Why can you squat MORE with a low bar position vs. a high bar position? – A matter of TORQUES!
Primary movement requirements: Hip extension and knee extension DRIVE the squat movement. (Load stabilization is also required from the back, shoulders, and especially the feet!)
High Bar Squat: the bar typically rests on the muscle bulk of the upper traps.
Low Bar Squat: the bar rests on the bulk of the rear deltoids and upper back musculature (known as the “shelf”, because it creates a near form-fitting placement of a barbell), just above the spine of the scapula.
The Difference: The barbell in a low bar squat rests only a few inches lower than it would during the high bar squat. In effect, this decreases the lever arm of your torso. In order to maintain balance, the barbell must remain over your center of mass – the mid foot. To achieve this, a greater forward lean of the torso is required during the low bar squat. The torso comes forward so the hips must travel back, and as a result, the forward distance (lever arm) that the knees have to travel decreases.
Conclusion: The external resistance of the barbell resides CLOSER to the body’s FULCRUM in the low bar squat. Therefore, a low bar squat requires less muscular FORCE to create the necessary torque to overcome the resistance of the barbell.