Squat Biomechanics: Depth, Stance, Torque and the Truth About Butt Wink
Updated: Sep 8
The squat is where most gym arguments live: how deep, how wide, knees over toes or not, and whether the tuck of the pelvis at the bottom - butt wink - is a fault to be fixed or a feature to be ignored. Each of these has been measured, and the measurements tend to replace absolutes with trade-offs.
The evidence
This article draws on a review of knee and spinal loading across squat depths and loads (Hartmann and colleagues 2013), a laboratory study of stance width, foot angle and experience on knee, hip and trunk motion and loading in 42 participants (Lorenzetti and colleagues 2018) and a 2024 biomechanical review of the squat with clinical applications (Straub and Powers 2024).
What happened
Depth and the knee. Reviewing 164 publications, the authors found that peak retropatellar compressive stress occurs at about 90 degrees of knee flexion, not deeper; beyond that point the "wrapping effect" of the quadriceps tendon over the femur improves load distribution and lowers patellar stress. Concerns that deep squats degenerate the tibiofemoral joint lack scientific foundation, and menisci, cartilage, ligament and bone adapt to loading. Partial squats performed with equal or greater loads favour long-term spinal degeneration more than well-executed deep squats. Their conclusion: learned under supervision with progressive loads, the deep squat is an effective and protective exercise (Hartmann and colleagues 2013).
Depth and joint stress, quantified. Patellofemoral joint stress rises steadily from partial to about 90 degrees, and tibiofemoral compressive force rises progressively through deeper ranges; when depth exceeds available hip flexion, the pelvis tilts posteriorly and the lumbar spine flexes, increasing spinal compression and shear (Straub and Powers 2024).
Torque is a trunk-versus-shin story. A more inclined trunk increases the hip flexion moment and reduces the knee moment; a more forward shin (more dorsiflexion or raised heels) does the opposite. When trunk inclination exceeds shin inclination by 10 degrees or more the squat is hip-biased; when the shin leads by 10 degrees or more it is knee-biased. Forward lean achieved through lumbar flexion rather than hip hinge reduces the spine's load tolerance (Straub and Powers 2024).
Stance width and foot angle. Wide stances (150-200% of shoulder width) produce 23% greater knee valgus moments and 19-37% higher hip external-rotation moments than narrow or medium stances, increase gluteus maximus activation by 13-61% and reduce gastrocnemius activity by about 18%. Turning the feet out 30 degrees halves the knee valgus moment while increasing the varus moment by 80%, with little effect on sagittal moments or quadriceps activation (Straub and Powers 2024). In 42 participants, wider stances reduced sideways knee deviation while larger foot angles increased it, experienced squatters showed far less knee deviation than novices, added load reduced deviation in the experienced, and the extreme combinations - narrow stance with 42-degree foot angle, wide stance with feet straight - produced the largest joint moments (Lorenzetti and colleagues 2018).
The current answer
The squat is safe to a depth limited by your hips, not by a rule about the knee. Patellofemoral stress peaks around parallel and eases beyond it, the tibiofemoral joint adapts rather than degrades, and the real cost of deep squatting appears only when the pelvis tucks and the lumbar spine flexes because hip flexion has run out. Butt wink is therefore a hip-mobility and set-up signal, not a knee problem and not a mystery: it shows where your usable depth ends under that stance and load. Stance width and foot angle are dials that redistribute torque - wider and more turned-out shifts work to the hips and adds a sideways knee moment; narrower and straighter loads the knee extensors and calves - and the extremes in either direction are where joint moments spike. Trunk angle versus shin angle decides whether a squat is a hip exercise or a knee exercise, and lumbar rounding is the one shortcut that costs spinal tolerance.
How certain is this?
Certainty is high for the depth-related patellofemoral and tibiofemoral loading pattern and for the trunk-shin torque relationship (consistent across modelling, in-vivo and review evidence), moderate for the stance-width and foot-angle moment and activation figures (laboratory studies in healthy, mostly young adults with modest loads), and moderate that deep squats are safe for the knee with sound technique (extensive review evidence but few long-term trials). Who this covers: healthy adults and recreational to experienced lifters; people with existing knee or hip pathology should follow the condition-specific progressions the clinical review describes. What would change our mind: prospective data linking a specific depth or stance to injury rates in lifters, or in-vivo spinal load measurements during butt wink at heavy loads.
Depth is decided by your hips. Torque is decided by your trunk and your shins. The knee is mostly along for the ride.
What this means in practice
Squat as deep as you can with a neutral pelvis; the point where your pelvis tucks is your usable depth for that stance, and it is a hip-mobility target, not a reason to fear the knee.
To bias the hips, lean the trunk more than the shin (a low bar, wider stance, or a hinge-forward cue); to bias the quadriceps, keep the trunk upright and let the knees travel forward (high bar, heels raised, closer stance).
Set stance at roughly shoulder width with feet turned out 10-30 degrees as a default, and adjust from there; avoid the extremes of narrow-and-turned-out or wide-and-straight, where joint moments are highest.
If your knees drift inward, widen the stance slightly and reduce foot turn-out before adding load; experienced lifters and heavier loads both reduce knee deviation, so technique and progression matter more than a brace.
Never buy forward lean with a rounded lower back; hinge from the hips and keep the spine neutral, because lumbar flexion is what reduces the spine's tolerance under load.
Learn deep squatting under supervision with progressive loading rather than jumping straight to heavy partials; the review evidence favours full range for both knee and spine over time.
Want this applied to your own programme, your sport and your numbers? Book a Private Consultation - a 60-minute direction session with Coach Ohana.
References
Straub RK, Powers CM. A biomechanical review of the squat exercise: implications for clinical practice. International Journal of Sports Physical Therapy. 2024;19(4):490-501. https://doi.org/10.26603/001c.94600
Hartmann H, Wirth K, Klusemann M. Analysis of the load on the knee joint and vertebral column with changes in squatting depth and weight load. Sports Medicine. 2013;43(10):993-1008. https://doi.org/10.1007/s40279-013-0073-6
Lorenzetti S, Ostermann M, Zeidler F, et al. How to squat? Effects of various stance widths, foot placement angles and level of experience on knee, hip and trunk motion and loading. BMC Sports Science, Medicine and Rehabilitation. 2018;10:14. https://doi.org/10.1186/s13102-018-0103-7
Last reviewed: 3 September 2026. Re-review on the next biomechanical review of the squat or by September 2028.
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