Exercise intensity and VO₂max: a meta-analysis of 14 trials

A network meta-analysis in the Journal of Clinical Medicine (2025/2026) pooled fourteen randomised trials in people awaiting major abdominal or cardiothoracic surgery. It compared the components of prehabilitation (exercise, nutrition, psychological support and their combinations) for their effect on VO₂max, and split exercise by intensity, HIIT (high-intensity interval training) included.

The meta-analysis results

Exercise versus control showed an effect size of 0.44 on VO₂max. With exercise grouped by intensity, the largest effect was for HIIT versus control (ES: 0.51).

The authors note that a low VO₂max before surgery is associated with worse outcomes afterwards.

HIIT requires a good functional baseline.

The meta-analysis does not say which exercise suits a single person: the choice between intense training and more moderate activity, such as walking, is for the care team.

How much intensity before surgery

The meta-analysis measures the effect of exercise on VO₂max, not how much to train in the days before the operation. Progressive reduction of intensity in the final weeks (tapering) is decided case by case.

VO₂max estimated by a watch

Many sports watches give an estimate of VO₂max: it is an estimate, not the result of a laboratory exercise test.

The Digital Twin lines up the data your devices write to your phone, such as steps, heart rate and sleep, and shows you how they change over time. It does not set the programme and it does not diagnose: this research describes groups of people, not you. Preparation for surgery is decided and followed by the care team.