Question: why do some tendons damage more then others?

Jenni Tilley answered on 11 Jun 2011:

It’s all to do with what you’re asking your tendons (and ligaments) to do, how well adapted they are to do it and how much you’re using them.

For example, Knees and ankles are common problems for lots of athletes because they put them through extreme forces when they’re playing their sport. Tendons and ligaments are designed to be ‘pulled’ (tensile forces) not ‘pushed’ (compressive forces) or ‘twisted’ (torsion). Lots of tendons are damaged when they experience compressive forces or torsion, so when a footballer turns too quickly and twists his knee by mistake, the ligaments in his knee experience torsion forces they’re not adapted for and get damaged.

The other problem is when we ask them to behave much quicker than their used to. Tendons are made of a very strong material called collagen, and some energy-absorbing materials called proteoglycans. 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). This could explain why lots of tendon injuries seem to happen when athletes fall or change direction very suddenly – they are stretching the proteoglycans so quickly that there isn’t enough time for them to re-arrange and they ‘snap’ instead of flow.