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Knockout: Humans Can Sure Take A Punch

The human body can take a remarkable amount of punishment, given bones made of one of the strongest materials found in nature. At the same time, even an unarmed person can inflict an astonishing amount of damage with the proper training.

So how much does it take to crack a bone? And how much mayhem can a person deal out? In an era when "extreme fighting" has become a popular phenomenon, scientists are testing the extremes that athletes at the peak of their game can reach in order to help the rest of us.

"Understanding brain injury mechanisms all the way down to the cellular level will ultimately help everyone, not just athletes," biomedical engineer Cindy Bir at Wayne State University in Detroit explained. "If someone has a brain injury in a fall or motor vehicle accident, what we learn from athletes can help as well."

Bone is extraordinarily strong — ounce for ounce, bone is stronger than steel, since a bar of steel of comparable size would weigh four or five times as much. A cubic inch of bone can in principle bear a load of 19,000 lbs. (8,626 kg) or more — roughly the weight of five standard pickup trucks — making it about four times as strong as concrete.

Still, whether or not bone actually withstands such loads depends heavily on how quickly force is delivered.

"When you perform CPR, you can give chest compressions and not break any ribs, but if you apply the same amount of force quickly instead of slowly, and you can end up having rib fractures," Bir explained.

Force unleashed

When it comes to unleashing force quickly, Bir and her colleagues investigated boxers and found they could generate up to 5,000 newtons of force with a punch, more than that exerted down by a half-ton on Earth's surface.

When it comes to kicks, "they can obviously generate more force, since there's more body mass behind it," Bir said. After looking at kicks from several different fighting styles, they found that experts could generate up to 9,000 newtons with them, equal to roughly a ton of force.

A quick, sharp blow that delivers some 3,300 newtons of force has a 25 percent chance of cracking an average person's rib, she said. It takes more force to fracture the femur, Bir noted — maybe some 4,000 newtons — since that long thighbone is meant to support the body.

"That doesn't means that below those values you won't have a fracture or above them you will," Bir said. The amount of damage a blow inflicts also varies due to factors such as the amount of muscle or fat covering a bone and the angle at which the blow lands, as well as the age and health of a person, which can affect bone strength.

Although it makes sense that a massive fighter can unleash more powerful blows than a lightweight, "it's also about how much of the mass of your body you can recruit," Bir said. "You see some little guys hit with a lot of force because they know how to recruit their mass."

Roll with the punch

When it comes to knocking someone out with a punch, "it's less about the force of the blow than it is getting the head to whip around, to move in a rotational kind of way," Bir said.

The shear forces from a strike that whips the head back stress out neurons, and the brain shuts down as a protective response. A blow that gives the head enough spin to go from 0 to 43,000 rpm in just one second has a 25 percent chance of knocking a person unconscious.

"That's why you see boxers build up neck muscles — the thinking is that you can prevent that kind of motion then," Bir explained. "It's also about anticipating the blow — the ones that catch you off guard can be more of an issue."

Knocking the wind out of someone is also less about force "than the impact occurring just right for it to happen," Bir said. When it happens, the air isn't literally squeezed from the lungs, but instead it is a matter of getting the diaphragm — the sheet of muscle under the lungs — to spasm.

"A blow can cause your diaphragm to temporarily lock up — it's kind of like a cramp, and so it's hard for you to take a breath," she explained.

The Biophysics of taking a punch

The sternocleidomastoids (SCM) -- one on each side of the neck -- are paired muscles, composed of the sternomastoid component that runs from the sternum to the mastoid process of the skull, immediately behind and below the ear, and the cleidomastoid muscle that runs from the clavicle to the mastoid. When flexed, the SCM rotates the head toward the opposing side. Flexing both SCMs in alternation shakes the head “no,” as one might if waving off an overly concerned ringside physician. Flexing them simultaneously flexes the neck forward and extends the head -- in the right circumstance resisting the force of a blow to the face. It’s why fighters often seem to be ducking into a punch.

Moreover, arrayed against them are the muscles used in throwing a punch: calves, gluts, lats, pecs, triceps, etc. These are some of the most powerful muscles in the body. It is not surprising then that we rarely see the thrower of a well-placed punch to the head grasping his hand in pain and stumbling back in amazement as his opponent casually flexes his SCMs and smiles; the muscular arithmetic is firmly in the thrower’s favor.

When a punch of sufficient force strikes the face, it accelerates the front of the cranium back into the frontal lobes of the brain. This is the irreducible sweet science of brain injury. A gentle blow to the frontal lobes causes various degrees of central nervous system sedation -- it stuns the brain -- and a blow of sufficient force simply shuts the brain off. Seizures are not uncommon.

When a blow to the head comes from an angle, as opposed to straight on, only one of the SCMs can resist the force: The resulting acceleration of the cranium and damage to the brain are thus much greater.

Worse still, when a fighter is struck on the chin, the mandible creates leverage that magnifies the force and damage. This is the phenomenon of a fighter being hit “on the button.” Incidentally, this is an argument why, all things being equal, fighters with large heads and Cro-Magnon-like chins are at a theoretical mechanical disadvantage in withstanding blows.

The anatomy of the brain makes blows to the back of the head particularly dangerous. The extensor muscles of the neck are far stronger than the SCMs, but the part of the brain under direct assault is more delicate. The frontal lobes injured in a frontal blow control speech, movement and thought -- all the neurologic skills we see depleted in old boxers. The back of the brain, the hindbrain or rhombencephalon, controls respiration, heart rate, swallowing, blood pressure. Fighters who sustain injuries there never grow to be old.

A study would have to evaluate several variables

Due to the nature of our anatomy along with the biophysics of how a person gets "knocked out", there would likely be too many variables to ever scientifically determine just one cause or "infallible protection" against a knockout.

Firstly, many knockouts are caused by an instant somatosensory loss via the mandibular nerve, so you would first have to objectively study how the a muscle absorbs impact while maintaining the stability of the sensory root of this nerve.

Secondly, if we take the SCM as an example, you would need to integrate and measure kinesthetics, isokinetics, and several dynamic functions of the muscle during head impact. Since most knockouts come from an angle (usually lateral), you would also have to establish how the SCM, a muscle whose primary function is flexion and rotation, would have the isometric or eccentric strength to help resist the lateral impact of a blow in comparison to other muscles which are stronger lateral resisters such as the upper fibers of the traps and other cervical musculature, especially considering that some cervical muscles exhibit very strong lateral flexion when contracted unilaterally.

Also, an impact that produces a knockout has very little to do with the strength or power of the blow, so the study would also have to incorporate the dynamics and biomechanics of how a fighter is hit and how and the impact precisely affects the anatomical structures independent of the degree of force.

Wading into a fight

It can be hard to study how much damage a person can really give or take.

"We try as best as we can to study athletes in their native environment, so to speak, so more time in the ring, or during bouts or fights the better — that's when they're really fighting to peak potential," Bir said. "It can be difficult integrating equipment into that environment to measure them, since you don't want to interfere with their normal functioning, such as sensors that might decrease the protective effect of their gloves. The nice thing is that technology is advancing and getting smaller and wireless, to not get in the way of what people are doing."

The data that Bir and her colleagues might glean could help save lives.

"We joke that if someone is willing to get hit in the head, we should be measuring it," she explained. "If we know what causes an injury, you can do simple things like develop better protective gear and design bike helmets to help, say, 7-year-olds."

February 16, 2010

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