Pronation is a term used in biomechanics and physical therapy to describe a rotational movement occurring in certain joints of the body. This motion causes a segment of the body to turn inward or downward from a neutral anatomical position. Understanding pronation is important for analyzing human movement, particularly in walking, running, and physical rehabilitation. The concept is often discussed in relation to its counterpart, supination, which describes the opposite rotational movement.
Anatomical Locations of Pronation
Pronation is observed most prominently at two distinct anatomical locations: the forearm and the foot and ankle complex.
In the forearm, pronation is a rotational action that occurs at the radioulnar joints, causing the palm of the hand to turn from a front-facing position to a back-facing position when the arm is extended at the side. This movement is accomplished by the radius bone crossing over the ulna.
The movement in the lower limb is more complex and occurs primarily at the subtalar joint of the foot. Foot pronation is a natural, inward rolling motion that helps the foot manage forces during locomotion. Although the forearm movement is a simple rotation, the foot’s action is a coordinated, three-dimensional movement critical to gait.
The Three-Part Mechanics of Foot Pronation
The pronation movement of the foot is not a simple inward roll, but rather a complex, multiplanar action often described as a triplanar movement. The movement is composed of three simultaneous actions occurring at the subtalar joint: eversion, abduction, and dorsiflexion.
Eversion is the movement in the frontal plane, where the sole of the foot tilts outward, away from the midline of the body. Simultaneously, the foot performs dorsiflexion, an action in the sagittal plane where the top of the foot moves upward toward the shin. The final component is abduction, a movement in the transverse plane where the forefoot rotates and moves away from the midline of the body.
These three movements combine to “unlock” the midtarsal joints, transforming the foot from a rigid lever into a more pliable structure. This momentary flexibility allows the foot to absorb the ground reaction forces during the initial part of the stance phase of walking or running. The oblique axis of motion at the subtalar joint facilitates this coupled, triplanar movement.
Distinguishing Functional and Excessive Pronation
Functional pronation is a normal and necessary component of the gait cycle that occurs during the loading response phase after the heel strikes the ground. This movement typically involves approximately four degrees of inward roll, which is adequate to distribute impact and allow the foot to adapt to the terrain. The foot remains in this pronated, flexible state through the mid-stance phase before transitioning to supination for push-off.
Excessive pronation, commonly called overpronation, occurs when this inward rolling motion is too great or lasts for too long during the gait cycle. Instead of the foot stabilizing to become a rigid lever for propulsion, excessive pronation keeps the foot in a prolonged, flexible state, which can destabilize the entire lower kinetic chain.
When the ankle rolls too far downward and inward, it can cause increased rotation in the lower leg and subsequent misalignment that extends up to the knee and hip. Overpronation is often linked to the flattening of the arch and can place undue stress on the inner structures of the foot and leg. The distinction between the two lies in the magnitude and timing of the roll.