Anaerobic Exercise: What It Is, Examples & Benefits

Anaerobic exercise is any activity done at such a high intensity that your muscles need energy faster than your body can supply it using oxygen alone. Instead, they switch to quick-release fuel stores that work without oxygen. That's why anaerobic efforts, such as sprinting, lifting heavy weights or a hard interval on a bike, can only be kept up for short bursts before you need to stop and recover.

In everyday terms, aerobic exercise is the steady stuff you could keep going for a while, like a brisk walk or an easy cycle. Anaerobic exercise is the hard, short stuff that leaves you needing a breather. The NHS physical activity guidelines call this kind of effort "very vigorous" activity. Below, we explain how anaerobic exercise works, give examples, compare it with aerobic exercise, and look at how it can fit safely into a routine if you're managing your weight.

What does anaerobic mean?

"Anaerobic" simply means "without oxygen". It describes how your muscles get their energy during a particular kind of effort, rather than a specific type of workout.

Your muscles run on a molecule called ATP (adenosine triphosphate). Your body can make ATP in two broad ways:

• With oxygen (aerobic). This is efficient and can keep going for a long time, but it's relatively slow.
• Without oxygen (anaerobic). This is much faster, but far less efficient, so it runs out quickly.

A StatPearls review of anaerobic glycolysis, published by the US National Library of Medicine, explains that during high-intensity exercise where oxygen demand exceeds oxygen supply, muscles rely on anaerobic glycolysis (breaking down glucose without oxygen) to make ATP. The same review notes that although the oxygen-based process produces around 15 times more ATP, glycolysis happens roughly 100 times faster.

That trade-off is the whole story of anaerobic exercise. You get a rapid burst of energy, but it can't be sustained. Once you ease off and catch your breath, your body switches back towards its slower, oxygen-based energy supply and recovers.

Where does lactate come in?

When glucose is broken down without oxygen, the end product is lactate. The StatPearls review explains that, in tissue short of oxygen, pyruvate (a product of glucose breakdown) is converted to lactate, and that this final step is what allows the fast energy pathway to keep running. In other words, lactate is a normal part of how your body powers hard, short efforts.

Anaerobic vs aerobic exercise

The easiest way to tell the two apart is by how long you could keep going and how much you can talk.

The UK Chief Medical Officers' physical activity guidelines describe cardiovascular, or aerobic, activity as activity that "increases breathing rate and makes the heart and muscles work harder". They also give a simple guide to intensity: being able to talk but not sing indicates moderate activity, while having difficulty talking without pausing is a sign of vigorous activity.

In reality, most activities aren't purely one or the other. A game of football, a circuit class or a hilly bike ride will move you in and out of both systems. It's more useful to think of a sliding scale, from gentle and steady at one end to short and flat-out at the other. If you track your effort with a watch, our guide to heart rate zones explains how the higher zones line up with harder efforts.

Examples of anaerobic exercise

The NHS lists these as examples of very vigorous activities, which it describes as "exercises performed in short bursts of maximum effort broken up with rest":

• Lifting heavy weights
• Circuit training
• Sprinting up hills
• Interval running
• Running up stairs
• Spinning classes

Other activities that typically involve short, all-out efforts include:

• Sprints on foot, on a bike or in the pool, with recovery in between
• Jumping exercises, such as squat jumps or box jumps, done hard and in short sets
• Heavy resistance work, where a set of a few repetitions leaves you needing a rest
• Short, hard rowing intervals on a rowing machine

Is HIIT anaerobic?

Partly. The NHS says that very vigorous activity in short bursts with rest is also known as High Intensity Interval Training (HIIT). The hard intervals lean heavily on anaerobic energy, while the recovery periods and the session as a whole also work your aerobic system. That mix is part of why HIIT can feel so demanding in a short time.

Is weightlifting anaerobic?

Heavy lifting usually is, because each set is a short, intense effort followed by rest. The NHS also counts lifting weights, working with resistance bands and bodyweight exercises such as push-ups as strengthening activities. It says that to get health benefits from strength exercises, you should do them to the point where you need a short rest before repeating the activity, which is a good description of anaerobic effort in practice.

How anaerobic exercise fits UK activity guidelines

The UK Chief Medical Officers recommend that each week, adults should accumulate at least 150 minutes of moderate intensity activity, or 75 minutes of vigorous intensity activity, "or even shorter durations of very vigorous intensity activity (such as sprinting or stair climbing), or a combination" of all three.

The NHS adults' guidelines add that you should:

• Do strengthening activities that work all the major muscle groups (legs, hips, back, abdomen, chest, shoulders and arms) on at least 2 days a week
• Reduce time spent sitting or lying down, and break up long periods of not moving with some activity

So anaerobic exercise isn't required, but it's one legitimate way to meet the guidelines, especially if time is tight. Many people find a mix works best: mostly moderate activity, a couple of strength sessions, and some harder efforts if they enjoy them.

