by E. Paul Zehr
When we think of Bruce Wayne beginning his journey to become Batman, we want to know what led to the strengthening of his bones that would have occurred during his training. Well, let’s start with the scenario in which the strains are very large, causing small microfractures of the bone matrix. These microfractures are like little fissures in the bone, and they wind up separating and isolating some of the osteocytes embedded in the bone. This starts a cell death cycle for these osteocytes, initiating the cycle of bone remodeling described above. When the loading strains are lower, the pressure changes within the bone fluid stimulate the osteocytes to release hormonal and biochemical signals that activate the remodeling process.
The size, frequency, and repetition of the strain events are important for bone remodeling. Just as with muscle, a general principle is that the extent to which bone will remodel depends really on how much it is strained. And Batman not only has lazy muscles, as we learned earlier, but his bones are also lazy! To stimulate the bone remodeling process, we have to exceed some limit of strain size and repetitions. It is really important for the strain to be repeated and above a certain frequency. Just applying large loads to bone will not trigger the bone remodeling process. In fact, a problem with trying to stimulate a change in bone remodeling as an exercise adaptation is that bones typically have a very large safety factor. Batman’s bones (or anyone else’s for that matter) are able to withstand loads that are two to five times larger than the loads typically experienced every day. Great, you might say, that means my bones are safe from injury most of the time and that is why Batman hardly ever seems to have broken a bone. Well, yes, this safety factor helps keep the bones safe. But it actually means that training to make the bones stronger means significantly exceeding the daily loading cycles and pushing near the upper limit of the safety factor. This is why minor or trivial loading doesn’t yield a huge change.
What Kinds of Bone Are There?
Bone in Batman’s skeleton (and yours) comes in two types: compact (sometimes called cortical, which I described above) and trabecular (sometimes called cancellous). About 80% of Batman’s skeleton is compact bone, while about 20% is trabecular. Compact bone is found in the walls of long bones (like your humerus) and on the surface of flat bones (like those found in your skull). If you were to look at a close-up of compact bone, you would see that it is made up of tightly packed concentric layers of fibrous mineralized material as part of an interconnected network of osteocytes.
Trabecular, or cancellous, bone is found at the ends and inside of the long bones and, in contrast to the makeup of compact bone, is made up of a spongy-looking lattice. This lattice (shown in Figure 5.1) is composed of what look like supporting struts, and these struts orient themselves and thicken in the direction that will resist the loading patterns to which the bone is exposed.
Another example may be helpful. We subject our legs to many daily stresses and the long bones of our legs adapt accordingly. But the adaptation of the bone strength is still very much specified by the structure of the bone and what it does. The femur is strongest in compression (which is the main type of loading that happens during walking), three times weaker for shearing (imagine a sideways force applied to your femur to get the idea), and intermediate for tension (think bending forces).
At this stage it might be reasonable to wonder if the kind of training Batman needs to engage in could reasonably be expected to affect his bones. The answer is yes! And there are some pretty interesting examples of the specific way these adaptations occur. Let’s consider two kinds of athletic groups that face many of same the physical demands behind Batman’s prowess—martial artists and rock climbers.
Martial artists typically have much higher bone density than do untrained people, but you may be surprised to know that where this density is found in the body depends very much on the martial art. Training in judo, which involves a lot of grappling and throwing, and in karate, which involves a lot of punching, kicking, and striking, results in higher total bone density. Remember that your body responds and adapts in a very specific way to stresses. Given the main emphasis on grappling and throwing in judo and on kicking and striking for karate, these adaptations are specific and make a lot of functional sense.
This brings us to the other athletic group I mentioned above—rock climbers. Now, rock climbers also closely approximate one of the main activities Batman excels in and trains for: scaling buildings. This was something emphasized very early on in the Batman chronology. The panel shown in Figure 5.2 is from “The Batman Meets Doctor Death” (Detective Comics #29, 1939). In this story—and often throughout his adventures—Batman has to do a great deal of covert nighttime surveillance. We can see that Batman needs to do a fair bit of climbing to accomplish his tasks. Specifically, he needs to strongly grip the suction cups he uses for climbing. Compare this with the kind of climbing and gripping that rock climbers (maybe like you?) use routinely. Examples of these grips are shown in Figure 5.3. The main thing is that these activities require putting a lot of strain on the fingers. Advanced rock climbers commonly expose their fingers and hands to extreme stresses when using their fingers to support all of their body weight.
As you might guess after everything we have talked about to this point, mechanical loading of the fingers in rock climbing and bouldering should lead to increased bone density in the fingers. And yes, Climbers do have much higher cross-sectional bone areas than people who don’t climb, which corresponds to what you might predict. The mechanical stresses of these activities stimulate increased bone deposition, or the forming of new bone. We can rest assured that Batman’s skeleton adapts to the unique mix of loads he provides during training practice and while prowling the streets—and scaling the buildings—of Gotham City.
Figure 5.2. Stresses on the bones of Batman’s fingers and hands while climbing from “The Batman Meets Doctor Death” (Detective Comics #29, 1939). Batman needs to strongly grip the suction cups he uses for climbing (arrows).
