When the foot pronates and knee is rotated internally, any movement that requires flexion (bending) will occur at an oblique angle, as seen in the video above (the pronated foot is shown on the left, while the corrected foot is shown on the right). In other words, the knee does not move directly over the ankle, but rather off to the medial side (towards the midline of the body). Due to this oblique angle, people who pronate tend to excessively torque the knee—the degree of which depends on the degree of pronation. This puts additional strain on the anterior (front) and medial aspects of the knee, which may not be a significant issue when it comes to everyday activities such as walking, but it plays a huge factor in exercise and athletic performance.
Due to the additional torque on the knee caused by pronation, athletes in sports that involve explosive, rotational, and lateral movements—on top of rapid changes in direction—may be more prone to injuries. Since some individuals (particularly the young and/or fit crowd) may be able to compensate for their pronation, not necessarily everyone who pronates will experience symptoms of pain. However, regardless of injuries and/or pain, pronation can certainly affect athletic performance.
Having a “corrected” foot placed as close to subtalar neutral allows the knee and ankle to track properly, which translates to the optimal movement mechanics, as no energy is wasted in generating power. This significantly affects athletic performance, even in absence of injuries. When the knee doesn’t tract properly, more force is required to generate the same power and control the movement. This further exacerbates any issues caused by one’s pronation and can contribute to hip and knee stability. When the ankle and knee are properly aligned, the body is able to properly stabilize these structures and move more efficiently.