The question of how deep you should go in the squat is constantly debated. For example, many athletes are interested in the depth of the squat in relation to knee safety. Going deep so that your buttocks almost touch the heels is deemed dangerous to the knees for some athletes but for high-level athletes it is fine. Also of concern is the positioning of the knee in relation to the feet, i.e., whether the knees are forward of the feet or in vertical alignment with the toes. Some believe that the knee should not go beyond the base of the feet while others feel it makes no difference and is perfectly safe.
Others still debate whether it is safe to squat with weight plates or board under the heels. Some believe this practice makes the squat easier and that it is perfectly safe for the knees. Other strongly believe it is just the opposite; dangerous for the knees because it pushes the knees forward.
Few, however, look at prerequisite exercises that can help make the squat safe regardless of depth. For example, foremost in squat safety should be whether you maintain an arch in the lower back during the ascent and descent. Even though this is mentioned as being important, it is not given high priority. But yet, it should be the number one factor that determines squat depth.
Maintaining the lumbar spine arch is important for back safety and is also the critical element in regard to whether you engage the hamstrings and gluteus maximus muscles. For example, if you allow the lower back to flex (get rounded) as you lower the body, it gives more slack to the hamstrings. As a result, you have to go much deeper before the hamstrings and gluteus maximus come into play.
Flexion (rounding) of the lumbar spine while doing the squat is probably the number one reason for the occurrence of back injury. Because of this, the back arch should be the main concern when doing the squat, especially if you use heavy weights. If no weights are used, a rounded back may be safe, if the muscles are sufficiently strong. With ample resistance, the squat with a rounded back can cause severe problems to the spinal discs and vertebrae, even disc rupture.
If you understand that holding the arch in the lower back is the main requisite for a safe and effective squat, then maintaining the arch should be your number one concern. In my experiences, maintaining the arch is very difficult for most athletes and if it is truly maintained throughout the execution, most athletes do not get close to the thigh level position even if they have been doing the squat for many years. It is not only beginners who have difficulty holding proper spinal position, it occurs with high-level athletes as well.
Thus, main focus should be on whether the athlete maintains the arch in the lower back. Once the arch is lost or as the athlete is losing the arch, the descent should stop immediately. This point in the range of motion then determines the depth to which one should go in the squat.
If the arch is maintained and the athlete goes lower, other factors such as positioning of the knee become important. If the knees move too far forward beyond the base of support, then it is important to examine foot placement , Achilles tendon flexibility, amount of forward lean of the trunk, positioning of the hips, etc. All of these factors have multiple variants and depend on the objective of the squat.
For example, runners and jumpers should have the feet directly under the hips while powerlifters have the feet outside the hips. Some prefer the sumo style in which the feet are even further apart. A 45 degree forward trunk lean angle is usually considered ideal but it can vary up or down depending on the individual.
The problem of squat execution becomes even more complex when dealing with specificity of training and training transfer. For example, should the athlete do a deep, full (thigh level position), half squat or all variants to improve jump height, running speed, cutting actions, etc.? Or should the type of squat vary depending upon the phase in the periodization scheme? When should these variants be done (specific phase) in the periodization plan? Answers to these questions are complex and require a separate article to address each one.
Exercise technique can be quite complex even though most exercises are considered to be relatively simple. Most books and magazines that deal with strength training show and describe exercises in about one or two paragraphs accompanied with 1-2 photos. Only Biomechanics and Kinesiology of Exercise treats exercise execution in full detail. Not only is muscle involvement noted but also all details regarding execution such as what happens if you deviate from proper execution, if you use a different stance, positioning, grip are addressed.