MECHANICS FOR EQUESTRIANS II: Fatigue Failure – 11/7/2016 Paige LaBella, BSME
I was inspired by presentations at the 2016 SOM Conference, which as usual, were very informative and engaging. During a presentation, a question was asked regarding the presenter’s decision to analyze compressive forces on the navicular bone under dorsiflexion. From a mechanical engineer’s perspective, it makes perfect sense in the realm of material failure analysis and so I am motivated to share some engineering knowledge.
To determine the various strength limits of engineering materials, a sample of the material is destroyed and the forces are recorded during the destruction. The maximum force applied just prior to failure (breaking) is, in general terms, the strength of the material. I get a stomach ache when I think about testing living bones the way we test engineering materials, so let’s use steel as an example.
If a particular grade of steel has a tensile strength (meaning strength in tension) of 30,000 psi, this means, simply stated, it takes 30,000 lbs to pull apart a sample with a one square inch cross section. (If interested, this is a great video that goes into much more detail https://www.youtube.com/watch?v=D8U4G5kcpcM)
There are other modes of failure: compression, shear, bending etc. and the same principles apply to determining the strength of the material in these failure modes. To select a grade of steel for the purpose of building a machine of some sort, the engineer will determine the forces that the structural components are expected to experience and select materials that can withstand these forces within the constraints of the design.
It would be nice if it was just that simple but there is more to consider and that is fatigue failure and it’s a big one. The strength described above is for one destructive event. Materials also
experience fatigue which occurs through application of repetitive forces at magnitudes much lower than the force needed to break them in one single event. Think of how you would break a wire if you didn’t have a cutter. Are you strong enough to pull it apart? Probably not. You might repeatedly bend it until it breaks. The force you apply to bending the wire is very small but with enough repetitions it breaks. This is a familiar application illustrating the fact that the fatigue strength is much lower than the tensile strength. Again, the same concept applies to materials under bending, compression, and shear forces. When testing a material for fatigue failure, a force of known magnitude is repetitively applied to a sample until it fails. Typically there is a threshold force at which the eventual failure occurs with many fewer repetitions. With this information, you design your gismo to withstand the number of repetitions that you would expect to see in the lifetime of the product at the magnitude of force that is typical for normal operation. (Again, if interested, this is a great video that goes into much more detail https://www.youtube.com/watch?v=LhUclxBUV_E)
How does this apply to horses, specifically navicular disease? During the 2016 SOM Conference, Dr. Uhl and Dr. Osborne presented force analysis illustrating the compressive loading on the navicular bone during dorsiflexion. (Here’s a great video showing this loading on a cross sectioned specimen: https://www.youtube.com/watch?v=T_3qUJdsVQs)
We know that navicular disease occurs and that it is common, so how do we go about solving the problem? Let me jump back to steel for a second. If you have a steel part that fails by breaking in half, you can analyze the failure and determine the loading that caused the failure. The solutions might include changing to a stronger material, revising the design, or reducing the loading. Back to horses: Since horses are not yet made of steel and they are already extremely well designed, we are not going to change the design or materials. We are obviously limited to reducing the loading. While at first it may seem very difficult or even impossible to reduce the loading on the navicular bone, when you understand the contributors to the loading, it becomes clear that it is possible. The forelimbs carry approximately 57% of the horse’s weight and it increases with motion. We are not going to change that. Jean Luc Cornille reminded us in his presentation that studies have proven that collected gaits do not put more weight on the hind limbs. A contributor to loading of the navicular bone, that can be controlled, is the force created by the deep digital flexor tendon. We can teach the horse to minimize magnitude of the compression force from the deep digital flexor tendon on the navicular bone and avoid training that increases the loading. Exercises using long and low neck posture increase weight on the forehand and place the stance too far back (among other things!) – double whammy overload. “Horizontalization of the head and neck results in forward displacement of the center of mass of the horse. The immediate consequence of this is increased loading of the forehand…” and “overloading of the forehand results in additional stress on the joints and tendinous structures of the forelimbs.” (Jean-Marie Denoix, Biomechanics and Physical Training of The Horse, 2014, p.52).
A good start to solving this problem, or potential problem, is to refrain from working in low neck postures, but there is a lot more to unloading the forehand than just that. The benefits trainers perceive from working in low neck postures can be achieved without compromising soundness: by guiding the horse to achieve more precise control of balance through learning sophisticated control of their thoracolumnbar column. Not to throw a wrench into the works, but if you sighed in relief because you don’t work your horse long and low, hang on and read “Long and the law of physics” by Jean Luc Cornille first: http://scienceofmotion.com/documents/mechanoresponsiveness_20.html. It is possible to be riding with the neck too high or too short or both. The quick answer to the obvious question, how long and how high? is that with optimum coordination of the thoracolumnbar column, the resultant neck posture is long but not low. It’s right when the back is right…… most of the time. Wishy washy, I know, but they’re horses and we’re humans. Enough said.
Remember this fact: When talking about fatigue failure of a mechanical part, there is a threshold in force magnitude. Just a small increase in the magnitude above this threshold dramatically reduces the life of the part. The key is to reduce the loading (force magnitude) below the threshold. Horses don’t come with owner’s manuals specifying load limits like our pickup trucks do, so what we can do as riders is to learn how horses function and apply training that encourages soundness rather than overloading, specifically, reduce the magnitude of the forces that we can influence. Some pleasant results of unloading the forehand are increased suspension, a horse whose gaits are easier to sit, and a strong possibility of extending the life of the horse’s biomechanical parts. Wait. What? Easier to sit? Next topic: Elastic Strain Energy.