This will be thinking outloud, a random and not very scientific overview of damage. I went to get some words down to reread and research and clean up later.
There are two ways that heads get hurt by blunt trauma. Make that brains. The first is the contre-coup action. The head is moved very quickly and the brain slams against the inside of the skull. Imagine people without seatbelts in the back of a truck. The brakes slam on and they are still moving and are thrown against the front where they may hit hard enough to bounce back and strike the tailgate. Hit on both sides by the inside of their "protective shell" just like the same bruises that can appear on a brain after an impact.
In essence, this is a hard, fast push. Not a strike at all in the sense I normally use the word. This is the mechanism of injury that is forced by the use of boxing gloves and why the body mechanics of a strike in a gloved sport differ from those in an ungloved sport.
The second is by deforming the bone and literally pushing it into the brain case, eg a depressed skull fracture. That's hard as hell to do with a fist. The little bone/big bone rule usually prevents it (the hand is composed of small bones, the skull of fused big bones. In general, when the little bones hit the big bone, the little bones break with almost no damage to the big bone).
There are variations and refinements of these- strikes in certain areas increse the concussion and contra-coup effects by adding a circular or spiral vector to the motion of the skull or using the leverage to put shearing force on the top of the spine. Either bouncing or intrusion can tear the membrane around the brain or the blood vessels in the brain and cause further concussive symptoms or death.
So- first thing- there are systems of striking based on these two mechanisms of injury (MOI). Most grossly "time on target" or "follow-through" concepts apply to the contra-coup ideal, and "snap back on contact" apply to the intrusion system. (Research- heel up lends itself to bounce /ground contact to break?) The power generation system of one will not work for the damage system (or strategy) of the other.
The intrusion systems work far better for body shots, especially against bone and ribs, than the contra-coup MOI. There's nothing hard for the organs to bounce against.
Extremely vascular organs, such as the liver, can be seriously damaged by direct injury/intrusin. The surface of the organ essentially bruises and begins to leak. On the other hand, Contra-coup damage must literally tear the organ or the blood vessels free of their surrounding tissues, which is hard to do in a soft, massive and relatively liquid medium.
If the body has no where to "flow" with the strike, more damage is absorbed (that's a vague statement, try again). Though they are actually attached at many points, organs are suspended in a semi-liquid medium and are themselves extremely flexible. They are pushed aside by pressure. If space to be pushed aside is limited, the organ takes more of the damage because it can't get out of the way. Same if it gets hit from two directions. Also if it is hit too fast for it to flow. (Question- can a strike develop a wave action that does damage distinct from the physical impact? I have been told it can, but it may just be an attempt to explain contra-coup action. Hmmm....)
They can also be damaged by breaking the bones over the tissue and having the splinters of bone do what is so hard for blunt objects to do, directly tear the organs and blood vessels.
Joints can be torn by using the leverage inherent in the attached long bones to either force the bone out of the socket (ball-and-socket joints and some hinge joints) or to tear the ligaments holding the bones together (all three types of lockable joints).
The lungs can act as a big shock absorber for body blows, which is why striking at the bottom of the opponents respiratory cycle is so much more devastating. With training, the spine, knees and feet can also act as shock absorbers.
Knives are not the same as sticks and it is a fundamental mistake to train them the same way. Most telling is that knives damage soft tissue by severing it. Unless very heavy and used to shatter, they do very little damage against bone. Blunt object, on the other hand damage soft tissue by bruising (breaking tiny capillaries) unless they are penetrating to try to damage organs. Blunt objects do their greatest damage to bone. A knife to the meaty part of the forearm can sever tendons and arteries. A knife to the bony part causes surface bleeding. A stick to the meaty part cause bruising because the meat cushions the bone, but a good strike to the bony part can break the bone. With a knife you cut the inside of the elbow. With a stick you break the outside.
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