Leg extensions work the muscles of the thighs, mainly the quadriceps -- the muscles at the tops of your thighs. Proper form involves sitting on the leg extension machine with feet tucked under the roller bar. Grab the side handles and lift or lower the weight. Depending on the exact form you use, you can decide which muscles do the majority of the work in order to develop extreme, controlled definition in your legs.
Vastus Lateralis
The quadriceps muscles perform the extension work of leg extensions. Each of these muscles comprises four portions, two of which are integral to the exercise. The vastus lateralis is the outer portion of the quadriceps. You can perform leg extensions so that the vastus lateralis does the majority of the work. Point your toes slightly and rotate your hip outward while you raise the weight. This will develop definition and gain that rounded bulk of muscle above the knee that signifies superlative thighs.
Vastus Medialis
The vastus medialis is the second largest portion of the quadriceps muscles. While it appears to remain connected to the vastus lateralis throughout the length of the thigh, it actually bifurcates in the center at the appearance of another minor muscle. This means you can change your form during leg extensions, so the vastus medialis does the majority of the work. Rotate your hips so your toes point inward when you raise your legs. This creates a defined teardrop shape that tapers to a point at the inside of the thigh.
Hamstrings
The hamstrings are the muscles at the backs of your thighs. Leg extensions primarily work the quadriceps, but muscles work in coordinating pairs as agonists and antagonists. Agonist muscles pull your body in one direction and antagonists oppose that action by pulling the muscle in the opposite direction. When you perform leg extensions, your hamstrings are the antagonist muscles to the quads that let you lower the roller bar at a controlled rate.
Stabilizers
The upper and middle trapezius muscles act as stabilizer muscles when you perform leg extensions. The trapezius is a large triangular muscle group that starts at your neck and creates a cape-like cover over your upper and middle back before tapering down the spine. Stabilizer muscles do not move the way agonist and antagonist muscles do during exercise. Instead, they help the rest of the body to remain relaxed and absorb shock, so the primary muscles can do the necessary work to perform the action. The trapezius muscles stabilize your torso and hips so your thighs can lift and lower during leg extensions.
