Why: This exercise is as close to perfect as you can get, says Garber. It has the potential not only to build muscle and prevent injury but also to make you more agile and prevent stress fractures and osteoporosis (both more common in women than men). If you lunge with a barbell on your shoulders, you'll turn this into a weight-bearing exercise that can strengthen bones.
Do: With your feet hip-width apart, step forward and land with your heel first. Lower until the front thigh is parallel to the ground; then push off the front heel to stand back up. Repeat with the opposite leg.
Don't: Many people take a giant step forward, which Matthews says puts the emphasis on the quads. She recommends the same stride length as if climbing up stairs. "You're doing it right if you feel it in your glutes and hamstrings as well as your quads," she says. Lean forward slightly and never let your back knee hit the ground.