Box jumps bring a unique challenge to the muscles and test stability at the same time. Stand with legs straddling a raised step (or box), knees slightly bent, arms straight out to sides. Jump both feet up and in, onto step, as you bend elbows in front of chest. Perform a small (or deeper) squat at the top, and then jump back out to start, landing with soft knees and arms out. Do 15 reps. To modify: Hop up and step back down.
They may look intimidating, but "chains are an advanced tool to make gains on basic lifts: benches, back squats and deadlifts," says Dianna Scotece, a Tier 3+ trainer at Equinox in New York City. As a guideline, use chains that are roughly 10 percent of the barbell weight. (If the weight of the barbell is 300 pounds, each chain should weigh 30 pounds.) Introduce chains gradually to reduce your risk of injury.
Turning your squats into a plyometric move provides a power boost. Stand with feet shoulder-width apart, arms extended by sides. Squat, bending knees, sending hips behind you, and then press into the balls of your feet to jump straight up in the air; land softly back in squat position. Repeat.
The kettlebell swing starts with a squat. Adding the swinging motion, plus the unstable resistance of a kettlebell, ups calorie burn, improves power and stability, and increases hip mobility. Stand with feet slightly more than shoulder-width apart, holding a kettlebell with both hands, arms extended in front of you. Squat, pushing hips behind you, and lower kettlebell between legs. Drive hips forward as you swing kettlebell up to shoulder level in front of you, keeping shoulders down and abs engaged. Use control as you lower back to start; immediately repeat.
For inner and outer thigh burn, try doing narrow to wide squat jumps with a resistance band: Place the resistance band around your legs just above your knees with your feet a few inches apart (so that there's a bit of tension in the band). While squeezing your core, lower into a squat, push through your heels, and jump as high as possible. Land softly with your feet wider than shoulder-width. Pause, then repeat the squat jump landing back to narrow position. That is one rep. Do 10 reps.