It is well documented that larger Q-angles (the angle created by the width of the pelvis in relation to the center of the knee joint) play a significant role in the prevalence of ACL tears. Why do these larger angles cause ACL tears? A quick review of anatomy, physics, and human function can help explain.
The key anatomical features that need to be understood when talking about the Q angle and ACL rupture are the Gluteus Medius, the width of the pelvis, the direction of the ACL, and the function of the great toe. Humans are biomechanically challenged in terms of the available lateral hip musculature needed to stabilize the pelvis during the single leg stance phase of gait. This problem is accentuated with the higher body mass indices associated with modern culture. Females specifically have a wider pelvis in relation to their height to allow for childbirth. Recall, that the ACL runs from posterior laterally to anterior medially in the knee joint. The great toe helps to stabilize the foot, and therefore the tibia, especially when standing on one leg. The use of cushioned shoes makes it difficult for the great toe to purchase the ground.
While we are bipedal in the strictest definition of the term, in lower extremity movement, we function unipedally. There is a phase in the human gait cycle in which only one foot is in contact with the ground. This is where the challenges begin.
These challenges can be explained by physics. A wider pelvis actually creates more of a
rotational force that the gluteus medius has to stabilize when standing on one leg. If the hip and foot musculature responsible for countering this rotational force is not strong enough it requires one to laterally shift their center of gravity over their base of support to remain standing. Recall, that the base of support is only around the foot that is in contact with the ground. This lateral shift creates a valgus force at the knee joint (the knee is directed inwards). Adding to the problem is the lack of foot strength associated with modern culture. If the great toe cannot stabilize the foot, the lower leg bone collapses inward contributing to an increased valgus stress at the knee joint. The ACL is designed to resist this valgus force, however, if the force is too great the ACL will tear.
The solution to the problem begins with the understanding that it is very important to train your lower extremity while standing on one leg. This helps to strengthen the mechanisms needed to allow for hip, knee, and foot stability. Most multi-joint single leg exercises are very difficult to perform correctly. This is one of the reasons the CKC Trainer was developed. It allows for all users to perform single leg lower extremity exercises with proper form and load. By developing single leg strength one will go a long way in preventing ACL tears.