I don’t know how it started, but I am assuming that it probably started with physical therapists and rehab professionals who learned in school that when the knees pass the toes in a squat this places a lot of stress on the knee and can damage the structure and integrity of the joint.While I will agree that improper squatting patterns can get you injured and that an overly knee-dominant squat (not properly engaging the hips) is probably not the best thing for you, especially under load, I can’t see how you can squat down to an appropriate depth (femur parallel to the floor) without the knees slightly breaking the plane of the toes. I think the biggest problem with this myth stems from the sad fact that most rehab professionals have eliminated the squat from their exercise protocol, deeming it “to dangerous.” Because of this, most rehab professionals are clueless with regard to how to properly and safely coach a squat, which is silly because the squat is a fundamental movement of life. Telling someone not squat is like telling them “don’t sit on the toilet ever again” or “never pick anything up off the bottom shelf at the supermarket.” The problem isn’t the movement itself, it is that people don’t understand the movement (since it isn’t taught to them properly) so they end up getting hurt when they need to perform it in their daily routine.
While there may be certain instances that teaching a full squat would be unsafe, it is not appropriate to take information on people with specific injuries and pathologies and apply it to all clients/patients. Everything from research to exercise prescription is highly population specific and you need to understand just whom you are working with and what rules apply. For the majority of people, the squat should be a fundamental movement taught in both rehab and gym settings.
Don’t Let The Knees Pass The Toes!
The statement, “don’t let the knees pass the toes”, has been uttered by many doctors, coaches, trainers, and physical therapists. While a lot of strength coaches and personal trainers now seem to “get it” with regard to the knees passing the toes not being such a bad thing, this myth just wont seem to die.
The squat is a coordinated lift, which requires both mobility and stability at a variety of joints up the chain. The ankles, knees and hips go through active movements of flexion and extension (dorsiflexion and plantar flexion at the ankle) to perform the squat lift. However, the torso should remain stable during the squat, especially under load, and maintain an angle that is approximately parallel to the shin (relatively speaking since people will have variations in limb length and torso length).
In order to keep the knees from passing the toes, some practitioners advocate the shin remaining more vertical, which changes the mechanics of the squat, as knee flexion will be limited during the eccentric portion of the lift. In order to reach an appropriate depth, the client would then have to push their hips back even further than normal, forcing the trunk to bend over further. While this technique may seem to be more “knee sparing”, it tends to place greater load on the lumbar spine.
A 2003 study by Fry et al. evaluated the joint kinetics during a squat when forward displacement of the knee was restricted (IE, the squat I just described) vs. when forward displacement of the knee was allowed (IE, a normal squat where the knees can pass slight forward of the toes). While knee torque was slightly less during the squat where forward displacement of the knees was restricted, hip torque was increased almost 11 times greater than when forward displacement of the knees was allowed. The researchers concluded that, “Although restricting forward movement of the knees may minimize stress, it is likely that forces are inappropriately transferred to the hips and lower-back region. Thus, appropriate joint loading during this exercise may require the knees to move slightly passed the toes.”
Squatting Deep Is Bad For Your Knees!
The squat is used in weight rooms all over the world to prepare athletes for competition; however, safe depth is often argued between strength coaches and athletic trainers. It is typically understood that patellofemoral joint forces are increased during the eccentric portion of the squat at greater angles of knee flexion. A 2001 study published in Clinical Biomechanics looked at patellofemoral joint kinetics during three different squat depths (70 degrees, 90 degrees and 110 degrees of knee flexion). Researchers Salem and Powers hypothesized that patellofemoral joint reaction force would increase with increased knee flexion; however, patellofemoral joint stress would remain constant through the greater squat depths. They based this hypothesis on the idea that with increased flexion, patellofemoral joint contact area has also been shown to increase in previous cadaveric studies, leading them to believe that increased joint contact area would better distribute joint force over a larger area and maintain or potentially decrease patellofemoral joint stress.
Before continuing, I should note that a squat to 70 degrees of knee flexion is about a quarter squat, a squat to 90 degrees of knee flexion is a half squat and a squat to 110 degrees of knee flexion would be closer to a full squat (IE, femur parallel to the floor).
Out of the three depths, 70, 90 and 110 degrees, the full squat (110 degrees) produced the highest amount of patellofemoral joint reaction force (5097.1 N). However, this was not found to be statistically different than the joint reaction force of the 70-degree shallow squat (4319.2 N) or the 90-degree half squat (5065.7 N). Similarly, patellofemoral joint stress was greater in the 110-degree squat (12.3 MPa), however, this again was not statistically different than the 70-degee (10.8 MPa) or 90-degree (11.7 MPa) squat.
From this study, it appears that the consistent knee extensor moment created little variation in patellofemoral joint force or stress when squatting from 70 to 110 degrees of knee flexion. This led Salem and Powers to conclude, “The results of this study do not support the premise that squatting to 110 degrees places greater stress on the patellofemoral joint than squatting to 70 degrees. These findings may have implications with respect to safe design of athletic training regimens and rehabilitation programs.”
