Rønnestad 30/15s — Free Zwift Workout
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The 15 seconds is the mechanism

Fifteen seconds is too short for oxygen uptake to fall back to baseline, so each surge restarts from an already-elevated state. Measured directly in elite cyclists, the short format held riders above 90% of VO2 max 54% longer than an effort-matched long-interval session, at a similar rating of effort ^[3]. The recovery is doing work; treat it as a rest and you lose the reason the format exists.

That is also why the easy segment is an easy spin at roughly half your surge power, not a coast. Stopping pedalling lets the mechanism decay.

What the trials found, and what they did not

Over ten weeks, effort-matched short intervals raised VO2 max 8.7% against 2.6% for long intervals ^[1]. The elite replication ran the same 30-on, 15-off shape in three sets with 3 minutes between them, and pointed the same way on peak aerobic power and 20-minute power, though VO2 max itself did not separate the groups ^[2].

A 2025 network meta-analysis pooled 51 trials and found high-intensity intervals, sprint intervals and repeated sprints all raised VO2 max, with no significant difference between those three families ^[4]. It compared families rather than individual shapes, so treat 30/15s as a strong shape to rotate through rather than one proven to outrank the rest.

How to ride the set

Judge the session on accumulated time in the band, not on any single surge. Heart rate lags the first few reps and only settles near the ceiling around rep four or five, so chasing it early will make you start too hard and fade. Pick a wattage you can still hit on the last rep of the last set.