by E. Paul Zehr
Obviously, if free weights are used, training with a partner to act as a spotter is extremely important for safety. Alfred or Robin would be fantastic in this role . . . Well, Robin, anyway, Alfred is pretty slight after all. Batman and his crew would train with strength exercises at least twice per week and probably more like three times per week. This would cause his muscles to increase in cross-sectional area by about 0.2% per day in less-trained muscle like the elbow flexors and about 0.1% per day in muscle like his knee extensors. When training for power, the same exercises can be performed. But now the weight used should be reduced to a load such that 20 to 30 repetitions can be performed before failure, and the exercises must be performed ballistically. “Ballistic” comes from the Greek word for throwing and indicates movements that are as rapid and forceful as possible.
As a further wrinkle into how our physiological systems respond to stress, the adaptations to strength training appear to change while the systems themselves adapt. Put another way, there is a changing “dose-response” for building strength. What this means in practice is that just incrementing the load against which Batman works his muscles as a constant percentage of his maximum will not be effective in producing corresponding increases in strength. If we consider Batman as moving through three categories—untrained, recreationally trained, and then highly trained athlete—the way he has to go about his training will need to change. This performance progress is shown in Figure 4.5. I will refer to the untrained Batman as “Bruce Wayne,” the in-process Batman as “The Bat-Man,” and the finished product as “Batman” to reflect chronological changes in Batman’s name in his own comic book. A key issue is that Bruce will start off as “untrained,” and he could make maximal strength gains by training three days per week using a load equal to 60% of his 1 RM and doing about four sets of exercises each day. When he moves to “recreationally trained” (“The Bat-Man” status), he would need to train two days per week using a load equal to about 80% of his 1 RM. This means that the benefit of exercise training varies as the training status changes and fits with the overall framework of challenges to homeostasis and stress mentioned before. Finally, as “Batman” he needs to perform more sets at higher loads just to make any progress.
Figure 4.5. The time course of strength gains due to training. Shown are the contributions from changes in the muscle and changes in activity in the nervous system. Modified from Sale (1988). “Bruce” refers to Batman at the beginning of his training, “The Bat-Man” refers to the in-process Batman, and “Batman” to the finished product. This reflects chronological changes in Batman’s name in his own comic book.
Figure 4.6. Influence of multiple factors on muscle hypertrophy. Data from McArdle et al. (2005).
Bear in mind that, as mentioned, the development of strength comes first, followed by power training. Another thing to remember is that the training for strength can be of a more general nature, while that for power must be as specific as possible.
In addition to the very important mechanical signals associated with performing high-force contractions, the Ca2+ levels, metabolic byproducts, and circulating hormone levels will all be changed. These can significantly affect the hypertrophy response. We will return to this complex chemical interaction later.
How Strong Can Batman Become?
Overall there are a number of factors that contribute to Batman’s ability to get strong and powerful as a result of his training. Most of these are summarized in Figure 4.6 and relate to things we have already touched on, like nervous system activation, environment, the endocrine or hormonal system, and genetics. Other factors, related to metabolism and nutrition, will be discussed in future chapters.
There is an interesting story in the graphic novel Batman: Venom (1993) in which Bruce Wayne is pushing himself to train harder and get stronger after his “weakness” prevented him from saving a little girl from drowning. Bruce tries to lift 288 kilograms (635 pounds) in a “dead lift” (where you hold onto the bar and then stand up) and then tears his shoulder muscle in the process. Later on in the story, Bruce lifts 690 pounds in a clean and jerk (where you pull the bar up to the chest and then jerk it straight up over your head). It is reasonable at this stage to ask was that a realistic weight to lift, and how strong should and could Batman actually be?
As a reference for this, the current (as of 2007) Olympic record for clean and jerk in the closest weight category Batman would find himself (105-plus kg category) is 263.5 kilograms (about 580 pounds). This record was set by Hossein Rezazadeh of Iran on September 26, 2000, at the Sydney games. The world record for the dead lift is 455 kilograms (about 1,000 pounds) held by Andy Bolton of the United Kingdom. Maximum strength for weight lifting is roughly equal to the height of the person doing the lifting in meters squared. This calculation gives the total sum of the weight that could be lifted when combining two world weighlifting categories of the snatch and the clean and jerk. Using this for Batman gives him a maximum of about 223 kilograms (496.3 pounds). So, not quite Olympics but pretty good!
His muscles are not the only thing Batman needs to strengthen. Next we’ll next look at surprising ways Batman can build up his bones.
CHAPTER 5
Building the Batbones
BRITTLE IS BAD, BUT IS BIGGER
BETTER?
I choose this life. I know what I’m doing. And on any given day, I could stop doing it. Today, however, isn’t that day. And tomorrow won’t be either.
