The squat is one of the most fundamental and commonly performed exercises for strength gains, sports training, and rehabilitation. Many different variations of the squat are routinely prescribed in CrossFit workouts, and while the lower-body movement patterns of the front, back, overhead, and air squats might look the same, they each differ in degree and differentiation of total-body muscle recruitment. For the purpose of this post we will compare the front and back squats.
The back squat is performed with the bar resting on the upper trapezius and the posterior deltoid with the hands gripping the bar. The front squat is performed with the bar resting on the upper pectoralis major just below the clavicle and the anterior deltoid using a loose finger grip to secure the bar just outside of the shoulders; with the arms rotated forward so the elbows are pointing forward, and the upper-arm parallel to the ground. Other than the bar and arm positions, the exercises should ultimately be performed the same way by keeping the weight in the heels, maintaining proper lumbar curve, keeping the hips back and down, eccentrically decelerating down to point that the hip crease is below the knee, tracking the knees over the feet, and then concentrically accelerating back to full extension at the knees and hips.
The squat is generally considered as a Quadriceps exercise in the fitness industry, but limiting the exercise in this way undervalues what it accomplishes if performed correctly. In addition to the quads, the squat recruits and activates the gluteals, the hamstrings, stabilizers of the lower leg, and the core musculature including the abdominals and muscles of the back.
Interestingly, if both variations of the squat are performed with good form, the muscle recruitment and activity are almost identical with only minor variations. However, there are many factors that may influence the exercise, both decreasing efficiency and increasing the risk of injury. These factors include, but are not limited to: not maintaining proper lumbar curve, not tracking the knee over the foot, keeping the weight on the ball of the foot, or not getting the hips back far enough causing the knees to cross the plane of the toes.
Flexibility also plays an integral role in proper squat mechanics. For example, over-active hip flexors may rotate the pelvis forward causing the torso to fall forward, and put more pressure on lumbar vertebrae. If there is not full range of motion at the shoulder, the arms cannot be rotated forward making it easier for the torso to fall forward.
That being said, if there are no barriers to good form, there are a few differences that are observed both quantitatively through EMG data, and qualitatively through biomechanical movement observation. The back squat recruits more Hamstring activity, specifically the Biceps Femoris and Semitendinosus. The back squat also recruits less Quadriceps activity. Other than the minor differences of the lower body, the only other difference in regard to muscle activity is that the front squat requires more stability and therefore the muscles of the lower-back, specifically the Erector Spinae, are much more active in the front squat in order to keep the torso upright throughout the complete movement.
Aside from muscle activation, the other major factor that we need to look at is the potential for injury. During the squat this can most effectively be looked at through the forces that are acting on the joints throughout the movement of the exercise. The forces in regard to the squat are focused primarily on the knee and are compression and shear. (Keep in mind that these forces being discussed are when the exercises are performed correctly. If they are performed incorrectly, then other forces like torque come into play, and compression and shear are both increased, substantially increasing the risk of injury.) Compressive forces are significantly greater during the back squat than the front squat; shear forces are very similar in both squat variations. Both compression and shear are low in comparison isolated, open-chain leg exercises like the leg extension and leg curl machine. While compressive forces are found to be less in the front squat, it may be due to the fact that people front squat less weight than the back squat, and not that it is biomechanically more efficient.
In conclusion, there are few relatively small differences between the back and front squat. However the front squat requires more flexibility and skill to perform correctly, and puts less stress on the joints. So a person will achieve more in progressive strength and flexibility from front squatting, which is what CrossFit training strives to accomplish, getting the most out of every rep, of every exercise.