So, let us talk for a spell about the breakable bits of the human body. Yes, I can hear you raising an objection already... "All the bits are breakable"... and I'll grant you that's true. But some of them are much more breakable than others; we're here to figure out why that's the case. So let's dig into the subject of breakability a little more and see what we can come up with.
Cortical Bone Structure and Fracture Mechanics
At the smallest scale cortical bone is composed of tiny (~15 μm long and 50-70 nm in diameter) collagen fibers which are bound together with apatite crystals to form larger lamellar (plate-like) structures oriented longitudinally (e.g. parallel to the long axis) within the bone. These fibers don't tear easily under load, resulting in a material which is relatively resistant to transverse (e.g. perpendicular to the long axis) fractures1. A picture is probably in order:
So why do we care? Barring a few notable exceptions, strikes will result in the point application of force in the transverse direction. Both ends of the bone are fixed, so the application of a transverse force generates equal and opposing torques which generates a strain on the far side of the bone at the point of impact. The collagen fibers provide a counterforce to this strain, thus reducing the likelyhood that the bone will fracture. But the composition of the bone is only one variable; bone geometry also plays a significant role.
Consider for a moment what its like to break a stick across your knee. A thin stick is easier to break than a thick stick; the same thing is true of cortical bone. The force required to fracture a bone is (with some minor caveats) proportional to the cross-sectional area at the point of impact where the force is being applied2. Why is that the case? The cross-sectional area is proportional to the number of fibers resisting the applied force; the greater the number of fibers the smaller the strain to which each fiber is subjected.
Continuing with the stick analogy: long bones, like long sticks, are easier to break than short ones; the amount of force required to break a bone is inversely proportional to its length3. This is due to basic mechanics; the application of force to a bone fixed at two ends exerts a torque at the point of impact. Torque, in turn, is proportional to the length of the lever-arm with which it is applied, in this case the distance between the point of impact and the end of the bone.
Target Selection
All of the above indicates that long, skinny bones make the best targets. A quick survey of the human skeleton provides us with a short list of candidates:
- clavicle
- radius
- ulna
- fibula
There are a few other long bones, the humerus, femur, and tibia, but I feel safe in excluding them right off the bat. They didn't show up in any of the target lists that I found, and my personal experience suggests to me that they're simply too thick to fracture in the context of standard, unarmed combat4. But of the four bones on the list above only one, the clavicle, shows up on target lists. What's going on with the other three that make them look like good candidates on the basis of bone geometry but not-so-hot in real life?
If you look at the analysis above there are a couple of subtle assumptions which should be called out:
- Both ends of the bone substantively oppose movement.
- The strike applies force at a discreet point.
These items explain why the radius, ulna, and fibula are relatively difficult to break. The radius and ulna, in particular, violate assumption 1; neither the wrist nor elbow naturally provide substantial resistance to movement. If you strike someone in the forearm the entire arm will generally recoil from the strike; the force of the strike is translated into rotational motion rather than causing strain at the point of impact. The ends of fibula, on the other hand, do offer a fair degree of resistance; the knee can only move so far without your opponent falling over (in which case you've won anyway) and a properly planted foot is damn near immobile. But the fibula violates assertion 2; it is encased by muscle,front and rear, which protects the bone. The muscle serves as a cushion; a strike to the tibia will be dissipated over a relatively wide area by virtue of the presence of this muscule. This is true, to a lesser degree, of the radius and ulna as well; the various flexors and the brachio-radialis do a decent job of covering both bones. In order to effectively break one of these bones you need to anchor them and/or strike with a tremendous amount of force. Various grappling techniques use the former approach, while the later can be achieved through the use of weapons.
Compare this with the clavicle:
The clavicle screams "Break me! Break me!". It has no protection to speak of; there's just a thin layer of skin between it and the outside world. Additionally, both ends of the bone are fairly immobile. A downward strike to the clavicle is (mostly) transmitted to the spinal column by way of the sternum, scapula, and ribs. The spine is specifically configured to oppose this type of compressive force and provides exactly the kind of resistance needed to make such a strike effective.
But Wait, There's More!
Funny enough, we've gone through all of this dicussion but only crossed one item off the list from my original post. What about all of the other targets that were listed? I've got plenty on those as well, there's just too much for one post. Stay tuned for Part II, same bat-time, same bat-channel.
