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Knee Joint Stability

Controlling Knee Bend and Shin Rotation
(Relative to the Thigh)
Published: 2012 08 03
Updated: 2020 07 06

We tend to think of the knee as a simple hinge joint. As a result it's easy to suggest that all the knee does is bend and straighten. But when the knee is bent it also allows the shin to rotate relative to the thigh.
And so when stabilizing the knee it helps to think of stability both in terms of stabilizing or resisting changes in knee bend as well as changes in shin rotation.

Knee Joint Hinging

Knee Straightening Muscles

The main muscles that act to straighten the knee are the vastus muscles. These are:

These three vastus muscles make up most of the bulk at the front of the thigh. Normally these are grouped together along with the rectus femoris as the Quadriceps! However, the rectus femoris, unlike the vastus muscles, is also a hip flexor.

In addition to straightening the knee these muscles also resist the knee being bent.

Front view, the vastus muscles, vastus lateralis, vastus intermedius, vastus lateralis.  Neil Keleher. Sensational Yoga Poses.

Because these muscles are all relatively massive, they tend to press out against overlying muscles when active.

And so as well as acting to straighten the knee, these muscles may also help to remove slack from overlying muscles like the IT Band muscles (the tensor fascia latae and gluteus maximus) as well as the sartorius and gracilis both of which attach to tibia as part of the pes anserinus.

This assumes that muscles have an optimal length (or optimal length "range"). So for example, if, while standing, with the knees bent the IT Band isn't at optimal length (perhaps having too much slack), activation of the vastus lateralis helps to remove the slack so that the tensor fascia latae can activate.

Front view of thigh including vastus muscles and overlying sartorius, rectus femoris, it band. Neil Keleher. Sensational Yoga Poses.

Knee Bending Muscles

Muscles that can be used to bend the knee or resist is being straightened are the hamstring muscles. This includes the biceps femoris (both long head and short head) which attaches to the fibula, and the semimembranosus and semitendinosus, both of which attach to the inside of the tibia.

Rear view, Hamstring muscles: biceps femoris, semitendinosus, semimembranosus. Neil Keleher. Sensational Yoga Poses.

The semitendinosus is the third muscle that forms the pes anserinus.

Another muscle that can help bend the knee or resist it being straightened is the gastrocnemius, the larger of the two calf muscles. The two upper tendons of this muscle pass between the tendons of the inner and outer hamstrings before attaching the the bottom of the femur.

Stabilizing the Knee Against Hinging

To stabilize the knee against hinging the vastus muscles can be used against the calf muscles, particularly while standing. With weight pressing through the feet, the calf muscles are anchored at the heel. The quadriceps can then work against the gastrocnemius to stabilize the knee joint.

If the pelvis is stable then the hamstrings have a stable foundation and it may be possible to activate the hamstrings against the quadriceps to stabilize the knee joint against bending or straightening.

Either of these muscle pairs (vastus against gastrocnemius or vastus against hamstrings) could be used with the knee straight, or bent in various positions.

Shin Rotation at the Knee Joint

Outer Thigh Shin Rotators (IT Band)

In terms of shin rotation, the IT band passes over the vastus lateralis to attach to the top of the tibia in front of the fibula.

Side view, IT band, gluteus maximus, tensor fascia latae. Neil Keleher. Sensational Yoga Poses.

Inner Thigh Shin Rotators (Pes Anserinus)

Inside view, Pes anserinus muscles: sartorius, gracilis, semitendinosus. Neil Keleher. Sensational Yoga Poses.

Along the inside of the thigh the sartorius can act to rotate the shin outwards relative to the thigh when the knee is straight.

If the knee is bent then the sartorius can act, along with the gracilis and semitendinosus to rotate the shin inwards relative to the thigh.

Stabilizing the Shin Against Rotation at the Knee Joint

If the knee is bent, the pes anserinus muscles can work against the IT band muscles to stabilize the shin against rotation.

If the knee is straight then the tensor fascia latae can work against the gluteus maximus to prevent rotation.

Another possibility with the knee straight is that the sartorius could work against either the gracilis or the semitendinosus to prevent shin rotation.

Controlling the Knee

Part of controlling the knee can involve stabilizing either the femur or the lower leg bones.

Front view adductor magnus, including long head. Neil Keleher. Sensational Yoga Poses.
side view glutes minimus. Neil Keleher. Sensational Yoga Poses.

To stabilize the femur against rotation two important muscles are the adductor magnus long head and gluteus minimus. Both can cause internal rotation or at the very least resist external rotation of the thigh relative to the pelvis.

So that these muscles have an opposing force to work against, an external rotator like the deep fibers of the gluteus maximus are also important.

If the femur is stabilized against rotation at the hip joint, using the gluteus maximus (and other external rotators) against the adductor magnus long head and gluteus minimus, the hip bone can then act as an anchor for the shin rotation muscles to effectively control the lower leg bones.

By the same token, if the shin is stabilized against rotation via muscles of the foot and ankle, then the shin rotation muscles have an anchor from which to work on the hip bone. Muscles that attach between the lower leg and femur, for example, the short head of the biceps femoris, also have an anchor from which to act on the femur.

Read more about the biceps femoris short head in anchoring biceps femoris short head.

This is classified under the section for The foot but it's still relevant to knee stability.

Alternative Strategy for Knee Control

Since some of the shin rotator muscles attach to the hip bone, another way to make the knee joints more controllable (and/or more stable) is to anchor the hip bones via the abs. For example, creating an upward pull on the ASICs via the external obliques can give sartorius, rectus femoris and tensor fascia latae an anchor point from which to effectively control the lower leg bones.

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