Interval training modestly improves threshold power and time trials in trained endurance athletes, but no interval intensity (threshold, sweet spot or VO2max) has been shown to be clearly best
Interpretation
Three meta-analyses point the same way: intervals beat continuous training with a small effect in 302 trained cyclists (Norte 2024, SMD 0.33) with no clear winner by interval duration; trained athletes improved time trials about 2% with gains depending on work-bout duration and dose rather than intensity (Rosenblat 2021, 29 studies, 400 men and 91 women); adding HIIT improved most measures 1.1-2.3% in elite athletes, with interval type unclear (Wiesinger 2025, 34 studies). Single small trials show larger differences (Rønnestad 2015: 40-min power +12% vs +4% in 16 men; Stöggl 2014: threshold-dominated training less effective than polarised in 48 athletes). No controlled trial of 'sweet spot' training was found; most data are short (4-12 weeks), mostly in men, and not in triathletes.
Practical implication
With about 8 hours a week, one or two interval sessions on the bike with the rest easy matches the research; rotate interval types across blocks and prioritise completing sessions over finding a perfect intensity. Expect gains of a few per cent. Sweet spot is untested, not disproven; evidence is extrapolated from trained cyclists and other endurance athletes, mostly men.
Sources
6 sources, independently verified.
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.
Adding HIIT to conventional training improved most performance measures by 1.1-2.3%; more aerobic HIIT types raised VO2max by about 2.6%; the largest effect (12.6%) was in female endurance athletes after 6 weeks of traditional long intervals. Moderating effects of HIIT type, sex, duration and training phase were unclear for most measures.
Ten weeks, two effort-matched sessions a week: 3 series of 13 x 30 s/15 s vs 4 x 5 min. Short vs long: power at 4 mmol/L +12% vs +5% (between-group ns), 40-min all-out power +12% vs +4% (p = 0.056), VO2max +8.7% vs +2.6% (significant), mean of five power tests +10.0% vs +3.1% (significant). Very small groups, men only.
Nine weeks of four intensity distributions. Polarised (68% low, 6% threshold, 26% high) improved VO2peak 11.7% and power/speed at 4 mmol/L 8.1%; HIIT improved VO2peak 4.8%. The threshold-dominated group (54% at threshold) showed no significant change in VO2peak, peak power/speed (+1.8%) or power/speed at 4 mmol/L (+1.4%), and had the most dropouts. Small mixed groups; lab outcomes only.
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.