Joint Alignment and Rotations of the Knee

By Mary Ann Foster
[The Science of Movement]

The knee is the most frequently injured joint in the body for three reasons: it is the largest, most complex, and most unstable joint. The knee serves as a weight-bearing fulcrum and primary shock absorber between the two longest lever arms in the body—the femur and the tibia. It is made more stable by two large menisci, a substantial joint capsule and network of external strapping and internal cruciate ligaments, and balanced muscular pulls around it. When optimally aligned, these structures work efficiently to absorb shock and minimize wear and tear on cartilage.
During movement, multiple compressive and rotational forces exert crushing, grinding, and twisting loads on the knees, particularly on the meniscal disks. To minimize these mechanical stresses, the intricate calibration of each knee requires optimal alignment and precise tracking throughout range of motion. The femoral condyles need to move on the tibial plateau with the precision and balance of a stilt walker. Any slight variance from the optimal pathway of motion can cause repetitive strain and lead to tissue damage.

The knees are frequently injured during sports activities, usually from torque or direct impact in contact sports like football. Torque injuries tend to occur during quick turns, such as on a racquetball court, when the foot is fixed and the femur twists on the tibia beyond a normal range, tearing soft tissue. People with a knee injury often develop a vaulting gait, rising up on the toes of the unaffected leg to minimize weight on the affected one.

Functional Alignment of the Knee
We can measure the functional alignment of the knee by looking at the mechanical axis of the lower limb. In an ideal alignment, the centers of the hip, knee, ankle, and second metatarsophalangeal joint line up along a vertical axis. This alignment should be maintained during motion because mechanical stresses translate from one joint to another.
Another measure of knee alignment is the Q angle, which falls along the line of pull of the quadriceps on the knee (Image 2). An average Q angle is 15 degrees, although it can range from 13 to 18 degrees. The Q angle tends to be greater in women because the female pelvis is wider, which increases the oblique angle of the femur.
The oblique angle of the shaft of the femur causes a medial displacement of the knee called genu valgum. Five to 10 degrees of genu valgum is normal; a greater degree of displacement results in a knock-kneed alignment. Lateral displacement of the knee results in genu varum, a bowlegged alignment. Excessive valgus or varus will change the Q angle, placing undue stress on joint structures. As little as 5 degrees of genu varum can increase compressive forces on the medial meniscus by 50 percent.1

Rotations in the Knee
Two types of tibiofemoral rotations occur in the knee: axial and terminal.
Only possible when the knee is in a flexed position, axial rotation is important when quick direction changes are needed; for example, in sports such as tennis or basketball. When the knee is flexed at a 90-degree angle, the tibia can axially rotate an average of 30 degrees medially and 40 degrees laterally (Images 3A and 3B). If the foot is free, the tibia rotates under the femur; if the foot is fixed, the femur rotates over the tibia.
During the last 30 degrees of knee extension, the tibia and femur counter-rotate into the close-packed position called terminal rotation. In this position, the femoral condyles screw into a locked position, medially rotating against the lateral rotation of the tibia. The screw-home mechanism stabilizes the knee in the extended position, twisting and locking the joint structures into a tight fit.
When working with knee problems, you can access restrictions or hypermobilities by asking your clients to do these simple movements during a seated intake. Then assess their Q-angles in standing and/or supine position. To ensure safety, pay attention to rotations in this vulnerable joint during passive and resisted movement techniques.

Exploring Technique
Terminal Rotation of the Knee
1. Place one hand on your tibial tuberosity and the other hand on your quadriceps tendon. Notice how your hands line up vertically when your knee is bent.
2. Slowly straighten the knee and notice how the joint twists, causing the tibial tuberosity to shift toward the lateral side.
3. Slowly bend and straighten your knee several times to observe changes in alignment and feel the screw-home mechanism.
4. Stand facing a long mirror and assess
your Q-angles. Then, watch your knees while doing a set of small knee bends to assess joint rotations.

1. D. Neumann, Kinesiology of the Musculoskeletal System: Foundation for Physical Rehabilitation (St. Louis, MO: Mosby, 2002).

Mary Ann Foster is the author of Therapeutic Kinesiology: Musculoskeletal Systems, Palpation, and Body Mechanics (Pearson Publishing, 2013). She can be contacted at

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