Leg Press And Hack Squat Are Safer Than Barbell Squats!
Gym goers and personal trainers tend to favor things like the leg press, hack squat machine, and smith machine over the regular barbell squat stating that, “they’re safer than regular squats”. In addition, many rehab clinics have a leg press machine or some sort of total gym machine (which behaves similar to a hack squat), or they teach their patients to squat with their back against a stability ball placed against the wall (wall/ball squat). While I don’t deny that in a rehab setting these options may be necessary for some, it should be the goal to eventually progress the patient from the controlled environment of the leg press or wall/ball squat to a free standing squat (even if it is just body weight). The goal of rehab should be to rehabilitate the patient back into normal life. By having them only leg press or wall/ball squat and telling them to not perform regular squats ever again because they are “unsafe”, is doing the patient a total disservice since they will need to call on this pattern at some point in their daily activities.
Escamilla et al., looked at knee forces and muscle activity during a squat and leg press under a variety of technical variations using a load equal to the subjects 12RM. The squat variations included a squat with a narrow stance or a wide stance and with feet pointed either straight ahead or externally rotated to 30 degrees. The leg press variations included a leg press performed with the feet high on the footplate or the feet low on the footplate. In addition, the subjects were asked to use both a wide stance or a narrow stance in both the high and low foot placement.
Co-contraction of the quadriceps, hamstrings and gastrocnemius is extremely important for stability of the knee and has been shown to help decrease stress on the anterior cruciate ligament (ACL). During the knee extension phase of this study, the squat displayed a greater co-contraction of these muscles than leg press at either the high or low foot placement, implying that “the squat may be a more effective exercise for quadriceps and hamstring development compared with the leg press.”
However, while ACL forces were not produced in either the squat or leg press variations (which may be due to hamstring activity), posterior cruciate ligament (PCL) forces were greater in the squat compared to either leg press variation. This could have specific implications for individuals who are rehabilitating a PCL strain.
Patellofemoral compressive forces were increased at great angles of knee flexion for both the squat and the leg press. This was shown in the study discussed in the previous section by Salem and Powers. Escamilla et al. state, “Performing the squat, leg press high and leg press low within the functional range of 0-50 degrees may be most active for athletes or patients with patellofemoral pathologies. However, performing the squat, leg press high and leg press low at greater knee flexion angles may not be problematic for athletes with healthy knees as long as healthy loads are not used exclusively.”
I wonder about their 0-50 degrees of squatting recommendation, as that is an extremely short range of motion to go through. That is even less than the 70-degree squat used by Salem and Powers. In addition, if the athlete or patient is rehabilitated properly, should they not be able to squat to appropriate depths, given the fact that at the greater knee angles more muscle activation should occur. This would probably be up to the rehab professional or maybe even the trainer working with the client as to understanding progression and knowing when the individual is ready to move forward with their training program, rather than just doing the same thing over and over and becoming stagnant. Anecdotally, I have improved squat range of motion in clients I have worked with and myself, after suffering from some knee issues in the past. The quote above does imply that knee ranges of motion during the squat or leg press will vary given the individuals specific condition (IE, limitations for those with pathologies and normal ranges of motion with those who are healthy), however, would proper squat depth right off the bat, when the individual is not pathological, help to prevent these pathologies from occurring? I would think so!
The researchers go on to conclude that, “The greater muscle activity and knee forces in the squat compared with the leg press low and leg press high implies that the squat may be more effective in muscle development but should be used cautiously in those with PCL and patellofemoral disorders, especially at greater knee flexion angles. Because all forces increased with knee flexion, training within the functional 0-50 degree range may be efficacious for those whose goal is to minimize knee forces. The lack of ACL force implies that all exercises may be effective during ACL rehabilitation.”
A more recent study (2009) by Escamilla et al. compared patellofemoral joint force and stress between a one-leg squat, a wall squat with the feet close to the wall, and a wall squat with the feet further from the wall.
For those that are unfamiliar, the wall squat is a squat performed with the back against a wall. This is similar to the wall/ball squat and the position is also similar to a hack squat or total gym squat in that the feet are typically out in front of the torso rather than centered underneath it, which changes the mechanics of the squat greatly. This is also the way that some choose to perform their smith machine squats, which is nothing more than an accident waiting to happen in my opinion. In this study, the wall squat was performed while holding dumbbells down at the side. A towel was also placed between the lifter and the wall in order to prevent friction and allow for a smooth squat movement.
While discussing the one-leg squat in the introduction of this paper, the researchers note, “Performing a one-leg squat also causes the stance knee to translate forward beyond the toes at the lowest position of the squat. Clinicians and trainers often believe that anterior translation of the lead knee beyond the toes during squatting type exercises increases patellofemoral joint force and stress, but there is currently no evidence to support this belief.”
