Judge a class by what it measurably does,
not by its name.

It depends who is asking. Public-health guidelines set a low bar on purpose. The WHO's 2020 guidelines ask adults to do muscle-strengthening activity at moderate or greater intensity, working all the major muscle groups, on two or more days a week, and they name no class. The NHS goes further and lists examples: yoga and pilates sit on its list of muscle-strengthening activities beside lifting weights and working with resistance bands, with one proviso: to get the benefit, keep going to the point where you need a short rest before repeating.

By that measure most classes with any resistance in them count. An athlete usually means something narrower. The 2026 position stand from the American College of Sports Medicine, an overview of 137 systematic reviews covering more than 30,000 healthy adults, separates three things a class could build:

• Maximal strength, the most you can lift once. It grew most with loads of at least 80% of that maximum, and more again with heavier loads, over two to three sets on two or more days a week.
• Muscle size. It grew with weekly volume, from about ten sets per muscle group a week, and load mattered little: anything from 30% to 100% of maximum worked. With volume equal, one session a week did as well as more than five.
• Muscular endurance, how many repetitions you can keep going for. Resistance training improved it, though the data were too thin to say which load works best.

The stand adds that you do not need to progress to benefit, but continued progress likely does need it: the stimulus has to rise over time, through load, volume or something else. The same light weights every week can still do you good. They are unlikely to keep making you stronger.

The second yardstick: METs

Cardio is judged differently. The 2024 Adult Compendium of Physical Activities lists measured energy costs as METs, multiples of the energy you use at rest, averaged across adults aged 19 to 59. It sorts them into three bands: light from 1.6 to 2.9 METs, moderate from 3.0 to 5.9, and vigorous from 6.0 up. The figures describe an average adult, not you, and on this page they are an intensity scale and nothing more.

What METs cannot see is load on a muscle. They measure the whole body's oxygen cost, so a weights session of 8 to 15 repetitions rates 3.5 METs, moderate, while a Zumba class rates 6.5, vigorous. Only one of those is strength training. That is why every class on this page gets two questions, not one.

Three kinds of class put real resistance on your muscles: barbell classes, circuits and kettlebell classes, and core classes. They build different things.

Barbell classes such as BodyPump

Les Mills describes BodyPump as high-rep resistance training that builds strength endurance, with more than 1,000 repetitions in a 55-minute class. Two trials also found gains in maximal strength. In a 12-week trial of 19 women aged about 21, the nine who trained raised their one-rep squat by 33%, with no gain in running fitness. In 68 healthy, active adults over 55, 26 weeks of classes twice a week raised leg-press strength by 13%, against 3% in those who carried on as normal.

The third trial points the other way. Previously untrained women did 12 weeks of BodyPump, three sessions a week, and gained no more one-rep strength in the squat or bench press than a group who did not exercise, while women who trained individually with a personal trainer improved their squat by more. So a barbell class can build some strength, as it did in two of the three trials. What it cannot do is load you near 80% of your maximum while you lift for that many repetitions, and that is where maximal strength grows fastest.

Circuits and kettlebell classes

A circuit moves you from station to station with little rest, which makes it strength and cardio at once. A 2021 meta-analysis of 45 studies in healthy adults aged 18 to 42 found that resistance circuits raised upper and lower body strength, and VO2max by 6.3% on average, with the largest strength gains in untrained and active people rather than the already trained. The trade-off, in our view, is that a circuit is rarely as heavy as a dedicated strength session.

Core classes

Core classes build endurance more than strength. A 2022 meta-analysis of 31 trunk training studies in competitive athletes aged 11 to 37, so adolescents included, found a small effect on maximal strength and a large one on local muscular endurance, with moderate gains in sport-specific performance. A core class is worth doing for what it trains; it does not stand in for a heavy lower-body day.

If heavy strength matters for your sport, our view is that these classes add to lifting rather than replace it. Each one still counts as muscle strengthening for health.

Much of the timetable. Several common classes sit in or near the vigorous band, and for some of them research backs the label.

• Indoor cycling. A class rates 9.0 METs, one of the highest class figures in the Compendium. A 2026 systematic review of 17 studies in adults found it consistently associated with high-intensity demand, and in several studies with better cardiorespiratory fitness, though few of the 17 studies were controlled or long-term. It is hard leg work, but it is not a strength session: indoor cycling and weights covers how it sits beside lifting.
• HIIT. 7.0 to 11.0 METs; the evidence is in the studio HIIT section above.
• Step. A bench step class rates 7.8 METs; step aerobics on a 10 to 12 inch step reaches 9.0.
• Zumba and dance fitness. A Zumba class rates 6.5 METs. A 2020 meta-analysis of randomised trials in adults aged 18 to 47, mostly women, found Zumba may improve VO2peak, but only three trials could be pooled, too few for a firm conclusion. Dance as a sport is at dance.
• Combat cardio. Les Mills rates BodyCombat medium-high intensity and high impact, steady cardio with high-intensity peaks. The Compendium has no code for the class; kickboxing rates 7.3.
• Aqua aerobics. Water aerobics rates 5.3 METs, in the upper half of the moderate band, and the NHS lists it among moderate activities; a high-intensity water class reaches 7.5. A 2022 meta-analysis of 27 randomised trials in 1,006 young and older adults found water exercise also built some strength, with gains that seemed similar to exercise on land, though most trials reported their methods poorly. Swimming lengths is a different case: swimming and weights.
• Hot yoga. 3.0 METs by work done, but hard by heat and heart rate (see the table), which is why our hybrid training guide calls it hard, even when it sounds gentle.

Count hard days, not classes. Two gentle yoga classes add two easy days; one indoor cycling class adds a hard one. Your sport already brings hard days of its own: a match, a hard club night, a long session. The question is how many hard days your week can carry, and the answer depends on your sport, your fitness and how you feel the morning after, so we give no single number.

A short audit makes the count visible:

1. Write down your week as it is: matches, training nights, classes, gym sessions.
2. Mark each one hard or easy with the two questions. A load you could lift only a few times, or a pace at which you could not say more than a few words without pausing for breath, makes it hard.
3. Count the hard days. That number, not the number of classes, is what your timetable asks of your body.
4. Before you add a class, ask which column it goes in: another hard day, or an easy one.

Whatever the research says about a class, the one you go to every week is part of your training, and a plan that cannot see it is planning on top of it.