Starting Strength

of the hamstring muscles (attached to the tibia and to the ischial tuberosity of the pelvis), is primarily isometric, since they don’t necessarily change length on the way down. In the bottom of the squat, the tightened hamstrings, adductors, glutes, and external rotators provide a slight rebound, which will look like a “bounce”; this is the stretch reflex we discussed earlier. The tension of the stretch pulls backwards on the tibia, balancing the force produced by the anterior quadriceps attachment on the tibial tuberosity. The hamstrings finish their job, with help from the quads, adductors, and glutes, by extending the hips.

    Figure 2-9. Muscular actions on the knee. In the deep squat position (A), the anterior force provided by the quadriceps is balanced by the posterior force provided by the hamstrings. The depth is the key: partial (high) squats (B) predominantly work the quadriceps and therefore lack balance.

    A partial squat done with an upright torso and vertical back angle is typical of most people’s attempts to squat, because we have all been told that the back must be vertical to reduce shear , the sliding forces that occur along a segment in rotation. Shear between the vertebral segments is supposed to somehow disarticulate your spine, despite the fact that this cannot and has not ever occurred. But in a misinformed effort to protect the back, this advice results in a lot of unnecessary stress on the knees. As we’ve already discussed, however, the vertical back angle fails to fully load the hamstrings. Therefore, they cannot exert the posterior force needed to oppose and balance the anterior force exerted by the quadriceps and their attachments to the front of the tibia, below the knee. (In other words, there’s no force pulling backwards to balance the forces that are pulling the knees and tibias forward.) The result is an actual anterior shear on the knee. And like a front squat, the partial squat also forces the knees quite forward of the mid-foot – much more so than the low-bar squat form we will be using, which keeps the knees back and uses the hips as the primary mover of the load. This lack of posterior support produces an anterior-dominant force distribution on the knee: the further back the hips are, the more hip muscle you use, and the further forward the knees, the more quad you use. Many cases of patellar tendinitis have been caused by this incorrect squat technique. Even when partial squats are done with the correct back angle, they fail to work the full range of motion and therefore fail to perform to their potential as an exercise.

    Figure 2-10. The variation in squat depths commonly seen in the gym. Left to right: Quarter-squat, Half-squat, a position often confused with parallel, where the undersurface of the thigh is parallel with the ground, Parallel squat according to the criteria established in Figure 2-1 , and “Ass-to-grass” squat.

    The hamstrings benefit from their involvement in the full squat by getting strong in direct proportion to their anatomically proper share of the load in the movement, as determined by the mechanics of the movement itself. This fact is often overlooked when the medical community considers anterior cruciate ligament (ACL) tears and their relationship to conditioning programs. The ACL stabilizes the knee: it prevents the tibia from sliding forward relative to the femur. As we have already seen, so does the hamstring group of muscles. Underdeveloped, weak hamstrings thus play a role in ACL injuries, and full squats strengthen the hamstrings. In the same way the engaged hamstrings protect the knees during a full squat, hamstrings that are stronger due to full squats can protect the ACLs during the activities that we are squatting to condition for. With strong hamstrings and the knees-back position provided by the low-bar version of the squat, the hips bear most of the stress of the movement. So athletes who are missing an ACL can safely squat heavy weights because the ACL is under no stress in a correctly performed full squat (see Figure 2-11 ).

    Figure 2-11. Forces on the knee in the squat. The hamstrings and adductors exert a posterior tension on the tibia, and the net effect of the anterior quadriceps tendon insertion is an anterior force against the tibial plateau. With sufficient depth and correct knee position, anterior and posterior forces on the knee are balanced. The anterior (ACL) and posterior cruciate ligaments (PCL)stabilize