Adding LIT to HIIT: Is Low-Intensity Training Vital for Endurance-Trained Athletes during a 7-day HIIT Shock Microcycle?

PURPOSE: This study aimed to investigate the effects of a 7-d high-intensity interval training shock microcycle (HIIT-SM) with or without additional low-intensity training (LIT) on aerobic fitness and endurance performance compared with a control group (CG). METHODS: Thirty-three endurance-trained athletes (7 women, 26 men, mean ± SD: age, 30.2 ± 6.9 yr;maximal oxygen uptake (VO_2max), 59.8 ± 4.9 mL·min^-1·kg^-1) performed exercise testing at T1 and were randomly assigned to one of three groups: i) HSM, 10 running-based HIIT sessions (5 × 4 min at 90%-95% maximal heart rate) over 7 d; ii)HSM+ LIT, equal toHSMwith additional 30 min of LIT after each HIIT; iii) CG, regular training. Exercise testing was repeated 3 d (T2), 7 d (T3), and 14 d (T4) after the intervention. A 5-km time trial (TT_5km) was performed 3-4 d before T1 and 10-11 d after the intervention. Data were analyzed by two-way repeated-measures ANOVA. RESULTS: No interaction effect was found for VO_2max (P = 0.170, _pη² = 0.09), peak power output (P = 0.734, _pη² = 0.04), and work economy (P = 0.804, _pη² = 0.03). There was an interaction for velocity at lactate threshold (P = 0.006, _pη² = 0.18) with increased velocity in HSM at T2 (3.2%, P = 0.030), T3 (4%, P = 0.006), T4 (4%, P = 0.003), as well as in HSM + LIT for T2 (3.2%, P = 0.011), whereas CG showed no change. There was an interaction for TT5km (P = 0.044, _pη² = 0.19), with HSM improving 2.7% (P = 0.003) and HSM + LIT 2.3% (P = 0.010), whereas CG was, on average, -0.3% (P = 0.821) slower. CONCLUSIONS: HIIT-SM with or without additional LIT has negligible effects on VO_2max but improves other key endurance variables in endurance-trained athletes. No superiority of either intervention group was demonstrated. Therefore, additional LIT during HIIT-SM is not beneficial.