The squat is a foundational human movement, present in activities from sitting down to athletic performance. For decades, a specific instruction circulated in fitness circles: never allow the knees to travel forward past the toes. This concept originated from a concern that forward knee movement would place undue pressure on the knee joint. Modern biomechanical analysis reveals that this restriction is often unnecessary and can compromise the effectiveness and safety of the exercise.
Knee Travel Is Necessary for Depth
Achieving full depth in a squat requires coordinated movement at the ankle, knee, and hip joints. As the body descends, the knees must naturally track forward over the feet to keep the lifter’s center of mass balanced over the middle of the foot. Without this forward motion, the body’s weight would pitch backward, making a deep squat impossible. The depth of the squat directly correlates with the forward knee translation needed to maintain stability.
The belief that forward knee travel increases stress to a harmful degree overlooks the knee’s structural capacity. The knee joint is designed to withstand significant forces during deep flexion in activities like running, jumping, and lifting. While forward knee displacement increases the load placed on the joint, this force remains within the physiological limits of a healthy knee’s tendons and cartilage.
Trying to force a perfectly vertical shin angle simplifies a complex, multi-joint movement. The amount of forward knee movement required is not universal but depends on an individual’s body proportions, specifically the relative lengths of their femur and tibia. Individuals with longer femurs must allow their knees to travel further forward to maintain an upright torso angle and keep the weight centered.
The Importance of Ankle Mobility
The ability of the knee to travel forward depends directly on the ankle joint’s flexibility, specifically its capacity for dorsiflexion. Dorsiflexion allows the shin to move toward the top of the foot while the foot remains flat. A deep squat typically requires a substantial degree of dorsiflexion, often around 35 degrees, to accommodate the necessary forward knee path.
When the ankle lacks this range of motion, the body initiates a compensatory movement pattern to achieve depth. This limitation prevents the knee from traveling forward, forcing the torso to lean excessively toward the ground. This forward lean shifts the body’s center of mass back over the mid-foot, compensating for the restricted ankle.
Restricted ankle mobility alters the kinetic chain of the squat, moving it away from the most efficient biomechanical path. Improving the ankle’s range of motion allows for a more upright torso posture during the descent. This vertical position facilitates the natural forward movement of the knees, ensuring necessary joint angles are achieved without excessive leaning.
Shifting Stress to the Hips and Lower Back
Consciously restricting the forward movement of the knees to keep the shins vertical creates a significant mechanical trade-off. This restriction forces the lifter to compensate by moving the hips further backward to maintain balance. This exaggerated hip hinge increases the forward lean of the torso, placing stress on the posterior chain.
Research comparing squats performed with and without knee restriction highlights this redistribution of force. While restricting the knees may reduce peak knee joint torque by approximately 22%, it causes a significant increase in the torque placed on the hips. Studies have shown this compensatory movement can increase hip torque by over 1000%.
This increase in hip torque also translates to greater shear forces and compressive load on the lumbar spine. The lower back is not as structurally robust as the knee joint for handling the primary load of a heavily weighted squat. Attempting to “protect” the knees by restricting their natural movement shifts a greater, potentially problematic, force to the hips and lower back. Allowing the knees to travel naturally past the toes is often the safer, more anatomically sound approach for most healthy individuals.