Perry says that when it comes to squats, especially when the exercise is performed with a barbell on the back, the mistake he sees most often is caved in knees. "Proper squat form requires that the knees track directly over the toes, so they don't fall inward, or outward," says Perry. He says this mistake is most likely the result of weak glute muscles or tight inner thighs. "A combination of glute strengthening exercises and loosening up the inner thighs with stretching and foam rolling may help improve form.".