Weight versus Stress
When we discuss the intensity of hanging, we usually talk about the weight involved. Weight, after all, is directly related to discomfort and risk.
However, as far as the target tissues are concerned, the real issue is not weight, but rather stress. Stress is defined as force (such as weight) per square inch (Thanks, Mbuc!). A simple example of how this idea can be put to use is where the target tissue has a small cross-sectional area. This might be the case for a particular ligament bundle. To stretch this tissue, less weight will be required than it would take to stretch a thicker tissue. The point is, tissues deform as a function of the stress they experience, not the weight, per se.
This leads to a question: How can one increase the stress on a tissue while keeping the weight reasonably low?
Another principle of stress on materials is that stress increases dramatically around corners. This is why molded corners in most manufactured products are “radiused.” Indeed, for a material having a bend radius of zero, stress becomes infinite.
Can we use this principle in PE?
Well, fulcrum stretching may be based on this exact principle. Bend the shaft around a corner to increase the stress. If this is true, it’s reasonable to think that the smaller the radius of the fulcrum, the greater the stress concentration and the more effective the exercise. Very small radii will multiply stress many times over.
I have to run, so I’ll cut to the chase. I’m thinking that an upward stretch against a moveable rod, oriented perpendicularly to the shaft, will apply a greatly magnified force to the tunica, specifically in the area where it is the most thick—on the dorsal side.
I think it’s important to pull up against the fulcrum, rather than down, as it is usually done, because pulling down will reduce the effective radius of the rod due to the thickness of the shaft. I also think it’s important that one be able to move (preferably roll) the rod up and down the length of the shaft with the force applied, so as to cyclically provide increased stress all along the length of the shaft.
The promise of this idea is that great stress can be achieved with relatively little weight (force). Ample stress may be attainable manually, or with a simple hanger, formerly thought useful only as an ADS.
I have two concerns about this approach. First is safety. I haven’t tried the exercise yet, so I can’t vouch for its safety. One must be careful not to apply too much force, especially to the sensitive dorsal surface. Perhaps safety can be enhanced with a rod that has some relieved central portion though which the dorsal vein and nerve can pass. It’s really too early to think about this.
Another concern is that the structure of the tunica may resist the stress concentration. The tunica’s outer layer consists of longitudinally oriented collagen fibers. I don’t know whether the rules of stress concentration for solids apply to fibers. Do cables experience higher stress around pulleys? Maybe not. Even if the individual fibers resist the stress concentration, however, the tissue as a whole probably won’t. What I mean is that stress will still be concentrated between fibers in the area of the fulcrum. This will tend to break cross-linkages, which will effectively weaken the tissue.
Gotta run, but feel free to comment if you’d like and I’ll catch up later.
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