Question: how do tendons work?

Jenni Tilley answered on 11 Jun 2011:

Tendons attach our muscles to our bones. They must be strong to transmit the huge forces our muscle generate, but they must also prevent our those huge forces from damaging our bones. To do this, Nature uses some very clever engineering tools:

1) Tendon is a composite material (http://en.wikipedia.org/wiki/Composite_material). It’s made of a very strong material called collagen, and some energy-absorbing materials called proteoglycans. Its overall properties are a compromise of these two properties.

2) Proteoglycans are the body’s version of silly putty – stretch them slowly and they flow easily, but stretch them faster and they get stiffer. Eventually if you stretch them too quickly, they break. This is because proteoglycans are big chains of atoms – when you stetch the chains they re-arrange and flow past each other, absorbing energy through friction. Stretch them too quickly, there isn’t enough time for them to re-arrange so they can’t flow. You can try this at home by making silly putty (http://www.youtube.com/watch?v=J35rd3RC4vQ)

2) Collagen is very strong because it has a hierarchical structure, a bit like the Eiffel Tower: http://silver.neep.wisc.edu/~lakes/Hierarch.html. Collagen is made up of tiny fibres twisted up into bigger fibres, which are twisted up into even bigger fibres and so on, exactly like a rope. I spend my days imaging some of the smallest fibres (approx 100nm in diamter) – it turns out that the strength of tendon is related to the size of the fibres it’s made of. You can see a picture of the fibes I look at in my profile:/sportsj11-zone/profile

3) Tendon can adapt to it’s environment because it contains cells, nature’s main advantage! Cells can change the size of collagen’s fibres, and the amount of proteoglycans depending on whether we need our tendons to absorb more energy or be stronger. This means that the tendons we use more are much stronger than the ones we don’t use much – the tendons in a footballer’s foot would be much stronger than the tendons in his hands because he uses his feet more. This adaptability means that tendons can also fix themselves when we damage them, albeit only very, very slowly, and not as well as skin and bone fix themselves.