Squatting below parallel – is it safe?
Depending on what literature or dogma you subscribe to may determine your impression on whether an athlete should squat below parallel or not. However, the answer to the question of whether or not squatting below parallel is safe, like so many questions in the sports world, is not straight forward and should elicit the response: “it depends.” It depends on the sport each athlete is training for, any underlying injury or pathology, anatomy of each individual, strength, and finally movement mechanics. This two part series will focus on two main areas of interest; stresses at the knee, and lumbopelvic positioning (butt wink).
Squatting below parallel: Is it safe for the knee?
There is much debate within both the strength and conditioning world as well as the medical community when it comes to full depth squatting and its effect on the knee. A colleague surveyed the other orthopedic surgeons in his group about what they instruct their patients with regards to squats and we were given the following answers: “Be careful” “Get a good Trainer” “I recommend partial squats” “No hard yes or no” “All depends” “I worry about it in my meniscus and patellafemoral (PF) patients”. These sorts of vague and conflicting answers are a direct result of conflicting information and interpretation of available research as well as personal experiences. Medicine & Science in Sports & Exercise published a nice review by Escamilla of the available literature on biomechanics of a dynamic squat. This review showed that peak PF loads increased as knee angle increased with the greatest load occurring past 900. (figure 1) See chart below:
Knee Flexion Angle / Patella Femoral Loads600 / 4.6-5.1x Body weight (BW)
900 / 5.6x-5.8x BW
1300 / 6.3-6.5x BW
Based on these findings, many came to the conclusion and recommendation that partial squats are best. Unfortunately, these mechanical load figures were taken and interpreted by many without further research on the “Bio” aspects of biomechanics of the human body and specific training variables of athletes. When extrapolating this data it is important to keep in mind things like tendo-femoral contact, the influence of reversal of motion as well as an individual’s ability to squat greater loads at partial depth.
Damage to tissue is often a result of excessive stress on the tissue, not necessarily force/load. Stress=Force/Area. To the best of my knowledge it is currently unknown how much PF compression load and stress is detrimental. Even though PF loads continue to increase beyond 900 of knee flexion, the PF stress remains relatively constant due to what is known as the “wrapping-effect”. (figure 2) With increasing flexion, the additional contact between the quadriceps tendon and the intercondylar notch form to become the tendo-femoral support surface (wrapping effect) which contributes to enhanced load distribution and enhanced force transfer. PF contact is further enhanced by the contact area moving more cranially with greater surface area.
The highest dynamic peak loads occur immediately after the change from eccentric to concentric muscle contraction. As noted above, peak stress occurs near 900. During a “half-squat” that reversal happens at 80-1000 where peak stress occurs and no tendo-femoral support.
Finally, it has been shown that athletes are able to lift ~4x more weight with quarter squats (squats to 600) compared to full depth squats. These extra loads are typically not factored into research and load calculations at the tibiofemoral and PF joints. (figure 3) Let us take a look at an example of someone who weighs 200lbs and full depth squats 225lbs. Now using the figures from the table above (600;5.1xBW and 1300;6.5xBW).
6.5x(BW+barbell)
6.5x(200+225) = 2,673 at 1300
5.1xBW+(4xfulldepth weight)
5.1x(200+(225x4)) = 5,610 at 600
Now I understand there are additional factors that play into where the load is positioned (front rack vs low bar back squat), and external loads are not directly correlated to BW loads, the previous example is simply to make a point that extrapolating data, like loads at the PF joint during a body weight squat, can be problematic when applied to sports and athletics.
Up to this point we have focused on PF loads and stresses. Squatting affects additional structures in the knee as well including tibiofemoral (TF) joint, cruciate ligaments, menisci and extensor mechanicsms. Speed and fatigue have also been shown to affect forces across multiple structures within the knee. Performing squats at faster speeds increases both shear and compressive forces at both the PF and TF joints. Similarly, excessive fatigue can increase shear and loading forces across the joints as well as the cruciate ligaments. This is important to consider when programming HIIT style workouts, like CrossFit, for athletes who may or may not currently have knee problems. Additionally, at a full depth squat, athletes often have a decreased ability to control rotational forces and demands at the hip and knee resulting in increased shear. If mechanics breakdown below parallel, the resulting shear forces while loads at the knee are high, can increase risk of cartilage and meniscus damage.
The above biomechanical information and recommendations are based on healthy knees, but most of the time we see athletes to address a current or previous injury or impairment. These impairments may affect ones ability to safely squat to full depth.
Interestingly, machined squats (eg: Smith Machine) result in 30-40% greater shear forces at the knee compared to free barbell squats, however, consistently we continue to see surgeons and PTs prescribe machine or ball against the wall squats during knee rehab protocols. Is this really best practice? I don’t think so.
Meniscus: The meniscus structures within the knee function to shock absorb and distribute loads as well as provide some secondary stability to the knee. When these structures are injured or repaired they lose their ability to effectively perform these tasks. A full depth squat engages the posterior aspects of the menisci and caution may be warranted with athletes with known meniscus pathology. The menisci attenuate compressive forces well, but are subject to damage with shearing and rotation/angular velocity stresses that can be seen with speed squats, fatigue and often the Olympic lifts.
ACL/PCL: Anterior and posterior shear stress and load are well below the tensile strength of native/intact ACL/PCL throughout the squat motion. Anterior shear during squat is greatest between 0-60o with loads well below the tensile strength of intact ACL (500N vs 2160N). This is important to remember during phase 1&2 of ACL recovery (first 6 weeks) due to graft incorporation and decreased tensile strength. Knee posterior shear increases with increased knee flexion. Again the loads should not be an issue for an intact PCL (2704N vs 4000N). However, full depth should be avoided a minimum 3-4 months following PCL reconstruction. (figure 4)
Extensor Mechanism: Tendons respond to stress by increasing cross sectional area, increasing stiffness, and enhancing tensile strength. These are all normal adaptations in a healthy knee. During all squat variations there are high loads across the extensor mechanism (Quad tendon, Patella tendon). Heavy squats, particularly to full depth should be a concern following Quad or Patella tendon repair or significant inflammation of these tissues.
In summary, full depth squats are safe in a healthy knee so long as the athlete can maintain good mechanics. Regular squatting on a normal knee has functional adaptations to tendon, meniscus and cartilage that may be advantageous to athletes across a number of sports. A standard squat depth might not be appropriate for every athlete. As a coach or healthcare provider it is our responsibility to understand the mechanics of squatting, loads across structures in the knee, how an injury may affect the mechanics and then work with athletes to modify as needed. The most important thing when determining squat depth is making sure the athlete can maintain good mechanics, pain free. When comparing squatting to many of the other sports and activities people participate in regularly (skiing, mountain biking, contact sports, etc) I think squatting has been given an unwarranted bad reputation. (figures 5)
Keep an eye for part two of this series when we discuss squatting below parallel and the “butt wink” as it relates to hip and low back health.
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