The Active Knee Extension Test (AKET) is a common clinical assessment used by healthcare professionals to evaluate the flexibility of the hamstring muscles. The test measures the degree to which the knee can be straightened while the hip is held in a flexed position. The result is a numerical degree of movement that helps determine if muscle shortness is contributing to movement limitations or pain.
How the Active Knee Extension Test is Performed
The procedure requires precise positioning to ensure the measurement is accurate and standardized. The individual lies flat on their back on a firm examination table. The leg being tested is positioned with the hip bent to 90 degrees, so the thigh is perpendicular to the table.
To isolate the hamstring muscles and prevent compensatory movements, the pelvis and the thigh of the tested leg must be physically stabilized. The person is then instructed to actively straighten their knee as much as they can without allowing the bent thigh to move away from its starting position. The test is often stopped at the point the person feels a strong stretch sensation.
A goniometer is used to measure the angle of the knee joint at the maximum point of extension achieved. The goniometer’s axis is aligned with the lateral epicondyle of the femur, and the arms track the midline of the thigh and the lower leg. The measured result is the angle of remaining knee flexion, which represents how many degrees the leg is short of achieving a fully straight, or 0-degree, position.
Criteria for Normal Mobility
Normal mobility is defined by the degree of knee flexion that remains when the leg is fully extended. Normal hamstring extensibility is associated with the ability to actively straighten the knee to within a small range of 0 degrees of extension. Scientific studies have established normative values, often with differences between males and females.
For example, the average active knee extension angle has been reported to be approximately 17.8 degrees for males and 13.4 degrees for females. For a person to be classified as having hamstring tightness, the remaining knee flexion angle is higher than this average range. Cut-off values indicating hamstring shortness are sometimes defined as knee flexion greater than 33.0 degrees for males and 23.4 degrees for females.
A practical assessment is achieved by comparing the measured angle to the opposite leg. A difference of more than a few degrees between the two legs can signal an imbalance or issue, even if the absolute number falls within the general normal range. This bilateral comparison helps account for natural individual differences in anatomy and flexibility.
What Abnormal Results Indicate
An abnormal result, characterized by a significantly high angle of remaining knee flexion, primarily indicates a limitation in the extensibility of the hamstring muscle group. This muscle shortness can restrict daily activities like walking and running, and it is often associated with an increased risk of muscle strains.
In some instances, a limitation during the Active Knee Extension Test can also be related to increased neural tension, such as involvement of the sciatic nerve. Because the sciatic nerve runs closely to the hamstrings, the movement can place a stretch on the nerve, which may be misinterpreted as muscle tightness alone. Professionals may use other specialized tests alongside the AKET to differentiate between a purely muscular restriction and a nerve-related restriction.
Clinicians use the AKET measurements to track a person’s progress through rehabilitation programs. Following an injury, the goal is often to increase the range of active knee extension, and a decreasing number of degrees signifies improved flexibility.