# Intensity

> Mix hard and steady minutes; both build fitness.

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Canonical: https://boddy.app/exercise/cardio/intensity  
Last reviewed: 2026-10-28

## What It Is

How hard you work during your aerobic exercise.

## Why It Matters

Doing some of your aerobic exercise as hard intervals, rather than all of it at a steady, moderate pace, probably makes little to no difference to your cardiorespiratory fitness. Trials that compare the two find intervals raise VO₂ max a little more while taking a little over half the weekly time, mostly in people who were inactive or unfit to begin with.[1,2] The longest of them had adults in their seventies replace two of their five weekly sessions with either intervals or longer steady sessions, kept it up for years, and found a smaller edge still.[3] That is less than the gain most researchers treat as clearly worthwhile,[1] though some would count even a change this size.[3]

The two seem to get there by different routes, which makes them complements rather than rivals: hard efforts mainly push how much blood your heart can pump, while the metabolic gains come from the total work you do, which steady exercise reaches through volume and intervals through intensity.[1] What intervals add is speed: across the session lengths studied, gains from intervals seem to level off after fairly short sessions, while gains from steady exercise keep building with longer ones, so intervals reach a similar gain in less time.[1] Within the moderate range, how hard people worked made no clear difference to their gains.[1]

A mix of both is a sensible default, tilted by your schedule and by what you will keep doing, since intervals are harder to keep up: in the longest trial, fewer of the people assigned to them stuck with their training than of those on steady sessions.[3] If time is short, a couple of interval sessions a week, with efforts of a few minutes at close to the hardest pace you could hold for that long and easier recovery between them, get you to about the same place sooner, and every hard minute counts double toward your weekly total. If you would rather keep it mostly steady, that works too; just put in the minutes. If you are not used to hard efforts, build up to them over a few weeks, and if you have a heart condition, talk to your doctor before you start.

## GRADE

- Certainty: Moderate
- Magnitude: Trivial

Magnitude here is the effect on cardiorespiratory fitness, not on lifespan.

## References

1. Mattioni Maturana, F. et al. (2026). Optimising Exercise Prescription: A Meta-Analysis Examining the Dose Response of Exercise Duration on Cardiorespiratory Fitness Following HIIT and MICT. Sports Medicine. https://doi.org/10.1007/s40279-026-02528-y
2. Strauss, J.A. et al. (2026). High-intensity interval training for reducing cardiometabolic syndrome in healthy but sedentary populations. Cochrane Database of Systematic Reviews. https://doi.org/10.1002/14651858.CD013617.pub2
3. Stensvold, D. et al. (2020). Effect of exercise training for five years on all cause mortality in older adults—the Generation 100 study: randomised controlled trial. BMJ. https://doi.org/10.1136/bmj.m3485