Edward R. Laskowski, M.D.: The squat is an exercise you can do with resistance tubing to work a variety of muscles in the legs. Specifically, the squat targets the quadriceps, here, and the hamstrings, here. It also works the gluteal muscles, here. The squat is a great exercise for strengthening the leg muscles in general.
Nicole L. Campbell: To do a squat with resistance tubing, start by standing in the middle of the tubing with your feet slightly greater than shoulder-width apart and your knees slightly bent.
Grasp both handles and place the tubing midway between your shoulder and your elbow. Slowly descend, bending through the hips, knees and ankles. Stop when your knees reach a 90-degree angle. Then return to the starting position. You'll feel tension in your legs.
When you're doing squats, keep your back in a neutral position. Do not flatten the curve of your lower back, and don't arch your back in the other direction. Remember to keep your knees centered over your feet on the way down. Don't let your knees go ahead of your toes, and don't let your knees roll inward or outward. If you can't lower your knees to a 90-degree angle, simply go as low as you can. Stop when you're fatigued or your form begins to suffer.
For most people, one set of 12 to 15 repetitions is adequate.
And remember, for best results, keep your back in a neutral position during the exercise and keep your knees centered over your feet on the way down. Keep your movements smooth and controlled.