A 6-day high-intensity interval microcycle improves indicators of endurance performance in elite cross-country skiers.

Purpose: The aim of this study was to compare the effects of a 6-day high-intensity interval (HIT) block [BLOCK, = 12, maximal oxygen uptake (V̇O = 69. 6 ± 4.3 mL·min·kg)] with a time-matched period with usual training (CON, = 12, V̇O = 69.2 ± 4.2 mL·min·kg) in well-trained cross-country (XC) skiers on physiological determinants and indicators of endurance performance. Furthermore, the study aimed to investigate the acute physiological responses, including time ≥90% of V̇O, and its associated reliability during repeated HIT sessions in the HIT microcycle.

Methods: Before the 6-day HIT block and following 5 days of recovery after the HIT block, both groups were tested on indicators of endurance performance. To quantify time ≥90% of V̇O during interval sessions in the HIT block, V̇O measurements were performed on the 1st, 2nd, and last HIT session in BLOCK.

Results: BLOCK had a larger improvement than CON in maximal 1-min velocity achieved during the V̇O test (3.1 ± 3.1% vs. 1.2 ± 1.6%, respectively; = 0.010) and velocity corresponding to 4 mmol·L blood lactate (3.2 ± 2.9% vs. 0.6 ± 2.1%, respectively; = 0.024). During submaximal exercise, BLOCK displayed a larger reduction in respiratory exchange ratio, blood lactate concentration, heart rate, and rate of perceived exertion ( < 0.05) and a tendency towards less energy expenditure compared to CON ( = 0.073). The ICC of time ≥90% V̇O in the present study was 0.57, which indicates moderate reliability.

Conclusions: In well-trained XC skiers, BLOCK induced superior changes in indicators of endurance performance compared with CON, while time ≥90% of V̇O during the HIT sessions in the 6-day block had a moderate reliability.