The knee joint bends. To avoid bending it in the interest of injury prevention is folly, since we know from experiment and experience that the dangerous exercise regimen is in fact the one that does not use the knee to its full capability. Like anything else in our body and mind, it's Use It Or Lose It.
The half-squat-half-deadlift, whether single leg or not, is designed to avoid the full use of the knee for people who have pre-existing conditions that prohibit full use of the knee. Everyone who is healthy enough to squat fully should do so. And like any fundamental movement, if you're not healthy enough to squat, you should avoid loaded squats while you work towards fixing those problems.
If you do choose to forgo deep, full squats in favor of the half-squat-half-deadlift, it will be tough to fully replace the squat. The benefit of the exercise is inherent in the loaded full range of motion of the knee, so any good solution will run into the problem of avoiding full knee bends. You can certainly go down that path, but the deep knee bend is what squats are and why they're useful.
The History of "Dangerous" Squats
Brad Schoenfeld's paper, The Biomechanics of Squat Depth (PDF) is an excellent review of the history of this topic:
Squatting safety continues to be a concern amongst some practitioners, particularly as it relates to performance at high knee flexion angles. The theory that deep squats heighten injury risk can be traced to studies conducted by Karl Klein at the University of Texas. Using a self-developed measuring device, Klein noted that weightlifters who frequently performed deep squats displayed an increased incidence of laxity in the collateral and anterior cruciate ligaments compared to a control group that did not (8). Klein concluded that squatting below parallel had a detrimental effect on ligamentous stability and should therefore be discouraged. Soon thereafter, the AMA came out with a position statement cautioning against the performance of deep knee exercises because of their potential for severe injury to the internal and supporting structures of the knee joint.
Subsequent research, however, has refuted Klein’s findings, showing no correlation between deep squatting and injury risk (13,15,18). In fact, there is some evidence that those who perform deep squats have increased stability of the knee joint.
Those references in the sentence I bolded are for the following:
- Meyers E. Effect of selected exercise variables on ligament stability and flexibility of the knee. Research Quarterly. 42(4):411 – 422. 1971.
- Panariello R, Backus S, Parker J. The effect of the squat exercise on anterior-posterior knee translation in professional football players. American
Journal of Sports Medicine. 22(6):768 – 773. 1994.
- Steiner M, Grana W, Chilag K, and Schelberg-Karnes E. The effect of exercise on anterior-posterior knee laxity. American Journal of Sports Medicine. 14(1):24 – 29. 1986
To go back in time to review Klein's anti-squat message, try reading this Sports Illustrated article, The Knee Is Not For Bending. A more plainly, on-the-face-of-it-wrong hypothesis I cannot imagine. Among its strange pronouncements:
- The squat is "of little conditioning value". No one who has ever squatted heavy and deep could ever say this.
- The blame for knee injuries in football is place on squats weakening players' knees, as opposed to, say, the dangerous nature of the game itself. The lack of correlation between teams that squat and teams that have high injury rates is ignored.
How Squats Are Safe
We do find that the knee does undergo stress at the bottom of the deep squat. This stress is in the form of a protective compression, which keeps the joint stable by reducing movement of individual components. To continue from Schoenfeld:
The reduction in ACL and PCL forces associated with deep squatting is believed to be a result of an impingement between the posterior aspect of the upper tibia with the posterior femoral condyles as well as compression of various soft tissue structures including menisci, posterior capsule, muscle, fat, and skin (9). This helps to constrain the knee joint, significantly reducing anterior and posterior tibial translation and tibial rotation compared to lesser flexion angles. Hence, tolerance to load is enhanced in the deepest portion of the squat with a protective effect conferred to ligamentous structures.