Squatting is a freaking wonderful strength training exercise. Let’s start from there why don’t we? Squatting involves the coordinated action of a boatload of muscle groups surrounding the hip, knee ankle, and core - so it provides a huge bang-for-buck in terms of body wide gains in strength and muscle mass.
Furthermore, the action of squatting mimics a ton of different motions commonly found in daily life and athletics, like sitting, lifting, climbing stairs and jumping (among others). So it turns out to be an enormously function method of training for performance in sport and life.
In the deep ranges of a squat, there is evidence to show that there is greater muscle activation, especially in the glutes, versus shallower ranges.
Fig 1. Shallow (a) at <80 degrees of knee flexion, medium (b) at 90-110 degrees and deep (c) squat at 110 degrees and more.
So we get that it’s a great exercise. Sometimes though my clients express concern around engaging in the deep ranges of a squat (think greater than 110 degrees of bend at the knee). This is the lowest end of the movement, past the “thigh is parallel to the ground” position.
The question surrounds whether the forces borne at the knee in a deep squat are damaging for ligaments, cartilages or other structures.
It’s a relevant question, as it is true that compressive and tensile forces when squatting start to elevate considerably as we approach and exceed 90 degrees of knee bend. But are these forces damaging? The short answer is not really... for most people with a healthy knee the compressive and tensile forces at the knee in a deep squat are completely tolerable and in fact may be useful for improving the strength of the stabilising ligaments and cartilages.
But there is a more nuanced discussion around the topic with useful considerations for people who are dealing with injuries to meniscal or articular cartilage, or who have sustained a ligament (particularly and ACL) injury and are managing that while still working out.
Let's consider first tensile forces at the knee. Tensile forces resist shearing, that is, sliding of one bone against another. These forces are resisted by ligaments, connective tissues that connect bones to other bones. As a side note - tensile forces are also resisted by muscles in addition to ligaments, and this stabilising benefit is yet another reason why strength training is useful.
These tensile forces are maximised in deep squat ranges. So that’s bad right? Well not so fast… There is good evidence to suggest that the ligaments of the knee have resistance to tensile force that is much higher than that produced under even heavily loaded deep squatting conditions.
So while heavy deep squatting does challenge knee ligaments, the forces produced by the heaviest, deepest squat that you or I might consider trying are well within the tolerance of a healthy ligament.
Fig 2 - The ACL, in much gloried yellow highlight. Kneecap is removed in this diagram so you can see it. I used the Essential Anatomy 3 app to help visualise the anatomy.
But what about compressive forces? Think of the knee as having 2 sets of surfaces that can be compressed.
First, the Patellofemoral joint (fig 3.a) is the articulation between the underside of the kneecap (patella) and the end of the thighbone (femur). Problems that may arise from compression here surround irritation of that cartilage that lines these surfaces. Patellofemoral Pain Syndrome (PFPS) is a very common knee problem of this kind.
Maximal patellofemoral compression is achieved between 90 and 130 degrees of knee flexion. So if you are already dealing with a PFPS type injury, deep squatting into these ranges can potentially irritate the problem.
Second, the tibiofemoral joint (fig 3.b) is the articulation between the end of the femur and the top of the tibia (the bigger shinbone). Here we find articular cartilage, and also the menisci that can be irritated under a compressive condition. Anyone who has experienced living with tibiofemoral arthritis, or an otherwise damaged meniscus can attest to the frustration of working out with such an injury.
Fig 3 - Compression forces exist at the knee across the patellofemoral (kneecap to thighbone) joint (a) and tibiofemoral (shin to thigh) joint (b). I used the Essential Anatomy 3 app to help visualise the anatomy.
If you are dealing with tibiofemoral arthritis or a damaged meniscus, squatting into deep ranges may irritate your injury and should be modified in some manner.
Thankfully, there are squat modifications that you might consider to mitigate these issues:
- Avoid the deepest rage of squat when injured!: As already discussed above, maximal compressive and shear forces are generated at the knee in the deepest ranges of squat. So unless you are operating on a healthy knee, I would suggest staying out of the deepest ranges of a squat. Keeping your depth to south of 80 degrees will be safer until your injuries have cleared up.
- Try front squatting: Loading the bar in a front rack position ( See Fig. 4) reduces compressive forces on the knee. If you are dealing with PFPS or arthritis, this style of squat will likely be worth trying. It’s a tricky form though, start training the movement with low loads initially and only add more significant weight when you have the movement pattern mastered.
- Practice slow controlled speed: Compressive forces at the knee are higher when moving with a faster cadence. Practice slow, controlled movements. Think 2-3 second descents (“one steamboat, two steamboat…” etc).
