Is zone 2 cardio worth the time?
What the studies we have published say — strongest evidence first. Every claim below links to the paper it came from.
A single endurance workout keeps resting fat oxidation elevated on the day of exercise and into the following day, a meta-analysis finds. Total energy expended in the session and the exerciser's aerobic fitness, not exercise intensity, predicted the size of the effect.
well supported · human dataMeta-analysis pooling varied exercise-study protocols; the abstract does not detail individual sample sizes; the sex difference appeared only in the fasted (postabsorptive) state; measures fat oxidation, not fat loss.
In a 6-week randomised trial of 45 physically active male university students, twice-weekly 4 × 4 min HIIT improved VO2max and time to exhaustion about as much as three weekly sessions, while once weekly produced only a small VO2max rise; completion rates were 100%, 93% and 80% for the 1-, 2- and 3-session groups.
preliminary · human dataOne 6-week RCT, n=45 physically active male university students (15 per group); homogeneous sample and short intervention period, so the 'optimal frequency' conclusion needs testing in larger, more diverse groups.
Six weeks of home-based Thai-boxing HIIT improved VO2 max, resting blood pressure, resting heart rate, grip strength, trunk endurance and body fat more than a general health-education programme in 58 sedentary young adults. Muscle mass and fat-free mass did not differ between groups.
preliminary · human dataOne 6-week RCT, n=58 sedentary young adults in Thailand; short duration, no long-term follow-up, and the comparison group received a different active programme.
A network meta-analysis of 29 randomised trials in 1,301 middle-aged and older adults with type 2 diabetes found high-intensity resistance training produced the largest HbA1c reduction versus usual care (MD -0.62), ranking top of 10 exercise interventions, while moderate-intensity resistance training led on fasting glucose and high-intensity aerobic training drove the biggest gains in peak oxygen uptake. No intervention significantly reduced resting heart rate.
well supported · human dataNetwork meta-analysis of 29 RCTs and 1,301 participants with type 2 diabetes; only the HbA1c result for high-intensity resistance training reached moderate certainty, the fasting glucose and blood pressure networks were rated low or very low, and the VO2peak network was sparse and described as exploratory.
Pooling 53 randomised trials in footballers, strength training ranked top for jump height, squat one-rep max, 5m, 10m and 30m sprints and agility, while endurance training ranked best for VO2max. The rankings come from probability scores across a network of trials, not head-to-head certainty.
well supported · human dataSystematic review and network meta-analysis of 53 RCTs totalling 1,580 footballers; rankings rest on SUCRA probabilities rather than direct comparisons, trial quality and dosing varied, and findings apply to trained football players rather than general gym-goers.
A meta-analysis of 21 studies found endurance training done alongside lifting reduced gains in hypertrophy, strength and especially power versus lifting alone, with interference worsening as endurance frequency and duration increased. Running, but not cycling, was linked to significant decrements in hypertrophy and strength in this analysis.
well supported · human dataMeta-analysis of 21 studies; the moderator findings on frequency, duration and modality are correlational across studies, not head-to-head trial comparisons.
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This page collects what our published, cited studies say; it is information, not medical advice. Have a different question? Ask it here.