The fitness industry is full of rules, and while many are created with the best of intentions, you’ll find that the bulk of them are formed without a single shred of scientific evidence, sound reasoning or even a basic understanding of how people actually train. My recent read of the book Risk: The science and politics of fear by Dan Gardner (1), and a few posts online discussing knee position in the squat got me thinking about the parallels between perception of risks and exercise prescription.

In Risk, Gardner took the media to town over the fear mongering reporting that dominates the news today, and questions our ability to calculate risk when making decisions. The media often reports statistics in absolute amounts, focusing on single individuals and events, and in doing so fails to give proportion. While stories of individuals obviously tug on the heart strings, in doing so we’ve skewed our ability to determine and account for risk.Using the horrific terrorist attacks of 9/11 as an example, he shows the shift away from air travel back to the car, and how the ensuing spike in car travel resulted in an estimated 1600 extra unnecessary deaths that year, just from the new-found fear of flying. We focused on the single, inexplicable event, and in doing so, put ourselves at greater risk: we didn’t consider the proportion, the actual likelihood of it happening again, and we failed to account for the relative risk between the situations (driving vs flying).

So what do terrorist attacks have to do with squats and patellofemoral force?

Well, very little directly, although if they get caught they will definitely have a little more time to spend in the squat rack. The same human behaviour that makes us act irrationally and worry without taking into account the proportion when considering statistics seems to have crept over to the exercise world when prescribing and teaching specific lifts. Anyone who has completed one of the common certification courses, regardless of organization, or has an internet connection for that matter, has probably seen the written-in-stone rule that the knee should never cross the toe when squatting. While solid evidence for this advice is never given, it is usually associated with a smorgasbord of catastrophic events leading to the complete obliteration of the knee. Once people get out in the real world they see numerous examples of lifters squatting with their knees over their toes, with no pain or disastrous consequences. This usually gets the gears grinding and as they think a bit more about it they find other examples, such as going up and down stairs, where the knee passes the toe with no ill effects. Better yet, how many of these trainers taught the squat with this golden rule, only to turn around and prescribe lunges without thinking twice about it, where the knee passes the toe and then some?

Patellofemoral compressive force explained

The argument against the knee passing the toe usually hinges on patellofemoral compressive force and the potential for this force to contribute to both acute pain and long-term degenerative changes at the knee joint. Patellofemoral compressive force (shown below) is due to the interaction of the patella (knee cap) with the femoral condyles, which is facilitated by the positioning of the patella in the quadriceps and patellar tendon. As the knee moves into flexion (the descent of the squat), the patella moves onto the femur, and first contacts between 10-20 degrees of flexion (2), and increases in contact area as the knee flexes. Patellofemoral compressive force generally increases as the knee flexes, peaking around 85 degrees of flexion (4) and remaining relatively constant as the knee moves deeper into flexion (3).

Knowing what patellofemoral force is, why would we expect the knee going over the toe increase it? If you squat so your thigh is parallel to the floor (a common range of motion recommendation, although I prefer full squats if possible), and allow the knee to pass the toe you will have a greater degree of knee flexion when you achieve the parallel position than if the knee stays over the foot. We know that patellofemoral force increases with knee flexion to a point (3), and based on this, a slightly higher PF compressive force could occur with the knee over the toe.

How much force is too much force?

Excessive or repetitive patellofemoral femoral forces are thought to contribute to knee pathologies and pain, as it provides stress to the articular cartilage of the patella and the patellar surface of the femur; however, we have no idea just how much force is too much. Just like we saw with the examples from the book Risk, considering the absolute value of a force really doesn’t tell us much about the risk of injury. Just because one position, in this case the knees over the toes, could be associated with elevated patellofemoral compressive force doesn’t necessarily mean that the lifter will be exposed to injury or that the exercise is any more dangerous than when performed with the knee remaining over the foot.

Looking at that interaction between the patella and the knee cap further, Escamilla (3) demonstrated that during 20 to 90 degrees of knee flexion, patellofemoral stress increased. Stress (force/area) is important in this case, as it considers both the patellofemoral compressive force we’ve been talking about, but also considers the actual surface area of contact between the patella and femur, which we know to increase as the knee flexes (5). The case above suggests that, during knee flexion of 20-90 degrees, the rate of increase in patellofemoral compressive force is greater than the increase in contact area between the two surfaces, which leads to the increase in stress. At higher degrees of knee flexion, patellofemoral stress can actually plateau, or even decrease, as patellofemoral compressive force peaks around 90 degrees while patellofemoral contact area can increase further, allowing the force to be applied to a greater area and ultimately reduce stress. So going back to the knee over the toe squat, the higher knee angle encountered when squatting to a parallel position could be associated with the same (6), if not lower patellofemoral stress (3). Better yet, this paper concluded that athletes, from a patellofemoral perspective, are actually safe to perform deep squats past 90 degrees of knee flexion. So we dropped the ball again, not only did we consider absolute forces with no understanding of the tolerance of the tissue or the amount of force required to produce injury, but we also neglected to consider changes in contact area and how those interact (stress).

Without a clear idea of the amount of force or stress required to produce an injury, it’s reckless to make sweeping statements about exercise form. The same loading that we claim could be the cause of injury could be a sufficient dose to produce positive adaptations that ultimately improve function. If you’re teaching appropriate hip hinging (sitting back) in the squat, a little forward knee translation in the end likely won’t be problematic. If you’re doing quarter squats by shuffling your knees forwards and backwards, then you probably have bigger issues to worry about than patellofemoral force anyway. So as someone who performs full squats pain-free, I’ll continue to do so and won’t be worried about any patellar issues down the road. All biomechanics aside, while the numbers tell one story, any pain or dysfunction experienced in the squat needs to be addressed irrespective of what the numbers might say.