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The principle of complementarity of structure and function states that function is dependent on structure.

So form dictates function, in other words – how something is built will determine how it works.

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This is quite a fundamental principle we not only find throughout the body but at everything we look at.Square Peg in a Round Hole

You know that a cube does not roll very well, especially not when compared to a ball.

And you also know that it is difficult to get a round peg through a square hole (unless the hole is disproportionally bigger of course).

So always look out for how things are shaped, that will tell us how we can expect them to function.

If there is some structural change let’s say due to postural imbalances, the effects are often dramatic.


Think of a joint that is out of alignment, or in an extreme case even has severe damage. Part of the meniscus in the knee has come off and is now floating inside the knee joint. That’s like skel02_ 214having a cube in a ball bearing, the proverbial spanner in the works. It will be difficult for the knee joint to function according to plan if its structure is damaged.


Or someone has a bad distortion of the spine, let’s say due to (or causative of) being hunched over all the time.

Not only might their musculoskeletal system be affected, but in extreme cases other organs and structures as well.

Extreme kyphosis might restrict the lungs from fully expanding and so jeopardize the blood flow to all tissues and organs.

Extreme rotation and/or lateral flexion of the spine might have the rib cage exert pressure on vital organs and affect let’s say the liver.


Next we will look at how you can classify joints according to structure and function.