—Bruce Wayne, in the graphic novel Identity Crisis
No matter what Batman does, it involves some sort of movement. Fast or slow, a lot or little. All of that movement can only happen because Batman’s muscles pull on his bones. In the last chapter we looked at how muscle generates force. Now we will look at how force must also be transferred across the bones. This means that bones are subjected to both the stresses arising from active muscles and those created by the body’s interaction with the environment. Think of the forces that affect your skeleton when you run. Your muscles are pulling on your bones, and your bones also experience large forces every time your feet hit the ground. Let’s explore how bones work and how bones adapt to what we do.
It’s likely that you haven’t spent much time thinking about how your bones can change or even considering the complicated job that our bones actually do for us. This isn’t surprising since we don’t really pay much attention to our bones generally, unless we injure them! Bones are, after all, buried deep in the body and can be felt or their appearance guessed at only by looking at the shape of the skin and muscle that cover them.
You were probably aware of your bones changing in size as you grew up, and you may be able to think back to that one summer you grew three inches in three months. (For those of you not yet in your teens, don’t worry. Your time is coming!) You may have also have had some experience with an elderly friend or family member with osteoporosis who broke a hip from a minor fall. The word “osteoporosis” means porous bone. You can imagine that porous bones would be weak and break easily. Batman certainly wouldn’t want that to happen while he was fighting criminals! Weak bones have another disadvantage. When bones weaken they are said to demineralize, losing essential minerals such as calcium, and then they don’t have the structural content they need to support the body and its movements.
In this chapter we examine bone and bone mineral density. Before we consider the mineral content of bone, let’s think about the stresses that could maintain and change bone. Remember that biological tissues respond in a compensatory way when subjected to stress.
An example used in the last chapter had Batman beginning to do strength training exercise. We talked about the bicep curl, which begins with the arm extended straight down to the front and holding a weight (or weights). Batman flexed his arm by generating motor commands that led to the excitation of his muscles. His contractile fibers interacted and generated force; this force was transferred throughout the muscle to the tendons at either end. The tendons are connected t
o bones and cross over joints—in this case the elbow joint. Because joints are really a kind of lever, activation of the muscle causes rotation of the forearm about the elbow, and this leads to bending stress to the bone because of both the weight and the force of gravity acting on the forearm. In many ways this bending stress arises following on the third law of motion formulated by Sir Isaac Newton (1642–1727): for every action there is an equal and opposite reaction. When muscles produce force to move parts of the skeleton, forces are acting on the skeleton.
So, what are bones good for anyway? Or, more grammatically correct, what is the function of our bones? Well, the bones in our skeleton provide a rigid frame upon which all our other body tissue is laid. In other words, bones give our bodies a shape and structure and provide the actual framework for our muscles to move. Bones also help protect internal organs—think skull and brain here—from impact forces in the environment. Although impact protection is only occasionally called into use, a more imperative feature of bone is to be strong and resilient so that it won’t easily fracture or fail during normal use. Your skeleton is repeatedly subjected to stresses every time you move. Even while you sit here reading this book, the force of gravity is providing a strain load on your bones. Repetitive bone loading during physical activity and exercise can lead to bone damage at the microscopic level, and that damage must be repaired, as you will read more about later in the chapter.
As long ago as during the life of Galileo Gallilei (1564–1642), physical activity was known to affect bone. Galileo suggested an association between mechanical forces and the structure of bone when he noted that bone size was related to body weight and activity. It turns out that key issues for bone are the stresses generated by forces occurring during muscle contraction as well as the force of gravity acting on the skeleton. This is a crucial problem when gravity is reduced—during spaceflight, more than 10% of bone mineral density can be lost after a six-month mission.
The concept of physical activity changing mineral content and therefore structural properties of bone was suggested in a formalized way by Julius Wolff (1835–1902). Wolff stated that, in a healthy animal, bone will adapt to the loads that are applied to it. This was actually articulated in what has become known as Wolff’s law: every change in the function of bone is followed by certain definite changes in internal architecture and by external confirmation in accordance with mathematical laws.
Without going into specifics about what Wolff meant by “mathematical laws,” one thing has proved true: bone remodeling—or modification of the bone over time—increases the strength of bone specifically related to the stress and strain applied to it. (By the way, now might be a good time to remind you that stress and strain are not the same. Stress is the reaction of the body to forces that affect its balance, whether they are physical or chemical. Strain is when a part of the body is injured by overuse.) Conversely, if the loading pattern and stresses are reduced or changed, the bone will remodel to the lowered demands as well. So, the general adaptation response of bone fits well within the framework already established for muscle. This regulation of bone remodeling is quite specific. We first encountered the concept of specificity of physiological changes with Batman’s adaptation to strength training. It is a theme that will recur again and again.
The skeleton is a very interesting structure to consider from the perspective of mechanical loading and forces that act upon it. Compared with muscle, there is a much larger range of structural and mechanical loads that our bones must bear. For example, I mentioned the skull as an example of a bone (22 bones, actually) that protects an internal organ from impact loading. In the case of Batman, that means getting hit on the head. The bones in the skull are not normally subjected to repeated strains or stresses during daily activity and the skull doesn’t really accumulate much mechanical microdamage. The skull of Batman, however, experiences impact loads seemingly on an almost daily basis. As with many other things, Batman might be an exception!