The wall squat long and short exercises both produced higher patellofemoral joint forces and stress when compared to the one leg squat. The wall squat short also produced a higher amount of force and stress at a 90-degree knee angle when compared to the wall squat long. This was thought to occur because the knees in the wall squat short went further passed the toe, changing the direction of patellar tendon force and potentially increasing patellofemoral force, than in the wall squat long, where the shins remained fairly vertical.
However, the knees also migrated passed the toe a similar amount in the one-legged squat, but this exercise produced less joint force and stress than the wall squat short. The cause of this was the greater amount of quadriceps force during the wall squat short. The quadriceps force was approximately 30-40% greater at 70-90 degrees of knee flexion in the wall squat short compared to the one-legged squat. In addition, at those same knee angles (70-90 degrees) hamstring force in the one-leg squat was 60-70% greater in the one-leg squat! The researchers made some extremely important observations:
1.) The increased quadriceps force and decreased hamstring force it the wall squat short is due to the fact that the trunk is erect in the wall squat and is tilted approximately 30-40 degrees in the one leg squat.
2.) The wall squat required a greater amount of knee extension because the erect trunk produced a line of force from the center of mass (the lifters mass plus the dumbbells held at the side) decreasing the ability of the hip extensors to contribute.
3.) The forward trunk tilt in the one-leg squat produced a line of force from the center of mass which allowed for a larger hip extension contribution
These three points are critical because they highlight the importance of maintaining the center of mass over the base of support (which takes place in a normal squat). In a hack squat, wall squat, wall/ball squat, total gym machine squat, or the way some perform a smith machine squat, the center of mass is displaced behind the base of support (the feet are out in front) which does not adequately allow the hip extensor mechanism to contribute to the movement.
The researchers went on to conclude that, “Except at 60 and 90 degree knee angles, patellofemoral joint forces were similar between the wall squat short and the wall squat long. Between 60 and 90-degree knee angles, wall squat exercises generally produced greater patellofemoral compressive force and stress compared with the one-leg squat.”
In the video below Patrick demonstrates proper squat technique:
Clearly it appears the myths surrounding squats are not founded on scientific information, but rather anecdotal evidence passed down from teacher to student in classrooms all over the world.Squatting is safe and is an extremely important movement for everyone to learn as it is performed frequently in our daily lives (just think about how many times a day you sit down to your computer desk and then get back up).
Obviously there are specific instances where squatting may be contraindicated for certain individuals and it is up to the clinician or trainer to understand their client and screen out any potential factors that may prevent the client from safely performing a squat.
As the research suggests, those with specific knee pathologies may benefit from a reduced range of motion. However, I would argue that the rehabilitation program should focus to rehabilitate the patient to safely performing a squat down to a depth that would be needed in daily life, for example, sitting down to a chair, which is probably right around 90 degrees of knee flexion if not a little more.
I should also note that back squatting might not be appropriate for every client. Often times, physical therapists and rehab professionals here the word “squat” and immediately think of someone with a load of weight on their upper back, which is why they may shy away from this exercise for their patients. Squats can be performed a variety of ways – front squat, back squat, trap bar, DB squat, kettlebell squat, as well as any of the several single leg variations, such as split squats, lunges, Bulgarian squats, or 1-leg squats. It is important to emphasize that people should first be taught how to squat with just their body weight, as this is an important pre-requisite to loading the client.
If you are still doing curls in the power rack, chances are this paper just went way over your head.
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1. Fry AC, Smith JC, Shilling BK. Effect of knee position on hip and knee torques the barbell squat. J Strength Cond Res. 2003;17(4): 629-633.
2. Salem GL, Powers CM. Patellofemoral Joint Kinetics During Squatting in Collegiate Women Athletes. Clinical Biomech. 2001;16:424-430.
3. Escamilla RF, Fleisig GS, Zheng N, Lander JE, Barrentine SW, Andrews JR, Bergemann BW, Moorman CT. Effects of technique variations on knee biomechanics during the squat and leg press. Med Sci Sports Exerc. 2001;33(9):1552-1556.
4. Escamila RF, Zheng N, Macleod TD, Edwards WB, Imamura R, Hreljac A, Fleisig GS, Wilk KE, Moorman CT, Andrews JR. Patellofemoral joint force and stress during the wall squat and one-leg squat. Med Sci Sports Exerc. 2009;41(4):879-888.
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About the Author
Patrick Ward holds a Masters Degree in Exercise Science. He is a Certified Strength and Conditioning Specialist (CSCS) through the National Strength and Conditioning Association (NSCA) and a USA Weightlifting-Certified Club Coach. In addition, Patrick is a licensed massage therapist focusing on Neuromuscular therapy and Active Release Techniques (ART). He lives in Chandler, Arizona and is the owner of Optimum Sports Performance and the Co-founder of Reality Based Fitness. He can be reached at firstname.lastname@example.org. Visit website: optimumsportsperformance.com