A

Why these moves work
Strong quads will give you the sleek thighs you've been wanting, but this is no mere gorgeous-gams plan. These muscles also support your knees, so the stronger they are, the less likely you’ll be to get injured. The exercises here challenge your legs to work independently to propel and stabilize your body. That strengthens the quads and also trains the muscles for real-life movements, like bending down to pick something up, running, and walking. Follow this plan and in a few weeks you'll have sexier legs—the kind you can show off in a short skirt or use to help you fly through a 5K.

Anatomy lesson
Your quadriceps (or quads) include four muscles on the front of your thigh that help extend your knee. The three vastus muscles—the vastus lateralis, intermedius, and medialis— connect at the top of your femur (thighbone) and your knee. The rectus femoris attaches at your pelvis and knee and also helps flex your hip.

Details
At the gym, you'll need a bench or step, an adjustable cable pulley with a handle attachment, and a leg extension machine. To do these moves at home, use a bench or step, a handled resistance tube, and a chair. Twice a week, warm up with 5 minutes of easy cardio, then do 2 sets of each move in order, resting for 30 seconds between sets.

B

TRAINER'S STRATEGY "To get stronger, you have to fully fatigue a muscle," says Melissa DeLancey, a trainer at Clay in New York City, who designed this routine. "By jumping, balancing, and working one leg at a time, you increase the challenge to get your legs feeling the burn fast."