Squatting variations

Like pressing, squatting can be performed in a variety of exercise variations. Since the squat involves multiple prime movers (the quadriceps, gluteus maximus, adductor magnus, and hamstrings), each of these variations can be used to target different muscle groups.

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#1. Bar position (front or back)

Although some studies have found minimal differences between front and back squat variations in terms of the levels of muscle activation of the prime movers, those that have found differences tend to favor front squats for the quadriceps and back squats for the hip extensors. Thus, when using the squat to develop the quadriceps muscles for bodybuilding, the front squat is likely a better option.

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#2. Bar position (high or low)

Traditionally, when using back squats, it has been assumed that the high-bar position allows greater quadriceps activation, while the low-bar position allows greater hip extensor muscle activation. Nevertheless, more recent research indicates that the differences in quadriceps activation between the high-bar and low-bar positions are quite small (despite the heavier weights used in the low-bar squat variation), but the differences in hip extensor activation are substantial. Thus, when using the squat to develop the hip extensors muscles for bodybuilding, the low-bar squat is likely a better option.

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#3. Range of motion

Historically, it was believed that deeper squats required a greater contribution from the gluteus maximus. However, this conclusion was drawn because the contribution of the adductor magnus muscle was not measured. Indeed, more recent research has shown that the gluteus maximus actually reduces its contribution with increasing squat depth, while the contribution of the adductor magnus increases greatly.

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This occurs because the gluteus maximus has poor leverage in higher degrees of hip flexion (at the bottom of the squat) but very good leverage close to full hip extension (standing upright, at the top of the squat). In contrast, the adductor magnus has  good leverage in higher degrees of hip flexion and poor leverage close to full hip extension.

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#4. External resistance type

The external resistance type used during a squat variation affects the relative contributions of the various muscles in the same way as the range of motion, because it similarly alters where in the exercise range of motion the exercise is most difficult. Using elastic resistance moves the difficult part of the exercise to the top of the exercise range of motion, and therefore requires the greatest challenge from the gluteus maximus. Also, because the quadriceps are the limiting factor at the bottom position of the squat, using free weights instead of elastic resistance increases the ratio of quadriceps to hip extensor contribution. 

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#5. Stance width

The stance  width used during back squats influences the ratio of hip extensor-to-knee extensor joint moments, such that the wide stance exercise variation is more hip-dominant than the narrow stance exercise variation.

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#6. Loading

The relative load used during back squats influences the ratio of hip extensor-to-knee extensor joint moments. As loads are increased, the relative contribution of the hip extensors increases, while the relative contribution of the knee extensors decreases.

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Conclusions

During squat variations, various factors are able to affect the relative contributions of the prime movers (the quadriceps, the adductor magnus, the gluteus maximus, and the hamstrings), including [1] bar position (front/back or high-bar/low-bar), [2] range of motion (ROM), [3] external resistance profile, [4] stance width, and [5] loading. Using a front squat or a high-bar position tends to involve slightly greater quadriceps muscle activation, while using a back squat or a low-bar position tends to involve slightly more hip extensor activation. The differences caused by bar position occur despite the heavier loads that can be used in back squats and low-bar positions. When performing squats to greater depths, the adductor magnus increases its contribution and the gluteus maximus decreases its contribution, contrary to popular belief. Similarly, when using accommodating resistance such that peak external forces are required near to full hip extension, the gluteus maximus increases its relative contribution, but when using free weights, the quadriceps and erector spinae display an increased relative contribution. Finally, both stance width and loading during back squats influence the ratio of hip extensor-to-knee extensor joint moments, such that wide stances and heavier load exercises are more hip-dominant than narrow stances and lighter loads.