When selecting the best exercises for strength (increased production of force against an external resistance), we need to consider what exercises:
1. Involve the largest amounts of muscle mass
2. Have the longest effective ranges of motion
3. Enable us to lift the most weight
The low bar squat satisfies this criteria more so than the more conventional high bar squat for a number of reasons:
Firstly, the more bent over back angle in the low bar squat is designed to put more of a load on the hamstrings. Therefore, we can use a larger amount of muscle mass to contribute to the movement. The very vertical back angle required to keep the bar over the middle of the foot in a high bar squat puts the hamstrings in a position where they cannot contribute as much to the movement.
Secondly, squatting to a position just below parallel enables us to maintain an effective range of motion. Effective is the key word here, a squat that it taken too deep requires the low back to relax in order to permit this depth. Thus, undoing our first criteria for enabling the largest amount of muscle mass to be involved.
Thirdly, as illustrated by the above picture, the low bar squat places the barbell in line over middle of the foot. This is one of the most important factors as to why more weight is able to be lifted in the low bar squat. With the barbell over the middle of the foot, the trainee can exert a force on the barbell in a straight vertical line upwards directly against gravity. Since the force of gravity acting on the bar is always straight down in a vertical line, the most efficient way to oppose this force is by acting on it vertically as well. If the bar is over the front of the foot as is often the case in a high bar or front squat, extra effort is spent keeping the barbell in balance and therefore is not efficiently spent moving the bar upwards against gravity.