Wear and Tear

This one has been creeping up on me a bit these last few months and has come to a head during the last week…

After a couple of heavy days climbing I have been finding myself really struggling with the tendons of both lower arms and my elbow joints. This is quite uncomfortable and in the evenings it can leave me feeling as though I have hands like a Lego man and can also result in a numb, tingling sensation in the hands.

I find stuff like this really worrying, when your body does not behave as it should, yet am confident it can be sorted out with care and attention. Part of it is climbing smart, not overdoing things and using ergonomic techniques and equipment – but also listening to signals that the body is giving you and giving it a physical break when necessary.

After twenty five years of intensive climbing all this does not come as a huge surprise, after all, wear and tear is to be expected in view of the type of work being performed. I am actually glad that it is not more than this, not to say that there is an inevitability to it, yet in my mind this does have to do with cyclic wear.

Cycles to Failure is an engineering term to evaluate the effects of material fatigue on the life span of a given material.

This is expressed in a S-N curve, Sª on the vertical axis representing the alternating stress amplitude, versus the horizontal axis which expresses the  number of cycles (Nf) to failure.

The higher the load, the lower the cycles to failure, inversely, the lower the load, the higher the number of cycles to eventual failure. This is complex stuff, not quite as easily summed up in rules of thumb as some would have you believe, yet the concept is simple enough: if you load a system close to failure, you can load it less times than if you always stay well below a critical loading point. However, even if the load is low, if the cycles are very high, in due course, this can also lead to failure, as has been the case with connectors in paragliding rigs for example.

One of the questions I have been asking people I meet who work with health and safety organisations and/ or have a medical background is whether the cycles to failure model is also applicable to the human body. Can the same concept of high loads = low cycles and low loads = high cycles be transferred to us?

Watching kids down in town doing Parkour, doing big jumps and other funky moves, I always wince a bit and wonder what sort of damage this is doing to their bodies and especially their joints long-term. Or have I got it all wrong, and actually it is all down to your genes and the way your body is built, some knees fail faster than others? I suspect it is a combination of both. But not just that… a key difference between a machine and a body is a body’s inherent resilience and an ability to repair itself.

re·sil·ience
rəˈzilyəns/
noun
1.
the ability of a substance or object to spring back into shape; elasticity.
“nylon is excellent in wearability and resilience”
2.
the capacity to recover quickly from difficulties; toughness.
“the often remarkable resilience of so many British institutions”

Obviously, a machine can be engineered to have a defined degree of resilience, but once a point of maximum tolerable loading is passed, will suffer irreversible damage and/ or deformation. Once this point of no return is passed, the damage is permanent. Animal cells however have an amazing ability to regenerate. Obviously the same holds true here, continuous, repetitive movements can result in repetitive strain injuries or musculoskeletal disorders, or worse, a single extreme incident can lead to failure, but within a range of tolerance, a body is able to repair damage – which is quite amazing, if you stop to think about it. Even more amazing is the fact how we take this resilience for granted.

So… the answer is, I am not sure whether the Cycles to Failure model – or a variation of it – is applicable both to machinery as well as the human body, but will keep on pestering people and will let you know should I find out more.

In the mean time I intend to go easy on my elbows, looking forwards to a bit of rest over Christmas and New Year.