The knee connects the femur (thigh bone) to the tibia (shin bone), providing stability for weight-bearing and mobility for movement. This mobility is quantified by the joint’s range of motion (ROM), which describes the extent of movement possible before limitation by soft tissue, bony structures, or pain. A healthy ROM is fundamental to performing nearly all daily physical activities and is a primary indicator of joint function.
Understanding the Movement: Flexion vs. Extension
Knee movement occurs primarily in two directions: flexion and extension. Flexion describes bending the knee, which decreases the angle between the femur and the tibia, and is powered by the hamstring muscles.
Conversely, extension is the act of straightening the leg, which increases the angle between the two bones until the leg is fully straight. Extension is driven primarily by the quadriceps muscles. These movements are coordinated for fundamental activities like walking, sitting, running, and climbing stairs.
The Standard Range of Motion in Degrees
For a healthy adult, the standard range of motion for knee flexion extends from 0 degrees (full extension) to a maximum of 135 to 140 degrees of flexion. This range represents the joint’s potential, though highly flexible individuals may reach up to 150 degrees. This maximum measurement is usually passive ROM, meaning the movement is achieved with the help of an external force or by the clinician, rather than the patient’s own muscle power.
The movement an individual can generate independently is active ROM, which is slightly less than the passive measurement. While achieving full ROM is ideal, functional ROM is the range needed for common tasks. Walking requires less than 90 degrees of flexion, while sitting or climbing stairs requires between 90 and 120 degrees. Because demanding tasks, such as getting into a bathtub, may require closer to 135 degrees, a minimum goal of 110 degrees is often targeted in rehabilitation settings.
How Clinicians Measure Knee Flexion
In a clinical setting, knee flexion is measured using a goniometer, a tool that functions much like a protractor. The goniometer consists of a stationary arm, a moving arm, and a central axis point, or fulcrum, marked with degree measurements. The patient is positioned lying supine (on their back) or prone (on their stomach) to stabilize the limb for an accurate reading.
To measure flexion, the clinician aligns the fulcrum over the lateral epicondyle (a bony prominence on the outside of the knee). The stationary arm aligns with the greater trochanter on the hip, and the moving arm aligns with the lateral malleolus (the bony protrusion on the outside of the ankle). The patient bends the knee as far as possible, and the angle is read from the scale, providing an objective measurement of mobility.
Causes of Variation in Normal Range
Numerous factors can cause an individual’s knee flexion to deviate from the standard 0 to 140-degree range. Age is a determinant, as joint flexibility often decreases over the lifespan. Body structure also plays a role; a larger thigh or calf muscle bulk can cause soft tissue compression, physically limiting the maximum degree of flexion.
Underlying medical conditions, such as osteoarthritis, restrict movement by causing joint stiffness, inflammation, and the development of bone spurs. Injury to internal structures, such as a torn meniscus or an Anterior Cruciate Ligament (ACL) reconstruction, can also reduce ROM. Following total knee replacement surgery, the goal is often to achieve 115 degrees or more, but scar tissue or inadequate rehabilitation can prevent the joint from reaching full flexibility.