Much talk have been given to knee positioning and biomechanics during a squat, although there are studies that demonstrate increasing shear forces with excessive anterior translation of the knees (Myer et al., 2014), this could be a contribution to an increasing knee flexion angle as purported by Schoenfeld (2010), who showed that increasing knee flexion angle increases posterior, tibiofemoral, and patellofemoral compression forces. An anterior translation of the knee during a squat increases the speed at which the knee enters knee flexion and this increase in deceleration forces generates high joint forces at the knee (Schoenfeld, 2010). I believe it is due to this reason that there has been a lot of talk and theory behind why knee over toe squats are a taboo. However, these studies do not account for anthropometric differences in individuals; an athlete who has a longer femur will naturally have to drive his/her knees anterior to the toes in order to maintain proper alignment (Vigotsky et al., 2018) to fit the rule of biomechanics. Furthermore, McKean (2012) demonstrated that much variability exists in the knees during a squat in the frontal and sagittal plane, and purported that “restricting the movement of the knee during a squat will alter the movement sequence and hence place undue strain on segmental joints during the squat”.
I highly believe in variable movement strategies in individuals and there is absolutely no “one-size-fits-all” when it comes to compound exercises and athletic movements. Considering many sports involve movements when the knees are required to pass the toes anteriorly; such as that of a cross-over in basketball, a sprint start, or any martial arts movement, it is silly to suggest that knees over toes in a squat or any other exercises should be avoided. One can easily search for slow motion videos demonstrating olympic weightlifters and powerlifter, who are arguably the best at performing compound movement such as a squat, and find athletes who have their knees passing their toes during either the accent of a squat, or a decent of a squat, or both.
In a practical setting, much will depend on one’s movement strategy, sport, anthropometry, training age, injury and health history status in order for me to decide whether knees over toes is deemed as detrimental or suitable for the athlete/client. A person who has had a PCL injury and is in the process of rehabilitating for it for example should not be squatting past 30 degrees as it has been demonstrated to reach its highest torque of 73.2N, similarly someone who is rehabilitating for meniscal injuries should also be wary of knee flexion angle passing 30 degrees as it too has shown to have its peak tibiofemoral forces (Schoenfeld, 2010). However, athletes with healthy knees should expose themselves to movement that are specific to their sport and allow specific adaptation to imposed demands. Parameters will also play a major role, optimal adaptation is always tricky as professionals always need to incur enough stress in order to allow for progression but be careful of not passing that threshold. This is definitely a nuanced topic and will likely depend on multiple factors.
Myer, G. D., Kushner, A. M., Brent, J. L., Schoenfeld, B. J., Hugentobler, J., Lloyd, R. S., Vermeil, A., Chu, D. A., Harbin, J., & 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), 4–27. https://doi.org/10.1519/SSC.0000000000000103
Schoenfeld, B. J. (2010). Squatting kinematics and kinetics and their application to exercise performance. Journal of Strength and Conditioning Research, 24(12), 3497-3506. DOI: 10.1519/JSC.0b013e3181bac2d7
Vigotsky, A., Bryanton, M., Nuckols, G., Beardsley, C., Contreras, B., Evans, J., & Schoenfeld, B. J. (2018). Biomechanical, anthropometric, and psychological determinants of barbell back squat strength. Journal of Strength and Conditioning Research, 33, S26-S35. https://doi.org/10.1519/JSC.0000000000002535