The Work of the Joker: Bone Stealing from Bone?
There is one more feature about bone metabolism that should be of great interest—actually concern—to Batman. It turns out that while bone density goes up in a very specific area as a result of exercise training, bone density may also go down in other regions. That’s right. I said go down, not just stay the same. When athletes aged 18 to 40 have been contrasted with untrained people, they exhibit the increased bone density described above. Runners, weightlifters, and dancers all have increased density in the long bones of the legs. Surprisingly, in addition to these useful adaptations, athletes may also have lower bone density in areas of the body not subjected to exercise loading. For example, athletes may have lower bone density in the bones of the skull than do untrained people.
Figure 5.3. Examples of stresses on the bones of the fingers and hands during rock climbing and bouldering (arrows). Courtesy Sylvester et al. (2006).
Although this isn’t much of a problem for a runner or a weight-lifter, it actually represents a big problem for a fighting machine like Batman! This finding may not seem to make much sense. One speculation is that when bone resorption is activated, there may be an overall “zero-sum game,” meaning that the body may maintain an overall level of bone density. So strengthening some regions may lead to corresponding weakening elsewhere. This is not always the case and, in fact, it may not apply to other physical activities like rock climbing. But Batman better protect himself from falls!
Now we will look at another part of Batman’s training, one that is very important even to us non-crime fighters: how much he needs to eat to maintain his strength scaling walls and otherwise battling bad guys.
CHAPTER 6
Batmetabolism
WHAT’S FOR DINNER
ON THE DARK KNIGHT
DIET
He was never blessed with instant super-powers—it took him fifteen years to hone himself into a physical marvel and scientific and deductive genius.
—Tales of the Dark Kn
ight: Batman’s First Fifty Years: 1939–1989 by Mark Cotta Vaz
How much energy does Batman use in a day, and how much food does he need to eat? Even if you have pondered the activities of Batman, have you ever considered what would be the real energy costs in a day spent fighting crime as the Dark Knight? The answers to these questions depend on how much and what type of activity he had that day. Was it a big training day or a big “fight-an-entire-squadron-of-henchmen-single-handed” night? The bottom line is he uses up more calories than you or I do. Oh, also, in case you are wondering, Batman does take his vitamins.
Let’s start with the concepts of energy balance and metabolism. Many people make comments about their metabolism. We have all heard someone say something like “I have a low metabolism but my wife has such a high metabolism that she can eat anything.” Certainly there are lots of misunderstandings about metabolism and metabolic rate. Metabolism is defined as the sum total of all the chemical reactions occurring in the body. This means all of the processes that are occurring in all cells of the body. Metabolic processes can be categorized as either those that result in building up (anabolism), or in breaking down material (catabolism). An example of anabolic metabolism is building skeletal muscle proteins as a result of exercise stress (as discussed in Chapter 4). An example of a catabolic metabolic process is that of taking energy from food to provide energy for our cells to function. It is tempting to think of anabolic processes as “good” and catabolic processes as “bad.” However, both occur continuously in our bodies, and both must be in balance for healthy function.
The balance part I mentioned above has to do with a very simple fact: in a physiological energy system (a.k.a. your body), total energy taken in must be balanced by total energy expended. We have come back to the concept of homeostasis and also take on a new concept that “energy in equals energy out.” This concept adheres to the first law of thermodynamics, as laid out by the English physicist James Joule (1818–1889). Joule’s research was originally on the relation between mechanical work and heat. In fact, the international unit for work—the joule—was named after him and is used as the measure of energy in all fields . . . except metabolism and food energy. For various historical reasons, the term “calorie” (literally meaning heat) remains in use for energy expenditures in the body.
The first law of thermodynamics comes from what is more commonly described as the “theory of conservation of energy.” The main point is that energy is constant and only changes forms within a fixed system. In a physiological fixed system like Batman’s body, this means that there is a direct relation between energy input and energy output. Energy input really means stored energy taken in the form of food and drink matched with energy expenditure in terms of work performed in the body.
By “work” here I don’t mean tasks. I also am not referring to what scientists or engineers might call work—mechanical work. The concept of “work” in biology is a bit less obvious. For example, if you take a break from reading this book (probably a very short one) and lay down the book, you clearly have used some force and moved the book some distance. It wouldn’t be difficult to realize that you performed both mechanical and biological work in putting the book down. However, if you are just sitting and reading the book, you clearly aren’t doing much mechanical work at all. In both cases, you are actually doing a lot of biological work. Even when you sleep tonight, and think contentedly of all the interesting things you have gleaned from this book, you will still be doing lots of biological work. In fact, while you are alive your body never stops working.
The work that is going on in your body can be basically grouped into chemical, transport, and internal mechanical work. The chemical work involves activities like the growth and repair of your cells (including protein synthesis and wound repair) and the continual struggle to maintain homeostasis, such as continuous regulation of body temperature. Transport work includes the continual movement of ions, molecules, and large particles across cell membranes. The internal mechanical work that you are doing right now includes moving cellular organelles and contracting muscles needed to hold the book, maintaining your posture as you do so, and moving your eyes to follow the words in this sentence as it unfolds majestically across the page.
