Should the knees migrate past the toes when performing a squat? I posted this question on social media, and the immediate response by most was “No!”. I expected this answer from most everyone, from novice to advanced lifters. To you, I happily say, you’re wrong! The debate on proper squat mechanics will never die, but I am going to steal a line from Randy B., an athletic training, and performance enhancement peer, who answered my question: “Absolutely, [the knees] should [go past the toes]. Don’t believe urban legends or follow sports med sacred cows!” I couldn’t have said this any better! Randy is spot on. This urban legend could lead to injury. The purpose of this blog is to shed some light on the debate and provide the rationale for proper squat technique.
Early studies demonstrated that squatting with external loads caused undue stress and damage to soft tissue at the knee joint (1). It was demonstrated that powerlifters (lifting 2.5 times their body weight) reached maximum values of 8,000 N (Newtons) at a deep squat (1). This research caused experts to change squat mechanics. Maintaining a vertical shin angle prevents excessive knee flexion, thus limiting the stress placed on the knee joint. This, in turn, would possibly prevent damage to integral knee structures such as the menisci, articular cartilage, and ligaments. There you have it, the fear mongers began screaming: “Don’t let your knees go past your toes!”
Proper squat mechanics requires optimal movement at the ankle, knees, hips, and spine during the descent of the squat. When these joints move in unison, forces will be disturbed equally throughout the kinetic chain. The body is an interconnected chain, and restricted movement at one joint will lead to compensatory movement and dysfunction in other joints (6). Restricting knee flexion elicits compensatory and excessive forward lean at the spine. Restricting the knees significantly increases the load placed on the low back and hips. This is NOT a good thing!
Low-back pain and degeneration is one of the most debilitating forms of musculoskeletal injury seen in the adult population, affecting nearly 80% of all adults (3). The annual costs attributable to low-back pain in the United States has been estimated to be greater than $26 billion (4). In addition, 6% to 15% of athletes experience low-back pain in a given year (5, 6).
Squatting with a flexed lumbar spine decreases the moment arm (leverage), increases compressive load posteriorly, and increases the risk of disc herniation! (2) In addition, shear forces significantly increase as lumbar flexion increases from the neutral position. It is advisable to maintain a neutral spine throughout the squat and avoid excessive flexion or extension and because lumbar forces are increased with lean, it is better to remain as upright as possible at all times (9). This is done by allowing the knees to move forward.
To keep it practical and simple, I created a mock scenario using a generalized equation and a subject squatting 135 lbs. Torque is a measure of rotational force about an axis of rotation. There are many forms of and equations for torque, but in its most general form, torque (T) equals the product of the vertical force (F), the length of the lever arm connecting the axis to the point of force application (r), and the angle (Sin θ); thus the equation (T=rF Sin θ). Take a look at my simplified version below using the two images and notice the torque values at the knee and low back when the knees are restricted versus unrestricted.
To take this further, a study by Fry et al. in 2003 examined the joint kinetics that occur when forward displacement of the knees is restricted versus when such movement is not restricted. This study demonstrated that while restricting forward movement of the knees may minimize stress on the knees, it is likely that forces are inappropriately transferred to the hips and low-back region (see the following table). Thus, appropriate joint loading during the squat may require the knees to move slightly past the toes while the center of gravity remains directly above the midfoot (8).
|Restricted knee movement||Unrestricted knee movement|
|Knee torque||117.3 N·m||150.1 N·m|
|Hip and low back||302.7 N·m||28.2 N·m|
The restricted squats also produced more anterior lean of the trunk and shank and a greater internal angle at the knee and ankles. The illustration below provides a more detailed description of the angular changes produced at joints during both variations of barbell squats. These subtle changes in the joint angles significantly alter loads upon those joints.
It makes sense to protect your knees from unnecessary forces, however, your patellar tendon and PCL are more than strong enough to handle the forces. In Olympic powerlifters peak forces at the patellar tendon (8,000 N) and the PCL (2,500 N). These loads are much lower than their max capacity of patellar tendon (15,000 N) and the PCL (5,000 N) (9). These structures can handle the forces.
Some take home points:
- DO NOT purposely restrict movement of any joint unless there is an injury or condition that requires you to do so. Allow the body to follow a normal path.
- If you have a pre-existing knee injury, such as a PCL tear or patellofemoral pain, then restricting knee flexion is conducive, but do so with caution!
- Squat while maintaining a neutral spine and allow the knee to move beyond the toes. When you do this, you distribute forces evenly and reduce the risk of low back failure and maximize muscle coupling.
Quadriceps development is maximized by squatting to parallel, with no additional activity seen at higher flexion angles (10.)
- A full-depth squat while allowing forward knee migration, increases muscle activity of the glutes and the posterior chain (9).
The front squat produces significantly lower knee compression and lumbar stress in comparison with back squats, making it a viable alternative for those suffering from various knee and back ailments (8).
Nagura, T, Dyrby, CO, Alexander, EJ, and Andriacchi, TP. Mechanical loads at the knee joint during deep flexion. J Orthop Res. 20: 881–886, 2002.
Matsumoto, H, Suda, Y, Otani, T, Niki, Y, Seedhom, BB, and Fujikawa, K. Roles of the anterior cruciate ligament and the medial collateral ligament in preventing valgus instability. J Orthop Sci. 6: 28–32, 2001.
- Walker BF, Muller R, Grant WD. Low back pain in Australian adults: prevalence and associated disability. J Manipulative Physiol Ther 2004;27:238–244.
- Luo X, Pietrobon R, Sun SX, Liu GG, Hey L. Estimates and patterns of direct health care expenditures among individuals with back pain in the United States. Spine 2004;29:79–86.
- Nadler SF, Malanga GA, DePrince M, Stitik TP, Feinberg JH. The relationship between lower extremity injury, low back pain, and hip muscle strength in male and female collegiate athletes.Clin J Sport Med 2000;10:89–97.
- Nadler SF, Malanga GA, Feinberg JH, Rubanni M, Moley P, Foye P. Functional performance deficits in athletes with previous lower extremity injury. Clin J Sport Med 2002;12:73–78.
- Powers CM. The influence of altered lower-extremity kinematics on patellofemoral joint dysfunction: a theoretical perspective. J Orthop Sports Phys Ther 2003;33(11):639–646.
- Fry, A.C., J.C. Smith, and B.K. Schilling. Effect of knee position on hip and knee torques during the barbell squat. J. Strength Cond. Res. 17(4):629–633. 2003.
Schoenfeld, BJ. Squatting kinematics and kinetics and their application to exercise performance. J Strength Cond Res. 24(12): 3497–3506, 2010.
Watkins, J. Structure and Function of the Musculoskeletal System. Champaign, IL: Human Kinetics Publishers, 1999.