“DEVELOPING STRONG, MUSCULAR legs is a two-step process,” says Brynn Putnam, founder of NYC’s Refine Method. “First, you need to build your body’s largest muscle groups—glutesquads, and calves—proportionally. Second, you need to reduce body fat levels enough that this muscle development is visible.”

That’s exactly why high-intensity interval training (aka “HIIT”) is so clutch. With this 20-minute, high-intensity workout you’ll not only hammer each of the major muscles in your legs—you can do it in less time, to boot.

Do these five exercises one after the other, repeating the circuit four times through. Rest only if absolutely necessary.

1. Reverse lunge to stepup

Set up a box that’s about knee height in front of you. Step one foot back, and lower into lunge position with weight shifted slightly forward over your front leg. Bring your back foot forward and directly on top of the box, and immediately step up onto it. Step down with the same lead leg, then lunge by bringing the opposite leg back. Repeat for time.

2. 90° squat jumps

Squat back deeply, then launch yourself into a jump, rotating 90° in the air before you land. Repeat, rotating the opposite direction. Alternate for time.