The workout is long and difficult, but it will make you stronger, fitter, and leaner. Midway through you’ll want to quit—but not quitting is the key to reaping mental-toughness benefits.

The workout is especially beneficial for endurance and strength athletes.

Doing repeated lunges and wall sits helps bulletproof your lower body. You’ll not only have more stamina on a run or ride, you’ll also potentially get rid of aches and pains. If you don’t have any issues running now, you’ll prevent them.

Most strength athletes get stuck in a rut because they don’t do enough reps. This’ll solve your problem. Just a few rounds in you’ll probably have blown your legs up more than you ever have in your life.

“Sore Legs, No Equipment” is just one of the workouts in my new Men’s Health book Maximus Body that are designed to harden your body and mind. The book features 100+ workouts, and chapters that cover the fitness and mental toughness methods I’ve used with Special Forces soldiers, pro athletes, and average guys who want to finally get shredded. Check it out if you’re ready for a real change.

Directions: Do 40 alternating bodyweight lunges. Then do a wall sit for 30 seconds. Then do 38 alternating lunges, followed, again, by a 30 second wall sit. Then do 36 alternating lunges, followed by a 30 second wall sit. Follow that pattern of doing two fewer lunges followed by a 30 second wall sit until you’re doing just 2 lunges. Try to make it through the workout without resting.

The article The Hardest, Most Effective No-Equipment Workout You’ll Ever Do? originally appeared on Men’s Health.