For most of us, the bones in our legs are subjected to thousands and thousands of “cycles” of loading and unloading each and every day. The specific load that a bone experiences relates to where it is located in the skeleton and what its job is during movement. This tends to mean that changes in bone remodeling really depend on the site where the mechanical stresses are applied.
Let’s think back to the previous chapter on muscle. If you do a lot of strength training with your arms, your leg muscles aren’t going to get much stronger. Similarly, bones that are subjected to strain loads get stronger and those that are not, do not! That is not the whole story, though. Sometimes bones that are not subjected to the loading strains get weaker. This is because bone remodeling is a response to mechanical loads. The physiological adaptation depends heavily on the availability of materials, specifically calcium, to change bone structure. To understand this better, let’s look at the cellular process for bone remodeling.
How Does Bone Remodeling Actually Work?
Bone has a certain cellular structure that is altered by the actual minerals inserted into it. Let’s break that sentence down and consider each part of it. Figure 5.1 shows the basic structure of bone, using the example of a bone similar to one of the long bones of the leg, such as the femur in your thigh. The basic functional unit of bone is the osteon. If you were to peer at a slice of compact bone (like that found in your leg or arm) under a microscope, it would resemble a bunch of plant stalks or tree stumps. The osteon is made up of rings (called lamellae). In the center of these rings, you would see a tubelike section, called a Haversian canal. These canals, named after English physician Clopton Havers (1657–1702), provide nerve and blood to bone.
Figure 5.1. The basic structure of bone, using the example of a bone similar to one of the long bones, like the femur. Moving from left to right in the figure we go from the basic functional unit of bone, the osteon, all the way down to individual collagen fibers at the far right. Courtesy Enoka (2002).
At the smallest level, bone is largely composed of collagen, which is the main protein found in connective tissue throughout your body. This collagen is then embedded with minerals. We can think of bone as being composed of three types of cells that all have slightly different jobs but all begin with “osteo” (for bone). Osteoblasts—think b for building—are the cells responsible for the formation of bone. They lay down the unmineralized matrix that forms the main bone content. The primary activity of the osteoblasts occurs during periods of bone modeling and remodeling. Modeling, which is the period of growth that occurs in childhood and adolescence, is not relevant to understanding Batman’s adult bones. Instead, we are focusing on the remodeling process. After the matrix is formed, osteoblasts eventually reduce their activity and either retire to the bone surface or become entombed in the mineralized bone matrix itself.
Osteoblasts that are entombed in the matrix are called osteocytes, which are the most abundant and also the longest-lived of the three bone cell types. Osteocytes are linked together by microscopic interconnecting canals (called, not surprisingly, canaliculi, or little canals) that make up the Haversian canal system discussed above (see Figure 5.1). (So remember c for canal.) Their main purpose is to sense and signal responses to mechanical inputs, such as those during loading.
Osteoclasts are the last cell type. The job of the osteoclast is to clean up the bone as part of maintaining bone integrity. Think of the cl in osteoclasts as the cl in cleaning. Picture the video game Pac-Man, and you will have the basic concept. Osteoclasts dissolve the organic and inorganic parts of the bone mineral. This occurs as part of the extensive remodeling that is always going on in bone.
In bone remodeling, osteoclasts and osteoblasts work in tandem. When bones are loaded, they experience microdamage—tiny cracks—that need to be filled and strengthened. In resorption (or dissolving of bone or other tissue), osteoclasts come along and break up very thin bits of bone in the microdamaged areas. Right after this, osteoblasts fill in the little divots left behi
nd by the dissolving action of the osteoclasts.
This is followed up by mineralization of the newly created organic matrix. In a way this whole process of activation, resorption, and formation is a bit like seeing a smallish hole in the drywall of your house. You first need to smooth out the hole with some sandpaper. Then the hole can be filled and will cure while it dries. The curing process (really, the hardening of the fill) is like mineralization in bone. Events of bone remodeling are really sensitive not only to mechanical loading but also to hormones. Two essential hormones that can influence the rate of bone remodeling are parathyroid hormone and estrogen.
Now that we have some grounding in the basics of how bone as a tissue works to remodel itself, let’s visit what mechanical loading events might occur during the kind of exercise that Batman would experience in his training.
Physical Exercise and the Bones
When Batman is performing his training exercises or when he is leaping, cavorting, and energetically fighting any of the members of his rogue’s gallery, he is constantly subjecting his skeleton to many “loading events.” Large impacts will be experienced when he jumps down from a height and the large forces he needs to generate with his muscles require strain on his long bones, such as the thigh bone (femur) and upper arm (humerus). This means his bones are experiencing tensile (bending) and compressive (squishing) strains. These strains give rise to changes in fluid pressure within the bone that trigger specific adaptations for bone strength.