by E. Paul Zehr
Concussion—and postconcussion syndrome, which we will discuss later—is considered a “diffuse” injury. This means that there isn’t any localized specific injury site but rather a very large and global injury. Imagine what happens when you are playing a DVD or a CD and then the player is jostled. Typically, there is a “skip” followed by a delay, then the movie or song resumes where it left off (or close to there). Concussion is a bit like this in that the jolt to the player can be considered to be the blow to the body (or head) that leads to the acceleration that causes the brief change in neural function. Most symptoms of minor concussion resolve after two to three days, but effects can linger longer.
A better analogy than DVDs or CDs is actually an old-style record player. When the record player gets bumped, it skips just like the CD player. However, it doesn’t resume exactly where it left off (kind of like the amnesia effect) and, in contrast to the digital CD with its laser that doesn’t really harm the CD itself, bumps to the record player that cause the needle to skip and jump actually do scratch the record. If you substitute “brain cells” for “record” you will begin to appreciate why concussion might be a concern after all and why repeated concussion could be a real problem.
This brings up the question of what exactly is happening to the neurons and Batman’s brain when he is hit. What is concussion at the cellular level? We can think of a concussion as inducing a harsh chemical imbalance in the neurons of the brain after impact. This is really a challenge to homeostasis (yes, it is back again!) in the metabolism of the neurons in the brain. Rapid accelerations induce these changes. Although the neurons in your brain aren’t meant to respond to impact, they can be affected by physical trauma.
Think of your eye. You have photoreceptors that respond to the photons of light hitting your retina. However, if you have ever been hit in the eye, even lightly, you probably saw a flash of light or sparks. You activated your photoreceptors mechanically in that case. Kind of like the example above of the skipping CD. The CD player wasn’t designed to respond to mechanical input; however, it does produce a response of sorts by resetting itself. Well, neurons are after all just cells, and all cells are squishy and have numerous pores in them. Through the pores (called channels) ions move in and out of the cell to maintain function. Physical trauma can therefore make cells do things they don’t normally do by changing the way the ions move.
An important thing about all cells is that the watery material on the inside is maintained with a different concentration than the watery material in which the cells live. A membrane—really like the skin of the cell—keeps a concentration of some ions on the inside of the cell different from those on the outside of the cell. Some key ions are potassium (which you find in bananas and some other foods) and sodium (which we find in too many foods!). Well, when the head is subjected to concussive forces, the main outcome is that potassium ions are ejected from neurons. This disrupts the function of the neurons and affects communication between neurons in the brain. Sodium ions are also affected, as are neurotransmitters that are used to excite neurons.
This leads to a synchronous activation in the brain. While your cortical neurons are always active, they are not usually active all at the same time. However, blunt mechanical trauma causes many neurons to synchronize and discharge all at once, kind of like a seizure. As such the neurons are “stunned” and slowly wake up again over time (and some don’t wake up at all). The normal concentrations of sodium and potassium ions are restored in several hours but other ions like calcium may take several days to return to a normal level.
Another negative consequence of the neurons’ discharging all at once is that they need a lot of extra energy in the form of glucose and oxygen to keep on doing their work. The problem is that the energy supply isn’t there because the demand was artificial. So, the normal “warm up” of blood flow and oxygen delivery was absent. Also, the presence of all that calcium actually reduces and interferes with the ability to provide blood and oxygen to the neurons that demand it.
This cascade leads to an energy crisis—kind of malevolent neuronal oxygen debt—that causes the neurons to fail (shown in Figure 13.1). The increase in energy demand coupled with reduced blood flow and reduced metabolism is the energy crisis. Some neurons may die but many will take several days to recover from this massive shift of ions. The memory problems and mental confusion seen during this time after concussion occur because of what has happened at the cellular level. Returning to normal concentrations of neurotransmitters can sometimes take up to two weeks. Note that the main areas affected by repetitive mild head trauma are the hippocampus as well as the frontal lobes. As we learned earlier in Chapter 7 on motor learning, these areas are very important for memory formation and storage.
With repeated concussion we enter the territory of secondary impact syndrome and postconcussion syndrome. The best-case scenario for Batman is that the blow he suffers doesn’t injure him unduly (remember what we learned in Chapter 11 about the cushioning provided by the batcowl) and that he has time to recover after he is concussed before suffering another one. It is quite dangerous when someone hasn’t fully recovered from the initial concussion and then suffers another blow to the head. In this scenario, even a very minor impact to the head can trigger a change in the regulation of blood supply to the brain. This is called “secondary impact syndrome” and can lead to swelling and blood pooling and is very often fatal.
Figure 13.1. Effects of concussion on energy demand, metabolism, and blood flow over time.
