Physiological, Perceptual, and Performance Responses to the 2-Week Block of High- versus Low-Intensity Endurance Training

PURPOSE: This study examined the physiological, perceptual, and performance responses to a 2-wk block of increased training load and compared whether responses differ between high-intensity interval (HIIT) and low-intensity training (LIT). METHODS: Thirty recreationally trained males and females performed a 2-wk 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 (T(1)) and after the block (T(2)), and after a recovery week (T(3)). In addition, weekly averages of nocturnal heart rate variability (HRV) and perceived recovery were compared with the baseline. RESULTS: Both groups improved their running time in the 3000 m from T(1) to T(2) (INT = −1.8% ± 1.6%, P = 0.003; VOL = −1.4% ± 1.7%, P = 0.017) and from T(1) to T(3) (INT = −2.5% ± 1.6%, P < 0.001; VOL = −2.2% ± 1.9%, P = 0.001). Resting norepinephrine concentration increased in INT from T(1) to T(2) (P = 0.01) and remained elevated at T(3) (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 with VOL across the block and recovery weeks (P < 0.05). CONCLUSIONS: HIIT and LIT blocks increased endurance performance in a short period. 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.