Retirement is often a time when people slow down, but in Christine Hobson’s case, she’s speeding up. When her daughter persuaded her to join a running club so she wouldn’t get bored, she had no idea she’d get the fitness bug and run 125 marathons in total, visiting all seven continents.

And the 72-year-old former teacher has plans to run the North Pole marathon in 2027.

CrossFit means a lot of things to a lot of people – because it’s made up of a lot of things.

Since the rise of the fitness giant, countless brands, events and training methods have sprung up around it – not claiming to be CrossFit, but looking suspiciously CrossFit-esque.

There are, however, a handful of things that are uniquely CrossFit: the ‘Girls’ benchmark workouts. The Hero WODs and, of course, its signature rep schemes.

Chief among them is ’21-15-9′.

The 21-15-9 rep scheme may just be the single most CrossFit thing in existence. But what exactly is it? Where did it come from? And why might it actually be better at building muscle in a hurry than its conditioning roots would have you believe?

Let’s have a look.

If you’ve never encountered it before, the format couldn’t be simpler. Choose two exercises (occasionally more) and perform 21 reps of each, then 15 reps of each, then nine reps of each, completing the entire workout as quickly as possible – with good form.

Probably the best-known example is ‘Fran’: 21 thrusters and pull-ups, followed by 15 of each, then nine. On paper it doesn’t look especially intimidating. In practice, it’s one of the most feared benchmark workouts in fitness.

Unlike many modern training methods, 21-15-9 didn’t come out of a study. It came from the gym floor.

CrossFit founder Greg Glassman has explained that the format emerged through years of coaching and experimentation in the 1990s. Rather than chasing a perfect sets-and-reps prescription, he was looking for a workout that allowed athletes to maintain a high power output from start to finish.

The thinking is surprisingly elegant. You begin with 21 reps while fresh. By the time you reach the set of 15, your ability to produce force has already fallen. By the final nine, you’re significantly more fatigued – but the workload has dropped by almost the same amount.

Instead of grinding through increasingly miserable sets of the same length, the workout ‘meets you where you are’, reducing the work required as your capacity declines. The result is a workout that encourages you to keep moving instead of standing around trying to recover.

The numbers themselves are also remarkably practical. Forty-five total reps per movement provides plenty of training volume without turning the session into an endurance slog, while every set divides neatly into thirds if you need to break it up.

(Although I’ve got to be honest, I’m a 20-15-10-5 man myself, just for the sake of round numbers.)

Although there isn’t research showing that 21-15-9 is somehow the magic formula, there are obvious reasons why it consistently produces brutally effective workouts.

Descending reps help maintain intensity. As fatigue accumulates, reducing the target allows movement quality, bar speed and overall work rate to stay higher than they would if you simply repeated the same number of reps over and over.

It also tends to land in a physiological sweet spot. Most 21-15-9 workouts take between three and eight minutes, depending on the movements and the athlete. That’s long enough to create a serious cardiovascular challenge while still requiring meaningful force production throughout. You’re taxing your anaerobic systems hard while relying on your aerobic system to help you recover just enough to keep going.

Finally, there’s the psychological trick. The hardest-looking part comes first. Once you’ve survived the opening 21, every remaining round appears more manageable. ‘Only 15 left.’ Then, ‘Just nine.’ In reality, you’re becoming more fatigued with every rep, but the shrinking target keeps you attacking the workout instead of pacing too conservatively.

Perhaps the biggest misconception about 21-15-9 is that it’s ‘just cardio with weights’.

Choose the right load and something interesting happens. Very few athletes complete every round unbroken. Instead, the workout naturally evolves into a series of short, broken sets separated by only a few seconds of rest.

Your 21 might become 11-5-5. Your 15 becomes 8-4-3. Your final nine might stay unbroken – or become 5-4.

In effect, you’ve accidentally turned the workout into a form of rest-pause training.

Those brief pauses allow just enough recovery to squeeze out more high-quality repetitions before fatigue catches up again. By the latter stages of each mini-set, you’re repeatedly working very close to failure, recruiting the high-threshold motor units with the greatest potential for muscle growth.

It’s a similar principle to rest-pause training, myo-reps and cluster sets: all methods used to accumulate hypertrophy-friendly volume while keeping the load relatively heavy and the rest periods brutally short.

You’re basically speed-running a large number of hard, growth-stimulating reps in a very small window of time. Could this help explain why elite CrossFit athletes often carry an impressive amount of muscle despite spending relatively little time performing traditional bodybuilding splits?

It’s certainly plausible, although the ‘elite’ part often selects for athletes with the greatest muscle-building potential.

Much of their training isn’t simply conditioning. It’s high-density resistance training performed under accumulating fatigue, with only fleeting recovery between efforts. In other words, they’re often doing something bodybuilders have deliberately programmed for decades: packing a lot of hard work into a very short period of time.

That’s not to say 21-15-9 is superior to a well-designed hypertrophy programme. If your sole goal is building muscle, there are more efficient ways to do it.

But if you’re looking for a workout that develops fitness, tests your mettle and still provides a meaningful stimulus for strength and size, it’s easy to see why this deceptively simple rep scheme has remained one of CrossFit’s defining fingerprints for more than 20 years.

If you’re looking for an interesting twist on the 21-15-9 format, look no further than Hero WOD ‘JT’, which concentrates the muscle-building potential of the format into a brutal upper-body workout.

Created in honour of Petty Officer 1st Class Jeff Taylor, who was killed in Afghanistan in 2006, the workout strips away barbells altogether and relies solely on three bodyweight movements:

21-15-9 reps of:

Don’t let the lack of equipment fool you. The volume – 45 reps of each movement, 135 reps in total – combined with the descending rep scheme makes this a brutal upper-body test, hammering the shoulders, chest and triceps while demanding serious muscular endurance.

Better still, it perfectly demonstrates one of the biggest strengths of 21-15-9. As fatigue mounts and the sets naturally fragment, the workout begins to resemble one giant rest-pause set, allowing you to accumulate a huge number of hard, near-failure reps in less than 10 minutes.

If your goal is building an impressive upper body while developing serious work capacity, there are few bodyweight workouts that deliver quite so much bang for your buck, making ‘JT’ one of my personal favourites.

If there’s one thing Kori Sampson knows, it’s how to optimise your body composition for performance. To tap into his knowledge as an elite athlete and coach, we asked him to create a 4-week plan to help you move faster, recover quicker and keep pushing when the fatigue sets in – all while improving your muscle-to-fat ratio.

Ready to build muscle, burn fat and come out the other side looking, feeling and performing better? Click here to get 14 days of free access to the plan via the Men’s Health app.

The concept of ‘exercise snacking’ has never been more popular. Not only is it convenient and accessible, but there is solid scientific evidence that short bursts of physical activity can yield real benefits for our health. But can a swimming workout be an effective ‘exercise snack’?

A study published in the European Heart Journal found that just 15 to 20 minutes of vigorous physical activity a week (almost as low as two minutes a day) was enough to significantly lower the risk of heart disease, cancer and early death. The study defined vigorous activity as any exercise that leaves you out of breath and raises your heart rate, including swimming.