A less severe outcome of multiple concussions occurs when there has been good recovery after each concussion but impacts are repeated. This is clinically defined as “postconcussion syndrome,” but anecdotally it is called “punch drunk” syndrome. It is called this—not surprisingly since scientists and physicians are often very literal—because this syndrome is often present in boxers (who do absorb many blows to the head). The issue here is that, while the person—including Batman—does recover and may seem “symptom-free” between concussions, the nervous system doesn’t really truly return to the preconcussion state. Damage has occurred but is kind of “covered up” so that when more injuries occur, there is less and less ability to make repairs.
It is kind of like patching the knees on your jeans when you were a kid. The patch covered the hole but if you kept getting more and more holes and applying more and more patches, eventually there was nothing left to attach the patches to. You had no choice at that point but to go and get some new jeans. This is a pretty easy fix. Getting a new brain is much more problematic.
Even though most people don’t routinely ram their heads into something hard and suffer no ill effects, some other animals do just that. The woodpecker is a good example. So are goats and rams. They all routinely smash their heads together or against another hard object. Yet we don’t see mounds of mountain goats lying concussed everywhere when we go to the zoo. Nor do we find our path through the forest blocked by red-crested woodpeckers strewn about. These animals can tolerate acceleration forces that can be up to a hundred times greater than what you or I could tolerate.
Probably the most dominant feature of the woodpecker is the straight beak that it uses to pound away forcefully on tree trunks. It has been calculated that velocity of the beak at impact may be as high as 7 meters (23 feet) per second. The negative acceleration that occurs can be as large as 10,000 m/s2, or a thousand times the force of gravity! As a frame of reference, to experience an equivalent negative acceleration, Batman would have to be able to run into a brick wall at about 315 mph without suffering a concussion. Of course, his grievous musculoskeletal injuries would be a bit of a problem.
The question, though, is why is that so? The short answer is that the long answer is not really clear. However, it seems likely to have a lot to do with the nature of the acceleration experienced. It turns out that the woodpecker mostly experiences linear (straight line) acceleration and very little angular acceleration. Minimizing the angular
acceleration of the head is a major job of helmets worn in sports like football and is incorporated into the cowl of the batsuit.
Protecting the Batbrain
Clearly, Batman’s line of work means he is likely at risk for not just single concussive events separated by months or years, but more often he will have multiple repeated “exposures” to concussion. So, what can be done to moderate the effects of being hit in the head? Well, wearing a helmet would be a great idea since it would help reduce the accelerations that Batman’s head experiences.
There have been many attempts to cushion blows to the head in sports like football and boxing by using padding in helmets and gloves. The batsuit includes a padded and protective helmet as part of the cowl. In combat sports that use heavy contact there is a real need to protect the head and prevent broken noses and other injuries.
In my own karate training we used to practice a full-contact form of fighting that made use of robust sparring gear called bogu. This included headgear that was similar to a modified kendo helmet, which was easily strong enough to withstand the hardest punch or kick that could be delivered. It was a sturdy helmet with a big metal grid on the front. So, of course, we hit each other pretty hard.
I remember vividly one of the first times my brother and I tried out the bogu gear for our sparring back in the mid-1980s. We had been training with this gear for only a very short time when I launched a punch that hit my brother and knocked him unconscious. He was only briefly concussed, but the main point is that he had no facial abrasions or broken bones. By the way, there is no direct relation between my knocking out my brother in this instance and the fact that he had broken my nose a few years earlier. Honest.
However, getting knocked out is more of a significant concern, because our bones heal much better than do our brains. Since that time the bogu helmets have undergone many design changes. This can be related to what has happened over the centuries with boxing. Boxing and boxing-related injuries have been around since the sport began. Back in ancient Roman times, boxers would often use hand wraps with spiky protrusions that caused more bloody injuries and eventually led to a ban on boxing. Fast forward to the seventeenth and eighteenth centuries when boxing was a very popular sports activity in the United Kingdom. However, it could be quite brutal with almost anything—including biting—allowed.
In an effort to make the sport less injurious and more palatable, the Broughton rules in 1743, the London Prize Ring rules in 1839, and finally the Marquis of Queensbury rules in 1867 were introduced. These rules all limited the techniques that could be used, how fighters should behave, and how matches would be conducted. So, biting was right out at that point! In particular, in attempts to make the sport safer, the Queensbury Rules called for the use of boxing gloves, ten-second knockout times, and three-minute rounds.
However, none of these measures actually eliminated the impact that boxing has on the nervous system. So, what kinds of design changes are needed and what considerations need to be addressed to make activities with lots of head contact safe? The best place to explore this is in the design and redesign of football helmets such as used in the NFL. Let’s spend a moment to look inside the origin and development of helmets and explore briefly concussion-related issues in football. Football is clearly a high impact sport where body contact is guaranteed. Over the history of this sport, injuries have been fairly common. In the period from 1869 to 1905, 159 serious injuries—including 18 fatalities—occurred during football games. Despite these serious injuries, helmets weren’t used in an American football game until 1893 in the Army-Navy game played at Annapolis, Maryland. Navy won 6-4, by the way. Helmet use was not enforced until 1939 in the NCAA and 1940 in the NFL. When helmets were first introduced, they were basically lightly padded leather skullcaps. Examining the period from 1945 to 1999, there were almost five hundred deaths as a result of injury in football. More than two-thirds of the deaths were because of brain injury with the remainder due to spinal cord injury or bleeding in the brain. These rates have dropped off in recent years as a result of prohibition on leading with contact from the head and face during blocking and tackling. However, the problems of concussion remain.
