How to Improve VO₂max: Building Aerobic Fitness Step by Step

VO₂max, the most oxygen your body can use during hard exercise, is a key measure of aerobic fitness, and it responds to training. The catch is patience: gains build over weeks and months of consistent work, and an Apple Watch estimate takes time to reflect them.

Here is a gradual plan, plus the My Bio Age signals that help you follow the change without chasing a number.

Start from the guidelines

The CDC's adult activity guidelines recommend 150 minutes of moderate aerobic activity a week, such as brisk walking, or 75 minutes of vigorous activity such as running, plus muscle-strengthening work on at least two days. The NHS guidelines for adults aged 19 to 64 match these and offer a simple intensity check: during moderate activity you can talk but not sing, while vigorous effort leaves you only a few words before you pause for breath. Its guidance for adults aged 65 and over also asks for activities that improve balance and flexibility.

If you are well below these levels, reaching them is your plan for now. If you haven't exercised for a while, or have a medical condition or concern, the NHS advises speaking to your GP first.

Build a base, then add some harder work

Most sessions should feel comfortable: brisk walks, easy cycling, swimming or relaxed runs at a pace where you could still hold a conversation. Our guide to easy aerobic training covers this kind of session and why heart-rate zones differ between systems.

Once that base feels routine, add some harder work, such as one weekly session with a few short uphill or faster efforts, each followed by easy recovery. A 2015 meta-analysis by Milanović and colleagues pooled 28 controlled trials in healthy adults aged 18 to 45. Both continuous endurance training and interval training produced large VO₂max improvements compared with no exercise, and intervals gave a somewhat larger gain on average. The participants averaged about 25, though, and the extra benefit was small next to the gain from training at all. The best plan is one you can repeat safely, week after week.

Increase one thing at a time

The American College of Sports Medicine's exercise position stand counts a slow build-up in how hard and how much you exercise among the steps that may make exercise safer. A simple way to build slowly is to change one of these at a time and give each change a week or two before the next:

• Frequency: add a session, such as a third walk or ride.
• Duration: lengthen an existing session by a few minutes.
• Intensity: add an interval, or make one easy session a little brisker.

My Bio Age's Training Load card offers a rough check on how quickly your overall activity is rising. It compares your average daily active energy over the last 7 days with the last 28 and labels 0.8–1.2× as Optimal. Active energy includes everyday movement, and the 28 days include the latest 7, so it is a big jump in your total weekly activity, rather than one extra session, that lifts the ratio into Building (above 1.2×) or Overreaching (above 1.4×). These are the app's categories, not validated limits, so use them as a prompt to check how you feel rather than as a target. A short interval session may barely shift the ratio. Our Training Load guide explains more.

Give your watch chances to estimate

Apple's Cardio Fitness guide lists Outdoor Walk, Outdoor Run and Hiking workouts as the sessions it uses, and says indoor workouts don't count. Apple's developer documentation adds that the route should be fairly flat, with adequate GPS and heart-rate signals, and that your heart rate should stay roughly 30% or more above resting.

So record some sessions outdoors. Treadmill and bike sessions still build fitness; they just won't produce estimates. My Bio Age reads the estimates saved in Apple Health and doesn't create its own from indoor workouts. Its Index data checklist looks for VO₂max readings on at least three separate days in the past 21 days, and suggests recording outdoor walks or runs with Apple Watch GPS when they fall short.

What to watch in My Bio Age

• VO₂max over three to six months. Compare months, not single readings. A week without a new estimate can reuse one from up to 14 days before the week ends, so an unchanged weekly value may simply mean no new readings.
• Walking heart rate. Apple's HealthKit documentation links it to fitness because it reflects how efficiently your heart works while active. It can update on any day you walk, so it keeps adding data in weeks without a new VO₂max estimate. Compare it on familiar routes.
• Resting heart rate. A 2018 meta-analysis of controlled trials found that endurance training lowered resting heart rate in healthy people. Apple's Vitals app guide notes that medication, elevation, alcohol and illness can shift heart readings too, so read your resting heart rate as a trend over weeks.
• Recovery and sleep. A run of short nights or a falling Recovery score after a harder week is a cue to keep the next week easier.

VO₂max, walking heart rate, resting heart rate and active energy are separate inputs to the weekly MyBioAge Index™, alongside sleep, HRV and others. The Index scores complete Monday-to-Sunday weeks, and Pace of Aging follows its trend over roughly 4, 12 and 24 weeks, so expect any effect of a training block to appear gradually, if at all.

Safety first

Stop and seek medical advice if you get chest pain, unusual breathlessness, dizziness or fainting during or after exercise. The NHS says to call 999, the UK emergency number, for chest pain that comes on suddenly and doesn't go away, or that comes with sweating, sickness, light-headedness or shortness of breath. Elsewhere, call your local emergency number.

An ACSM screening update noted that exercise is safe for most people, and that cardiac events during exercise are often preceded by warning signs. If you have symptoms or known heart, metabolic or kidney disease, check with a clinician before adding vigorous intervals. A VO₂max estimate is not exercise clearance, and Apple notes that medication or conditions that limit heart rate can cause an overestimate. Never change medication or treatment to move a number.

Where My Bio Age fits

My Bio Age brings VO₂max, walking and resting heart rate, Training Load and Recovery together on your iPhone. In AI Coach, you can choose VO₂max and ask a Status or Why question about your own readings.

Give your training block a consistent place to review: download My Bio Age free on the App Store and follow your Apple Health trends across weeks. The iPhone app is completely free, with no in-app purchases, subscription or account required. Your health data stays private on your iPhone, and optional AI Coach explanations run fully on-device.

For background, read what your Apple Watch VO₂max estimate means, how walking heart rate average works and how to balance Training Load and recovery. My Bio Age provides general wellness information, not medical advice.