You've probably heard that allowing your knees to travel past your toes during these exercises can lead to injury. But is that really the case? Let's dive into the science and set the record straight, and then use that knowledge to make the right lower body training decisions when we have knee issues.

Sports Illustrated and Bad Science

The idea that forward knee travel is harmful can be traced back to a 1961 study by Dr. Karl Klein, which suggested that deep squats could overstretch knee ligaments, causing instability. This theory gained traction after he was featured in Sports Illustrated, in an article with the incredible name: "The Knee is Not For Bending" and despite criticism from his peers, his theory became widely accepted and squats all but disappeared from American fitness routines. Klein's argument rested on the claim that deep squats overstretched ligaments, due to the shear forces generated during the movement.

A question of forces

Shear forces, in simple terms, push one part of an object in one direction while pushing another part in the opposite direction. In a squat, these two forces are the anterior shear forces, primarily restrained by the ACL, and the posterior shear force, primarily restrained by the PCL. The deeper you squat, the more these shearing forces increase due to the horizontal distance between your knees and the load.

This study discovered that keeping your shins vertical reduced knee forces by 22%, which sounds good, but it led to a massive 1070% increase in hip forces. Restricting the natural movement of our knees just leads to a biomechanically inefficient, and potentially dangerous movement pattern. Which means forward knee travel is kind of necessary for an efficient, full-range squatting, so the question then becomes: are the shear forces that are a result of this going to damage the knee over time?

A review of 12 studies assessing squat forces at the knee reveals that even the largest shear forces measured in any study aren't enough result in damage to healthy knee ligaments like the PCL and ACL. The maximum strain on the PCL amounts to just a bit more than half of its theoretical strength, so the risk squats pose to a healthy PCL are minimal. The risk is even lower for the ACL because there is minimal ACL stress throughout the squat. Maximum ACL stress is at about 15-30 degrees and drops off thereafter. This minimal loading is due in part to the activity of the hamstrings which help stabilize the knee.

So while it is true that as your knees go over toes, it can increase shear forces, mostly through the PCL, these forces are simply not enough to damage healthy ligaments. Plus, your tendons and ligaments adapt to resistance training by increasing their tolerance, so you'll be less likely to injure them anyway!

So what should you do?

For individuals with healthy knee joints, restricting knee travel is unnecessary and may even prevent you from squatting deeply in a safe manner. Squatting through a full range of motion is something that shouldn't be avoided for people without reason. In fact, concerns about degenerative changes in the tendofemoral complex, chondromalacia, osteoarthritis, and osteochondritis in deep squats have been debunked by clinical studies.

If you still think that you want to avoid these scary shear forces by doing half squats, just know that when people choose to do half squats, they tend to use more weight and overload the parts of the squat where shear forces are highest, which would only make any potential for degenerative changes much worse. That doesn't mean everybody is ass to grass either.

 It's crucial to consider individual differences in anatomy and biomechanics when determining the safest and most effective way to squat. Deep squatting with forward knee travel might not be suitable for everyone right away; for example, those with pain at the front of the knee might benefit from other variations of the squat that reduce knee travel, such as the low bar squat, or the box squat. But with patience combined with intelligent programming, it can become a beneficial part of your routine.

So, in summary, allowing your knees to travel past your toes during squats and lunges is not inherently harmful. Increasing forces at joints is a natural part of movement, and the squat is no exception. The key is to apply the right training stimulus to avoid overloading your tissues and risking injury. So, go ahead and squat with confidence, knowing that you're working towards a stronger and healthier you.

References:

1. Escamilla, R. F. (2001). Knee biomechanics of the dynamic squat exercise. Medicine and Science in Sports and Exercise, 33(1), 127-141.

2. Escamilla, R. F., et al. (2012). Anterior cruciate ligament strain and tensile forces for weight-bearing and non-weight-bearing exercises: a guide to exercise selection. The Journal of Orthopaedic and Sports Physical Therapy, 42(3), 208-220. doi:10.2519/jospt.2012.3768

3. Aune, K., et al. (1995). Hamstrings and gastrocnemius co-contraction protects the anterior cruciate ligament against failure: an in vivo study in the rat. Journal of Orthopaedic Research, 13, 147-150.

4. Schoenfeld, B. J. (2010). Squatting kinematics and kinetics and their application to exercise performance. Journal of Strength and Conditioning Research, 24(12), 3497-3506.