If you’re looking to build strength and muscle endurance, then you might want to try a 21-15-9 workout. The 21-15-9 formula is based on how many repetitions of an exercise you’re doing in each set, and stems from CrossFit — but can be used in any kind of fitness routine to level up your strength.
The idea behind 21-15-9 originally came from Greg Glassman, the trainer who founded the CrossFit movement in 2000. “CrossFit’s goal was to improve athleticism by varying functional movements and performing them at a high level of intensity,” says Ryan Kennedy, a NASM-certified fitness trainer. After experimenting with various combos, Glassman landed on the 21-15-9 to help exercisers improve their strength endurance, aka the ability to move a weight with higher reps in a shorter amount of time.
“Historically, most gym-goers gravitated toward heavy lifting or cardiorespiratory endurance,” Kennedy tells Bustle. “21-15-9 helps work on the middle ground. The idea behind these workouts is that as you fatigue, you are required to perform fewer repetitions, allowing you to keep pushing yourself.”
Whether you’re lifting a barbell, performing a bodyweight exercise, or swinging a kettlebell, this rep routine is considered to be one of the best and most efficient rep schemes, says Josh Schlottman, CSCS, a certified personal trainer and nutritionist. And yes, you can do it even if you aren’t a CrossFitter. Read on below for all the benefits of a 21-15-9, plus how you can incorporate it into your routine.
What Are 21-15-9 Workouts?
So, what is a 21-15-9 workout? “The workout involves selecting an exercise, or multiple exercises, and performing it for three rounds,” Kennedy says. “The first round you’ll do 21 repetitions, the second round 15 repetitions, and the final round 9 repetitions. The goal is typically to finish all rounds in the fastest time possible.”
According to Schlottman, this setup qualifies it as high-intensity interval training (HIIT). “With each set in the 21-15-9 workout, you'll be lowering the reps by 6,” he explains. “This allows you to keep the same power production throughout the sets.” If you were to go for the same amount of reps each set — say, 21 each time — you’d quickly fatigue and wouldn’t be able to finish.
In the 21-15-9, you complete the first 21 reps, then as your muscles get tired, the next 15 still feel as difficult and taxing as those first 21 reps, Schlottman says. Finishing out with 9 reps pushes you to your limit, as those last 9 reps seem really difficult after completing 21 and 15 reps previously. The result? A way more efficient workout.
What Are The Benefits?
Because the 21-15-9 workout is HIIT, Schlottman says it works to effectively increase your strength and muscular endurance. “It's also a fun way to mix up your workouts as well since it's quite a unique rep scheme.”
There’s a mental side to it, too, says NASM-certified trainer Brock Davies. While most 21-15-9 workouts last less than 10 minutes, they’re so strenuous that they require a lot of mental toughness to get through. Speaking of, it’s a great workout idea if you’re short on time!
How To Do A 21-15-9 Workout
According to Kennedy, you could do barbell thrusters followed by bodyweight pull-ups, aiming for 21 thrusters, 21 pull-ups, 15 thrusters, 15 pull-ups, followed by 9 thrusters and 9 pull-ups. This combo, in the CrossFit world, is known as “Fran.” Apparently, Glassman would name his workouts just like the National Weather Service names hurricanes, Kennedy says. Fran is what he chose for this benchmark workout.
If you aren’t as into lifting as a CrossFitter might be, you could also do a mix of kettlebell swings, burpees, and standing toe-touch ab crunches, Davies says. It doesn’t really matter what you do — just choose your fave exercise (or two or three) and follow the 21-15-9 format.
Kennedy suggests completing one to two sessions per week if you work out a lot, and if your goal is to improve strength endurance. For a more casual gym-goer, he suggests doing a 21-15-9 once a week, or once every fourth workout. You can combine this workout format with other types of exercise, too. If you’re a runner, for instance, you might do lower-body dominant exercises like squats, deadlifts, and walking lunges on off-days, which Kennedy says would be a great complement to your training routine.
What To Keep In Mind With 21-15-9
Since the 21-15-9 is so intense, you shouldn’t dive right in or go too hard, especially if you’re newer to working out. Kennedy recommends becoming proficient in your chosen exercise first, so practice those thrusters, kettlebell swings, and deadlifts before attempting speedy reps. “High repetitions at a fast pace can be dangerous if the lifter is not experienced with the movements,” he says. But once you get the hang of them, you’ll definitely feel the burn — in a good way.
Claudino, J. G., Gabbett, T. J., Bourgeois, F., Souza, H. S., Miranda, R. C., Mezêncio, B., Soncin, R., Cardoso Filho, C. A., Bottaro, M., Hernandez, A. J., Amadio, A. C., & Serrão, J. C. (2018). CrossFit Overview: Systematic Review and Meta-analysis. Sports medicine - open, 4(1), 11. https://doi.org/10.1186/s40798-018-0124-5.
Ryan Kennedy, NASM-certified fitness trainer
Josh Schlottman, CSCS, certified personal trainer and nutritionist
Brock Davies, NASM-certified trainer