- Keep foot placement at around 90-120% of hip width: There is evidence suggesting that keeping your foot placement closer to hip width rather than at wider stances reduces compressive and shear forces at the knee. Granted, there are other reasons why you might consider using a wide stance squat, including reducing shear forces at the low back. If you are also dealing with low back pain, foot placement is a consideration you may want to discuss with your physiotherapist so that you can make a decision best suited to your specific context.
- Don’t squat fatigued: Duh! Stability is compromised when you are muscularly fatigued. If your legs feel like jelly, end your set and come back fresh the next workout!
- Ensure that you are executing a balanced lower extremity strength training protocol!: The muscles of the calf, the front and back of the thigh (quads and hamstrings) and those that surround the hip (glutes) all contribute to knee stability. Choose a varied set of strength training exercises that train all of these muscle groups to help shore up your muscular stability. In addition to squats, choose deadlifts, heel drops, bridges/hip thrusters, lunges or even knee extensions and curls to round out your program. If in doubt, speak with your physiotherapist or trainer to help you choose.
Fig 4 - Front rack versus back rack squat position. The circle is where the plate on an olympic bar would be. Stick men illustrate only so well...
Form matters, so becoming proficient with the movement pattern of a quality squat is a prerequisite to executing under load and in the deepest ranges of the exercise. A deep dive into the minutiae of squat for is not the goal of this article (although maybe in a future article…), but in general consider the following brief form pointers.
Keep a neutral spine, including your neck: Yes there are natural “normal” curves in your spine, and you want to keep those through the squat. Avoid excessive flexion (forward bend) or extension (arching back). This includes at your neck. There is a temptation as you descend through the squat to crane your neck back, avoid this. Your neck should hold your head in neutral position relative to the trunk (not looking up or down).
Upright trunk: Work to keep your trunk as vertical as you can (See fig 5.a). This requires a lot of strength through the glutes, hamstrings and core. Weaker people will have difficulty maintaining this vertical position and will trend toward a horizontal trunk (see fig 5.b).
Fig 5 - Trunk in vertical position (a - on the left) and in a more horizontal position with the hips pushed further back (b - on the right). Shoot for position (a).
Knees over feet: Looking front on, keep your knees over your feet. This helps you avoid the dreaded valgus position (and the inappropriate hip and knee forces that go with it).
Looking from the side I am not so anal about the often cited rule to keep your knees from extending past the toes. The suggestion that creeping your knee past the toes places undue stress on the ACL is misplaced (it doesn’t) and indeed keeping your knees pushed far back may place excessive strain on the hips and low back.
Feet planted on ground: Keep your feet planted from heel to toe on the ground. Feet are pointing ahead or slightly outward. Don’t let your ankle collapse inward or outward. In other words don’t load excessively though the inner or outer (left or right) sides of your foot.
The take home message of this discussion is that squatting to deep ranges on a healthy knee is safe. And here’s a nice thought as well, there is some evidence to suggest that over the long term, squatting in general may help to stimulate an anabolic building effect in the ligaments and cartilages of the knee. So as long as you are executing with good form on a healthy knee, squatting deep can help build long term robustness and stability rather than damage and disability.
However, if you are dealing with moderate to advanced arthritis at the knee, a known ligament tear, meniscal damage, or are nursing patellofemoral pain syndrome or a similar condition, then modifying your squat may be the most appropriate choice. If you are unsure ask your physiotherapist.
Consider the suggestions presented above and demonstrated in the attached video. If you have questions about the appropriateness of squatting for you, ask your physiotherapist for advice specific to your situation. In the meantime, have fun in the gym!
Further reading (if you really want to impress your friends)
Comfort, P., McMahon, J. J., & Suchomel, T. J. (2018). Optimizing Squat Technique—Revisited. Strength & Conditioning Journal, 40(6), 68-74.
Escamilla, R. F. (2001). Knee biomechanics of the dynamic squat exercise. Medicine & Science in Sports & Exercise, 33(1), 127-141.
Gullett, J. C., Tillman, M. D., Gutierrez, G. M., & Chow, J. W. (2009). A biomechanical comparison of back and front squats in healthy trained individuals. The Journal of Strength & Conditioning Research, 23(1), 284-292.
Hartmann, H., Wirth, K., & Klusemann, M. (2013). Analysis of the load on the knee joint and vertebral column with changes in squatting depth and weight load. Sports medicine, 43(10), 993-1008.
Myer, G. D., Kushner, A. M., Brent, J. L., Schoenfeld, B. J., Hugentobler, J., Lloyd, R. S., ... & McGill, S. M. (2014). The back squat: A proposed assessment of functional deficits and technical factors that limit performance. Strength and conditioning journal, 36(6), 4.
Schoenfeld, B. J. (2010). Squatting kinematics and kinetics and their application to exercise performance. The Journal of Strength & Conditioning Research, 24(12), 3497-3506.