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.
In endurance athletes with more than 2 years of training experience, pooled results from 3 randomised trials showed a moderate advantage for polarised training, mostly easy work with a small share very hard, over threshold training for time-trial performance. Only 4 trials met the inclusion criteria in total.
preliminary · human dataSuggestive rather than settled: only 4 randomised trials qualified and just 3 had data that could be pooled, all in trained endurance athletes. Those trials scored 4 or 5 out of 10 for methodological quality, which is modest, and time-trial performance was the only outcome with enough data to combine.
In adults with type 2 diabetes, every 30 minutes per week of supervised aerobic exercise cut HbA1c by 0.22 percentage points, with the benefit climbing to a 0.96 point drop at 100 minutes per week. Beyond 100 minutes the curve flattened, so extra weekly cardio bought no further HbA1c reduction.
well supported · human dataAbout as strong as exercise evidence gets for the headline effect, which the authors rated high certainty: results pooled from 26 trials in 1253 adults with type 2 diabetes. The medication and side-effect numbers rest on far fewer trials and are much softer.
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.
Aerobic training done with blood flow restriction cuffs produced greater muscle growth and strength gains than the same training without them, in pooled results from 7 trials of healthy adults aged 18 to 60.
preliminary · human dataPromising but not settled: the pooling covered only 7 trials in healthy adults aged 18 to 60, and most carried a high risk of bias. The benefit is plausible, but its true size could shrink as better studies arrive.
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.
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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.