The use of single and double bars on the face mask didn’t come along until the 1950s. Since that time helmets have undergone dramatic transformation. They now incorporate light injection-molded inserts and are used along with neck bracing to help shield the spinal column from damage. In many ways the newer helmets are similar to what Batman has in his cowl. The cowl is padded and acts like a helmet and also incorporates some form of neck protection. A good helmet or batcowl acts to change the impulse (recall from Chapter 10 that impulse = force × time over which it is applied) so that lower peak forces are experienced by the head inside.
Despite that, concussion is still a serious issue in professional football. It was reported in 2000 that almost 50% of players in the Canadian Football League had suffered at least one concussion. Considering secondary impact syndrome (repeated concussions and brain damage), those CFL players who had already had a concussion were about six times as likely to lose consciousness with subsequent concussions than were players who hadn’t had one before. So, obviously further refinements to helmets might be in order!
Strains and Sprains
It is now time to have a look at the more routine injuries that Batman experiences: his normal bumps, strains, and sprains. These are the more painful injuries, but they aren’t usually as potentially debilitating as concussion.
We have all hurt ourselves doing some activity and said we “strained” or “sprained” something. Probably you have heard the phrase “repetitive strain injury.” These kinds of injuries can be understood by using the concept of “microfailure,” which means that small loads repeated many times can also yield injuries. The tissues that are being strained—particularly those with poor blood supplies like tendons, ligaments, and cartilage—are not able to fully repair themselves between subsequent events. This is a bit like the repeated concussion concept above except it is with other tissue. Commonly it turns up in the context of workplace-related injuries and might be included in the serious sounding carpal tunnel syndrome. The basic concept is that a consistent and repeated low-level load can lead to injury in the same way that a very large load can lead to injury with only a few (maybe even one) loading events (shown in Figure 13.2). High loads with low repetitions are shown at the top left while low loads with high repetitions are shown at the bottom right. The point to note is that injury can happen anywhere on that continuum.
Fighting techniques can also produce repetitive strain injuries. Even in my own martial arts training—which, obviously, is much less intense than that of Batman—repetitive strain injuries are a major concern. I cannot train day to day with full intensity on the same weapon, for example. Instead I have to rotate through different techniques, patterns, and forms and alternate empty hand training and weapons training.
Let’s look more closely at what a strain is. To understand what happens to our muscles, tendons, bones, and ligaments when we have an injury means first thinking about what these tissues are made of and what happens to them when we subject them to force. Now we have entered the arena of tissue biomechanics. This field shares many features with materials science and materials engineering. Keep in mind that biological tissues like muscle and bone have mechanical properties that help explain some of the weird things that seem to happen to us when we are injured. Remember that in mechanical terms “stress” refers to the forces applied to a material, whereas “strain” refers to the change in length or size of the tissue to which the force is applied. So, it is fair to say that repeated stresses lead to strains. If you were to measure the relationship between stress and strain of something like the long bone of your upper leg (your femur), you would find that at some high stresses the strain is too high and the bone would break (failure or rupture in mechanical terms). This is where you would have an obvious injury such as a broken leg. You co
uld really make it sound better by calling it a severe strain, but it is still very painful.
Figure 13.2. The number of repetitions and the size of loads leading to injury. High loads with low repetitions are shown at the top left and low loads with high repetitions are shown at the bottom right. Injury can result from many repetitions with a small load as well as from few repetitions with a large load.
Whiplash Injuries and Sore Muscles
I hope you haven’t been in a car accident and experienced an injury known as “whiplash.” However, you have almost certainly done a new activity that led to sore muscles the following day (and for a few days after that). There are some similarities between what is going on in the two scenarios, and the similarities have to do with how our muscles work.
We can think of our muscles doing three general kinds of contractions. When you pick up an object (like this book) and flex your elbow to bring it closer to your face, you are doing a “shortening” contraction—so called because the muscle fibers in your arm muscles are physically shortening. In contrast, when you lower the object (or book) back down to the table or wherever, you are doing a “lengthening” contraction. Last, when you are just holding the book in front of you and not moving it up or down, you are doing an “isometric” (literally no change in length) contraction. When your muscles are performing lengthening contractions they can be “over-stretched” by whatever you are doing. A good example is walking down a hill or down stairs. When you have gone on a mountain hike or even one in a very hilly area you probably found your leg muscles (particularly the ones on the front of your thigh, your quadriceps, or the front of your shin, your ankle flexors) to be very sore the next day. That’s because those muscles were doing lots of lengthening contractions on the way back down the trail from the mountain.