The Knee Adduction Moment (KAM) is a measure used in biomechanics to quantify the load passing through the knee joint during dynamic activities like walking. It represents the rotational force that acts on the knee, effectively pushing the lower leg inward and stressing the joint’s inner compartment. This biomechanical variable is widely recognized as a surrogate indicator for the compressive forces experienced by the medial side of the knee. Understanding the magnitude of the KAM offers insight into the mechanical environment of the joint. Research indicates that the level of this mechanical load is closely linked to the development and progression of joint degeneration.
Defining the Biomechanical Concept
The term “knee adduction moment” breaks down into two core concepts: adduction and moment. Adduction describes a movement that brings a limb segment closer to the midline of the body, and in the context of the knee, it refers to the tendency for the lower leg to be pushed toward the body’s center. This force creates a varus thrust, which increases the pressure on the knee’s inner (medial) side.
A “moment” in physics is a measure of torque or rotational force, calculated as the product of a force and the perpendicular distance from the point of rotation. The KAM is an external moment, meaning it originates from outside the body, specifically from the ground reaction force (GRF) experienced during the stance phase of walking. The GRF acts on the foot, and its line of action passes a certain distance away from the center of the knee joint, creating a lever arm that applies a rotational force.
The magnitude of the KAM is determined by the size of the ground reaction force and the length of this perpendicular lever arm in the frontal plane. A longer lever arm or a greater force will result in a higher moment, indicating a greater mechanical stress on the joint. KAM is typically measured in a gait lab using motion capture systems to track joint movement and force plates embedded in the floor to measure the ground reaction forces. This analysis provides a dynamic evaluation of knee joint loading throughout the walking cycle.
The Critical Link to Knee Health
A consistently high Knee Adduction Moment is strongly associated with the initiation and progression of medial compartment knee osteoarthritis (OA). The medial compartment, or the inner side of the knee, is the most common location for OA, and the KAM is considered a primary biomechanical risk factor for this condition. The external adduction moment acts to open the joint space on the outer side and compress the joint on the inner side. This sustained, increased compressive load leads to the gradual wear and tear of the articular cartilage and the underlying bone structure.
Studies have found that a higher KAM correlates with increased medial meniscus extrusion, which is the displacement of the meniscus from its normal position, further destabilizing the joint. The KAM is used by clinicians as a predictive biomarker for OA severity because it reflects the dynamic loading environment that drives the disease process. The moment generally presents as a double-bump pattern during the stance phase of gait, with a first peak occurring early in stance and a second peak later on.
The largest peak, often the first or second, is highly correlated with the peak medial contact force, which is the actual compressive force experienced by the joint surfaces. Therefore, reducing the KAM is a primary target in non-surgical strategies aimed at slowing the rate of joint degeneration.
Factors That Influence KAM
The magnitude of the Knee Adduction Moment during walking is influenced by a combination of physical and dynamic factors. Walking speed has a significant influence on the KAM magnitude; generally, increasing the speed increases the moment. The overall alignment of the lower limb, often referred to as the mechanical axis, is also a major determinant, with a varus (bow-legged) alignment increasing the moment arm and thus the KAM. Body mass is a fundamental factor since the ground reaction force is proportional to body weight, and KAM measurements are typically normalized to body mass and height to account for this.
The foot progression angle, which is the degree to which the foot is pointed inward (toe-in) or outward (toe-out), also alters the KAM. The choice of footwear can have a noticeable impact, with some types of shoes or walking barefoot resulting in different adduction moments compared to standard or specialized footwear.
Strategies for Reduction and Management
A primary goal in the management of medial knee OA is to intentionally lower the Knee Adduction Moment to reduce compressive loads and mitigate disease progression. Therapeutic approaches focus on either modifying the patient’s gait pattern or using external devices to shift the load. Specific gait modifications can be learned through specialized training, sometimes involving real-time biofeedback.
One common learned modification is increasing the lateral trunk lean, which shifts the body’s center of mass closer to the knee joint, thereby shortening the moment arm and reducing the KAM. Altering the foot progression angle, where walking with a slightly toe-out or toe-in pattern, can help decrease the KAM peaks. Increasing the walking base, or stance width, can also reduce the moment.
Assisted modifications include the use of specialized orthotic devices and bracing. Lateral wedge insoles, which are placed inside the shoe, are designed to shift the center of pressure underneath the foot laterally, reducing the moment arm of the ground reaction force. Valgus knee braces use a three-point pressure system to apply an external force that slightly pushes the knee into an abduction (valgus) position, directly counteracting the adduction moment and decreasing the medial compartment load. These interventions provide non-invasive options for achieving a sustained reduction in the dynamic forces that act on the compromised joint.