This study examined the physiological, perceptual, and performance responses to a 2-week block of increased training load and compared whether responses differ between high-intensity interval (HIIT) and low-intensity (LIT) endurance training.
Thirty recreationally trained males and females performed a two-week block of 10 HIIT-sessions (INT, n = 15) or 70 % increased volume of LIT (VOL, n = 15). Running time in the 3000 m and basal serum and urine hormone concentrations were measured before (T1) and after the block (T2), and after a recovery week (T3). In addition, weekly averages of nocturnal heart rate variability (HRV) and perceived recovery were compared to the baseline.
Both groups improved their running time in the 3000 m from T1 to T2 (INT -1.8 ± 1.6 %, p = 0.003; VOL -1.4 ± 1.7 %, p = 0.017) and T1 to T3 (INT -2.5 ± 1.6 %, p < 0.001; VOL -2.2 ± 1.9 %, p = 0.001). Resting norepinephrine concentration increased in INT from T1 to T2 (p = 0.01) and remained elevated at T3 (p = 0.018). The change in HRV from the baseline was different between the groups during the first week (INT -1.0 ± 2.0 % vs. VOL 1.8 ± 3.2 %, p = 0.008). Muscle soreness increased only in INT (p < 0.001) and the change was different compared to VOL across the block and recovery weeks (p < 0.05).
HIIT and LIT blocks increased endurance performance in a short period of time. Although both protocols seemed to be tolerable for recreational athletes, a HIIT-block may induce some negative responses such as increased muscle soreness and decreased parasympathetic activity.