What about older adults?

The Chief Medical Officers advise that older adults should do activities aimed at improving or maintaining muscle strength, balance and flexibility on at least 2 days a week. That doesn't have to mean sprinting or heavy weights. The NHS's strength exercises, such as sit-to-stands and wall press-ups, can be done at home, and it advises building up slowly by gradually increasing the repetitions over time.

Benefits of anaerobic exercise

Because anaerobic exercise covers both strength work and short, hard cardio efforts, its benefits overlap with both.

• It helps you meet your activity target in less time. UK guidelines recognise shorter durations of very vigorous activity as a way to reach the weekly target.
• It builds and maintains strength. Heavy lifting and other resistance work count towards the twice-weekly strengthening activities the NHS recommends.
• It adds variety. Mixing short, hard efforts with steadier activity can make a routine feel less repetitive, which may help you stick with it.
• It challenges your heart and lungs too. Hard efforts push your breathing and heart rate up sharply, and the recovery periods keep your aerobic system working.

It's worth being realistic about what exercise alone can do for weight. The UK Chief Medical Officers' guidelines are clear that "diet is the important factor in weight management", with physical activity playing "a supporting role in maintaining a healthy weight and improving our physical health whatever our weight". In other words, anaerobic exercise is valuable for health, strength and fitness, but it works best alongside changes to what you eat, how you sleep and how you manage stress. If you're curious about what happens to body fat as you lose it, see our explainer on where fat goes when you lose weight.

Anaerobic exercise and holding on to muscle

When you lose weight, some of what you lose can be muscle rather than fat. The UK Chief Medical Officers' guidelines note this specifically in relation to GLP-1 weight loss medicines, stating that when taking these "both fat and muscle decrease".

That's one reason strength-based anaerobic work matters if you're losing weight, with or without medical support. The NHS recommends strengthening activities on at least 2 days a week for all adults, and keeping these up while you lose weight is a practical way to look after your muscles. Our guide to maintaining muscle while losing weight on treatment goes into this in more detail, and our article on whether muscle weighs more than fat explains why the scales don't always reflect changes in your body composition.

How to add anaerobic exercise to your routine

If you're new to harder exercise, or coming back to it, a gradual approach is safest. The UK Chief Medical Officers say that starting at low durations and intensities and building up over time as the body adjusts is the safest way to progress from inactivity to meeting the guidelines.

1. Build a base first. Spend a few weeks getting comfortable with regular moderate activity, such as brisk walking or steady cycling, before adding all-out efforts.
2. Start strength work with your own body weight. Sit-to-stands, wall press-ups and step-ups are a sensible starting point. Add weights or resistance bands as you get stronger.
3. Warm up properly. A few minutes of easy movement before hard efforts gets your heart rate up gradually.
4. Keep the hard bits short. A beginner interval session might be a handful of short, hard efforts with generous recovery between each one. Stop each effort while your form is still good.
5. Rest between efforts and between sessions. Recovery is when your body adapts. Many people find one or two harder sessions a week is plenty alongside their other activity.
6. Cool down. Finish with a few minutes of easy movement to let your breathing and heart rate settle.

A simple example week

Here's one way a week could look for someone who's already used to moderate activity. Adjust it to suit your fitness, schedule and any health conditions.

Is anaerobic exercise safe?

For most healthy adults, short bursts of hard exercise are safe when built up gradually. The UK Chief Medical Officers' guidelines note that adverse cardiac events are rare, and that they are inversely associated with the amount of vigorous activity people regularly do. In other words, the risk is lowest in people who are used to it, which is another reason to build up slowly rather than jumping straight into maximum efforts.

The BHF also points out that for most people, moderate intensity is best, because exercising at maximum intensity can make you run out of energy too quickly and may make the activity less enjoyable. That's a useful reminder that anaerobic exercise is an optional extra, not a requirement.

Stop and seek advice if something doesn't feel right. The BHF advises seeing your doctor if you're concerned about how fast or slow your heart is beating, or if you get dizziness, palpitations or chest pain when exercising. The NHS says to call 999 straight away if you get sudden chest pain or discomfort that does not go away, pain that spreads to your arm, neck, jaw, stomach or back, or chest pain with sweating, feeling sick, light-headedness or shortness of breath.

Exercising while on weight management treatment

If you're taking any medicine to help manage your weight, mention your exercise plans to your prescriber, build up intensity gradually, and let them know if you feel faint, dizzy or unusually tired when exercising. Our article on the best time to exercise looks at fitting sessions around your day.

This article is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare professional before starting any treatment.