Tendon sees

The next time you tip toe down the corridor so as not to disturb the sleeping children, consider that you are employing a sophisticated biomechanical strategy which is known to organic tissue enthusiasts as the stretch-shortening cycle.
Essentially your Achilles tendon, that thick cord travelling from your calf into your heel, is a spring that stores energy and allows you to run, jump and creep down a hallway, without the heel crashing heavily on to the floorboards thus awakening the sleeping cherubs.

A miraculous collection of cellular features are packed into this fleshly rope, but it is in fact no rope, because a rope would slacken. It's an elastic band, but at the same no elastic band, because it also becomes firm and stiff, quite unlike an elastic band.

Tendon cells make the collagen to do the amazing stuff that we marvel at, from a basketballer slam dunk, to Usain Bolt's blistering sprint. From BJ Watlings back foot hook shot to my own skilful jaywalk, the tendon enables so much.

The Achilles tendon is a biomechanical wonder. PICTURE / GETTY IMAGES

Collagen is the building block of our body tissues. It comes in several varieties, each one specific for a different purpose. Type 1 and 3 populate the tendon which are built out of molecules that come together in bundles called fibrils. Type 1 is also found in all the connective tissues, muscle, ligament and bone. Type 3 is associated with these. Nature has seen fit to order the laying down of these fibrils in such a way that they behave as spring and support, mover and absorber of shock. Each fibre comes in at around 500 micrometres long, which is about the space two dust mites take up queuing for tonight's dinner. Fibres stack together into fascicles and then a bunch of those make up the tendon that you see. They align in parallel patterns that deal best with the loads they encounter. This is laid down in the womb so it's not adaptation to life.

Not every tendon is the same however.

There are some parts of the body where tendons are created differently and thus operate in specific ways. Positional tendons transfer load and are generally stiffer. The tendon that pulls your foot up is one of these. The build at a cellular level is different to the elastic tendons because the positional tendons have watery substance between the fibres enabling sliding which absorbs shock, while the energy storage and release tendons like your Achilles have lubricating and elastic goop between their fibres. The goop is called SLRP which makes sense as you usually slurp goop, and it's the watery substance that makes up 80 per cent of the tendon's mass. SLRP is actually a type of GAG which in this article is true both ways — (small leucine rich proteoglycan and glycosaminoglycans).
Another amazing thing about the tendon is that it is not just screwed on to bone, a very different tissue, but it morphs into a totally different tissue that fuses to the bone in two different ways. It then places a jelly pad between itself and the bone, presumably to protect itself, which coincidentally is what the jelly bursa does, but it also creates a hinge or fulcrum so that as the tendon stretches and wraps around bony contours, it does not get damaged.

Tendons have just the right amount of nutrition from blood supply, and if you point your foot as in ballet pointe, and force it against the wall, you have just increased the blood supply by seven fold.

Enough technical data or I may have lost you to Gareth Carter's dissertation on tender biennials, instead of tendon biomechanics.
From day one of Anatomy class in 1991 to 28 years later, it is impossible to look on the tissues of the body and be blasé. It is awe inspiring to me that we have a symmetrically identical double set of these specialised tissues throughout our limbs. It is continually fascinating to encounter the damaged ones and figuring out how to restore normality. I encourage you to sit down and feel your own tendons. Sense their gliding and elasticity and get out there and give them a bounce or two.

Greg Bell is a physiotherapist practising at Bell Physiotherapy. www.bellphysio.co.nz