Recruiting the hamstring during a back squat has been demonstrated to minimize the anterior shear forces along the knee and the ACL. Essentially, the hamstring helps to hold the bones in place while the knee bends under the load. EMG studies by Wright et al 1999 demonstrated that hamstring recruitment during a 70% max squat ranges from 35-50%. Not feeling any hamstring during your squat? Squat further back. The further back you reach with your glutes, the more your hamstrings have to pull (make sure your hips and spine are up to the challange). Powerlifters use the box squat to really take advantage of this mechanism. Elite powerlifters squatting in the range of 3x bodyweight will use 70-80% of their hamstring to cocontract on heavy squats. This cocontraction with the quads, abductors, lower back and stabilizers will move massive weight. While this is clearly an exception as professional athletes train at rates most do not, it demonstrates the need of hamstring training and proper recruitment during squating. Remember to reach back while arching, not to simply drop down.