) were both higher than baseline immediately post-exercise after both protocols, but the increases were larger following the 8x4 protocol. CONCLUSIONS: Doubling HIIT volume caused higher post-exercise concentrations of key hormones and an increase in plasma volume that was not observed following the 4x4 condition. There appears to be a minimum duration of interval exercise necessary to induce hypervolemia following a single session.