In the barbell back squat (figures A-D), the bar is carried on the back and the hips are lowered and raised through a complete range of motion, with the hips descending just below the level of the knee joint at the bottom position (figures C and D). In figure E, the trainee has not achieved sufficient depth; in figure F, the trainee has descended too far. Note how throughout the exercise, the back is kept in a neutral position (without rounding or hyperextension) and the knees and feet are kept in the same sagittal plane.
Reproduced with permission from Jonathon Sullivan, MD, PhD, FACEP, SSC.