Take Home Points

  1. Peak external knee flexion moment increases with squat depth and resistance.
The squat is a commonly prescribed exercise for sports team and general fitness. This multi-joint and multi-muscle exercise is popular as it targets many muscles around the hips and knees, but receives scrutiny as it may increase joint forces at the knee.
Studies have suggested increasing the squat depth results in increased gluteus maximus activation. Also, previous studies suggest knee laxity remains the same following a 8 week training program with thigh angle above parallel or below parallel squats.
However, conflicting studies exist on the amount of knee stress in squats of various depths.

Cotter (2012) had sixteen healthy, recreationally trained men

perform nine different squat loads and depths while monitoring peak external knee flexion moment (pEKFM). The three pre-determined squat depths were thigh above parallel, thigh parallel, and thigh below parallel to the ground. These three depths were performed unloaded, and at 50% 1RM and 85% 1RM. At the start of the investigation, depth-specific 1RM squat loads were determined.

 The largest 1RM was found for the above parallel squat.

Increases in either load or depth for the back squat significantly increased pEKFM.

This study found the typical decreases in squat load seen with increasing squat depth were not enough to offset the increases in pEKFM seen with increasing knee flexion. One main reason previous studies may not have found differences in pEKFM is they did not use depth-specific 1-RM predictive values.
This study suggests pEKFM increases with increasing load and depth during the back squat. If you are returning from a knee injury, deep squatting and heavy resistance will increase the joint stress at the knee. Therefore, progressing a return to these movements is important for prevention of re-injury. For athletes performing sports with high knee stress, consider replacing the heavy deep squat with other movements.
Reference
  1. Cotter JA, Chaudhari AM, Jamison ST, Devor ST. Knee joint kinetics in relation to commonly prescribed squat loads and depths. J Strength Cond Res. 2013 Jul;27(7):1765-74. doi: 10.1519/JSC.0b013e3182773319.
Written by Dr. John, DPT, CSCS