I like questions to be answered by science. Not that I think that research always defines fact but more because it gives you a trend of evidence to base how you train in the gym. One of the questions that constantly appear in the realms of strength and conditioning is squat depth.
The classic squat is a fundamental movement defined by near enough simultaneous flexion of the hip, knees and ankle. How this movement may look like varies dependent upon limb length and relative joint mobility. This exercise is fundamental to fat loss programmes, performance enhancement and is probably the most prominent body weight exercise taught to beginners looking to incorporate multi-joint exercises in to their fitness regimes.
Performing this movement until you can sit almost to the floor is even with body weight representative of great mobility and strength. With extra load though dumbbells or barbells the question remains that “Is this movement dangerous for the knees?” and specifically”Are the pressures too great in the knee complex to warrant squatting to this depth” or indeed “ass to grass” to be a bit cruder.
From a muscular activation perspective there is little extra effect on hamstring involvement from deep squatting and that if quad involvement is the primary goal then squatting deep may not give any extra reward. Where the benefits are seen is at the gluteus maximus, where there is increased muscular activation in the deep squat compared to a parallel squat. Therefore if you are targeting the hip musculature then this may be a technique that you could utilize.
The concerns around deep squating and knee pain relate to a basic study that suggested that deep squatting can cause laxity in knee ligaments including the anterior cruciate ligament. In turn though this has been refuted in studies that show improvement in knee stability and tighter joint capsules on the anterior draw test in deep squatters. Further studies have shown that ACL and PCL forces are reduced at full flexion whereas their greatest tension is found at parallel- the typical distance recommended for a safe squat. Interestingly the connective tissue, cartilage and meniscus are under increased tension in comparison to ligaments in the deep squat position. Obviously those who suffer from degenerative conditions around the knees at the cartilage, meniscus or patella may find that deep squats may not necessarily be beneficial when also looking to maintain healthy joints.
There is minimal evidence to suggest that squat depth is dangerous for those with healthy knees. It does suggest those that with chronic or acute knees issues may not benefit from deep squatting. As with any training programme the exercise needs to fit the person but those with healthy knees who want to maximise their glute development may find that deep squats are a worthwhile addition to their training programme.