This comparison between mechanical and biological descriptions of work also extends to the concept of energy. In mechanical systems we can think of energy of motion and stored energy. The usual way this distinction is made is to ask you to think of a simple task like rolling a big ball up a hill. As you start at the bottom of the hill, the ball has no energy. When you begin to move, it has energy of motion—kinetic energy—and slowly gains stored, or potential, energy. When you get to the top of the hill and come to a stop, the kinetic energy will be zero but the potential energy will be high. Then, when you let the ball roll down the hill, the potential energy will be transferred into kinetic energy. This goes on until the very bottom of the hill, where the ball is on a flat surface, full of kinetic energy and with no potential energy. That’s a simple mechanical example.
For our biological example, the energy of motion is contained in chemical bonding, transport across cell membranes, and the mechanical work that occurs in muscle contraction. Potential energy is found in the chemical bonds created in storing foodstuffs. In metabolism, cells transfer the potential energy of chemical bonds into kinetic energy for growth, maintenance, reproduction, and movement. So, work always involves movement, and energy can convert forward and backward from potential to kinetic. It is important to note—and this is especially relevant to you if you are sitting in a cool room—this conversion is never 100% efficient. That means energy is lost in the conversion process. For example, lots of heat is generated when energy is used (and lost) during muscle contraction.
For muscle contraction, as with other processes in the body, this means taking the energy in the molecule ATP, adenosine 5′-triphosphate, which you might recall from Chapter 4, and adding water. If you remember any chemistry you studied in school, combining chemicals often results in their changing into something else. In this case, ATP has three phosphate groupings. After water is added, one of these groupings splits off. This leaves ADP (adenosine 5′-diphosphate, with two phosphate groupings) and one phosphate grouping by itself, which is used to power muscle contraction. But also during this process, energy is released as heat.
We can trace the roots of ATP back to famous French scientist Louis Pasteur (most known for giving us safer milk to drink, through what we now call pasteurization). In the late 1860s Pasteur noted that yeast was able to convert the energy in sugar to create carbon dioxide and alcohol. Continuing this research, in 1929 the German biochemist Karl Lohmann discovered the energy source for cellular breakdown of sugar in the presence of yeast. He found that a substance called adenine was linked to the sugars and to three (there’s that number again) phosphate groups. This brings us to adenosine triphosphate (or ATP). The high energy bonds between adenine and the inorganic phosphate groups store the energy that we liberate for use in our cellular processes and energy that is evident in every living system. So, Batman has a lot in common with actual bats beyond his wing-like cape.
The Dark Knight Diet
What should be in Batman’s diet? An overview of his optimal diet is shown at the top of Figure 6.1. The numbers I provide for the “Dark Knight Diet” are based on the 2007 recommendations of the Institute of Medicine of the National Academies of Science. As such, the “macronutrient” (that’s the official way to describe what’s in the food you eat) breakdown in the typical diet should contain 40–65% carbohydrate, 10–35% protein, and 20–35% fat. The numbers for Batman—60% carbohydrate, 25% protein, and 15% fat—reflect the values within these ranges that are closer to those for trained athletes, which Batman clearly is.
Figure 6.1. The Dark Knight diet and the fate of food intake.
Carbohydrate is very important as a fuel source during exercise. Because the body has a low storage capacity for carbohydrates, t
hey make up a larger proportion of the diet of an athlete than for the average person. Athletes may need some slightly higher ranges of protein in the diet as well, which shows up in the Dark Knight Diet. Batman definitely doesn’t eat a high-fat diet.
What are macronutrients, exactly? You no doubt read at the detailed food labels on cans, boxes, and wrappers when you are shopping. Given carbohydrates should make up the bulk of your diet, let’s start with them. Depending on their biochemical structure, carbohydrates are classified as simple or complex. Simple carbohydrates include fructose, sucrose, and lactose and are found in foods such as fruits and milk products and in sugar. Complex carbohydrates are starches and glycogen and are found in foods such as grains, beans, and potatoes. When carbohydrates are eaten they must be changed into the simple form to be absorbed. That is, they must be broken down in your body to the smallest, simplest chemical structure.
When proteins are consumed, you are really eating something made from chains of amino acids (called polypeptides). There are twenty amino acids to be concerned with. (Do you remember the amino acid “letters” we talked about in Chapter 2?) Nine of them are considered essential. You have to get them from your diet because your body cannot make them from other amino acids or proteins. This group of nine is isoleucine, leucine, lysine, theonine, tryptophan, methionine, histidine, valine, and phenylalanine. These nine (and the other eleven) amino acids are essential because the body needs proteins to perform almost all cellular processes. If not getting them from the diet, the body will attempt to get them from its own muscles and other tissues. So, eating these proteins is a good idea for Batman and for you!
The last macronutrient to consider is fat. The “fat family” includes triglycerides (which form by far the largest percentage of fats), cholesterol, phospholipids, and fat-soluble vitamins. Fats are divided into four essential fatty acids and also classified as either saturated or unsaturated. An easy way to tell the difference is that saturated fats typically are solid at room temperature, whereas unsaturated fats are liquids.
