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"Squats are very efficient," Greene notes. "They work not only the buttocks but also the quadriceps—the muscles at the front of the thighs—and, as you go down slowly, resisting gravity, the hamstrings, at the back of the thighs."
Oprah is using a Smith press, a contraption that supports the weight if you lose your grip. Tips from Greene: Don't sit too low; that puts pressure on the knees. Keeping the toes pointed outward also helps protect the knees.