The difference in the position of the bar in the squat exercise creates slight alterations in our biomechanical leverages. These cause a big shift of emphasis of muscle activation of the prime movers. When the bar is held low across the back (rear delts and trapezius muscles), the lifter is forced to incline the torso forward during the descent phase. This forward lean occurs, so the lifter maintains their center of mass (balance) through the exercise.
Because muscle contractions surrounding a joint cause movement, in biomechanics muscular force is termed muscle “torque” about a joint.
A more forward inclination of the torso during the descent brings the weight horizontally closer to the knees. This reduces the resistive torque (muscle force) of the quadriceps about the knee-joint. At the same time, the weight is horizontally further from the hip-joint. This increases the resistive torque about the muscles of the hips –the lower back and the glutes. This means the hip and lower back muscles must work much harder (produce a greater amount of torque) to complete the exercise.
Placing the bar high on the upper back allows the lifter to maintain a more upright position during the descent phase, reversing the above scenario. The weight is now further away from the knee-joint, increasing the amount of resistive torque required by the quads and, because the bar is closer to the hip-joint, the muscular torque required by the glutes and lower back muscles is reduced.
The bottom line is that a low bar placement in the squat exercise forces the lifter to lean further forward and shifts the emphasis to hip muscles, making them work much harder during the lift than if the bar is placed high on the back (such as in a traditional bodybuilder’s squat). Depending on your goals, this may or may not be a good thing.
If you are purely concerned with moving as much weight as possible in the squat exercise, the low bar placement is the position of choice. However, if you are concerned with placing as much stress as possible on the quadriceps during the lift (to stimulate muscle growth), then the high bar placement is biomechanically a better choice.