Sprint interval training improves aerobic fitness on little time, but in trained men longer intervals improved time-trial performance more
Interpretation
In untrained and recreationally active adults the evidence that sprint intervals raise VO2max is reasonably consistent (Sloth 2013: 19 studies, 442 people, g 0.63), and the physiology supports large muscle adaptations from small volumes (MacInnis and Gibala 2017). In trained athletes, the head-to-head evidence is thin: 6 small all-male trials found long intervals (4 min or more) improved time trials about 2.0% more than sprints (Rosenblat 2020), while a network meta-analysis of 302 trained cyclists found both effective with no clear winner (Norte 2024), and a programming review found no dose-response for sprint training (Rosenblat 2021). No trial studied triathletes.
Practical implication
Triathletes can use occasional short sprint sessions for variety, top-end power or very time-poor weeks, but not as a replacement for longer intervals and endurance riding, especially for long-course racing. Get medical clearance first if you have heart symptoms or conditions. Evidence in trained athletes comes from young men.
Sources
5 sources, independently verified.
Sprint interval training (2-8 weeks) improved VO2max with a pooled Hedges' g of 0.63 (95% CI 0.39-0.87); individual study gains about 4.2-13.4%. Aerobic performance improved consistently (e.g. 750-kJ time trial +10.1% SIT vs +7.5% endurance training, ns) and anaerobic performance up to 17%. Only 4 RCTs; assessors generally unblinded; safety in unscreened people not studied.
Both HIIT and sprint interval training produce classic endurance adaptations, including higher VO2max and mitochondrial content. HIIT generally raises VO2max more than moderate continuous training at matched volume; sprint interval training increases mitochondrial content and VO2max about as much as moderate continuous training despite much lower volume. Effects of intensity on stroke volume, cardiac output and blood volume, and interactions of intensity, duration and frequency, are less clear.
HIIT improved time trials 0.9% more than SIT overall (not significant); long HIIT (work bouts of 4 min or more) improved time trials about 2.0% more than SIT (ES 0.88, p = 0.01); maximal aerobic power about 2.4% more with HIIT; VO2max did not differ. Mean PEDro score 4.7/10, no blinding, three non-randomised trials, small groups, cycling, running and rowing.
Exploratory, non-pre-registered network meta-analysis; interventions averaged 5.8 weeks and 2.3 interval sessions a week. Interval training beat low/moderate continuous training with a small effect (SMD 0.33, 95% CI 0.06-0.60); threshold outcomes 0.46 with a CI crossing zero. No clear best type by work-bout duration; both HIIT and sprint interval training judged effective; versus continuous training, short HIIT was significant while long HIIT and SIT were not in the subgroup analysis. Most estimates imprecise.
Trained athletes improved time trials by about 2% vs up to 6% in less-trained people. For HIIT, gains depended on work-bout duration rather than intensity, with a dose-response for sessions, weeks and total work; no dose-response for sprint interval training. Authors conclude programmes should match training status and that exceeding a minimum effective dose may not add benefit. Mixed sports, mostly men.