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Blue Sky Science: Why don’t joints bend both ways?

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Q: Why don’t joints bend both ways?

— Holden Taggart, 6, Merrimac Community Charter School

A: Joints don’t bend both ways because there are physical components within their design that prevent or resist motion. This could be bone, such as the elbow joint, or it could be ligamentous restraint, such as in the knee.

All joints in the body are a compromise between mobility and stability. Joints with greater mobility, such as the glenohumeral joint of the shoulder, have reduced stability. Joints with greater stability, such as the ankle joint, have reduced mobility.

If we have a joint that moves both ways or is hypermobile, that permits a greater range of motion at the cost of a substantial reduction in stability.

One of the most mobile joints of the body is the glenohumeral joint of the shoulder, which allows us to do things like throw a baseball or scratch a spot in the middle of our back. However, it’s also one of the most common joints in the body for injury, aches and pains due to instability or too much movement between the bones of the joint.

Joints like that of the shoulder, which don’t have as much bony or ligamentous stability, require greater muscle contraction of the muscles surrounding the joint to promote stability. Muscle contraction requires energy, so if each joint of the body required significant muscle contraction to provide stability, that would be a very inefficient strategy.

The hip joint, for example, is mechanically similar to the structure of the shoulder joint, but with bones that fit together more completely and big, thick ligaments that help to resist motion. This allows us to stand and bear weight through the hip joint without constantly having to provide a muscle contraction to prevent the joint from moving or collapsing one way or another.

Physical therapists frequently see patients who either have too much mobility or too little mobility.

For example, an athlete who tears an ACL has reduced stability of the knee as a result. An individual may have elbow pain from trying to throw a baseball with too little shoulder mobility. Treatments help to improve joint mobility or increase joint stability depending on what patients need for their desired activities.

Dan Cobian is a research scientist with Badger Athletic Performance and a faculty member in the physical therapy program at the University of Wisconsin-Madison.


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Blue Sky Science is a collaboration of the Wisconsin State Journal and the Morgridge Institute for Research. The questions are posed by visito…

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