From google....
In strength training, the squat is an exercise that trains primarily the muscles of the thighs, hips and buttocks, as well as strengthening the bones, ligaments and insertion of the tendons throughout the lower body. Squats are considered a vital exercise for increasing the strength and size of the legs and buttocks. Despite their popularity, squats have frequently resulted in injuries to the lumbar spine and knees, requiring modification from the standard squat with an olympic bar, with multiple variants now used.[1]