Everyone has a slightly different bony anatomy. Whether it’s a longer femur, bent shin (tibial torsion), or a rotated hip socket (acetabular retroversion), your anatomy, in addition to your functional goals, should ultimately drive squat depth. So how deep or low should you squat? From an injury prevention and biomechanical perspective, there is only one thing that should matter – posterior pelvic tilt.
Normal hip flexion range of motion is approximately 120 degrees. This is when the femur abuts against the hip bone, or pelvis. When we squat deeply past 120°, as shown below, where do we magically get more range of motion? The pelvis! The pelvis posteriorly tilts to allow more room for the femur to flex. Also known as the tuck under or butt wink, a posterior pelvic tilt is a naturally occurring phenomenon. However, an excessive posterior pelvic tilt can potentially become a problem when we add an excessive load, or weight, during a squat movement.
Because our legs (femurs), hips (pelvis), and spine (vertebral column) are all connected, movement of one bone will undoubtedly affect the others. When we posterior pelvic tilt, our lumbar spine flexes. The most important fundamental of good squat mechanics is to maintain neutral spine position. The moment we excessively posteriorly pelvic tilt, we automatically lose neutral spine position and dump into flexion. Significantly increased spinal flexion under load over time (YES, all these factors must be present) is detrimental for your back for two reasons:
1) Biomechanically speaking, it increases the lever arm of the back extensors. Thus, our back extensors must work much harder to keep our torso upright. Increased muscle activation of the back extensors increases the shear and compression loads on each vertebra, and over time, can lead to problems down the road.
2) The intradiscal pressure inside of each of your intervertebral discs increases tremendously in a flexed spine position. Additionally, the thinnest portion (posterior aspect) of the annulus fibrosis, or wall of the intervertebral disc, is tensioned in a flexed spine position. Our intervertebral discs are meant to withstand compression, not tension!
Remember, posterior pelvic tilt occurs naturally at end range hip flexion – so a little posterior pelvic tilt is expected during a squat movement (more than likely you will observe a return to neutral pelvis position). However, it is when posterior pelvic tilt occurs excessively during a squat movement under high loads and high repeittions, that you should begin to worry about the additional compression you are placing on your back.
Furthermore, from a functional perspective, will you ever have to perform a deep squat in everyday tasks? Training specificity is important. Unless your occupation or sport requires you to perform a deep squat, there’s no point in training your body to squat all the way to the ground and risk back injury.