VO2 sub-max protocols estimate VO2 max from heart rate response at sub-maximal workloads — without requiring an all-out effort.
Mechanism — At a given submaximal pace or power, fitter athletes have lower heart rates. Protocols (YMCA cycle test, Astrand-Rhyming, Rockport walk test) extrapolate to predicted max. Less accurate than true testing but useful for tracking.
Use case — Athletes who can't or shouldn't max-test (deconditioned, cardiac history, frequent testing). Watch sub-max heart rate at fixed pace over time — drop = improvement.
Caveats — Heart rate is affected by sleep, heat, caffeine, stress — standardize conditions or trends are noisy. Less precise than VO2 max measured by a metabolic cart.