Knees past toes: a bad idea? This belief originated from a study that is more than 30 years old (1978 Duke University study that found maintaining a vertical lower leg as much as possible reduced shearing forces on the knee during a squat). The truth is that leaning forward too much is more likely what is truly causing the problem or even injury.
In 2003, University of Memphis research confirmed that knee stress increased by 28% when the knees were allowed to move past the toes while performing a squat. However, hip stress increased nearly 1,000% when forward movement of the knee was restricted.
Our knees are actually not as weak as most people think. Research investigated the level of knee degeneration in weightlifters with a background of ~17 years of experience. The study showed no worsen level of degeneration of an age matches population.
Actually, weightlifters have been shown to have higher cartilage thickness than non-weightlifters: the thought behind it is that when squatting there are anabolic, biochemical and structural adaptations of the cartilage tissue, causing increased mechanical stress tolerance and hence protective effects against degenerative changes in both cartilage and meniscus.
Squats below 90 degrees stress cartilage of the odd facet preventing cartilage degeneration and atrophy of the odd facet.
So, what should you do?
If your most comfortable squat set up doesn't show much knee travel that's fine.
If your most comfortable squat position shows meaningful knee travel.. that's fine too.