by E. Paul Zehr
One last point to make about motor units is that they don’t just come in one-size-fits-all form. In our tug-of-war analogy, think of your team as being composed of different-sized people. Just for simplicity let’s say you have 10 small-sized and 10 large-sized people. Obviously, the large-sized people will be able to produce more force. Well, your motor units come in basically two types based upon speed of contraction and how long they can keep contracting (called fatigability). The types are called, rather unimaginatively, type I and type II, or slow and fast. The type II, or fast, units are broken down a bit further into fast fatigue resistant and fast fatigable. In contrast, type I units are all fatigue resistant. The strongest and fastest contracting units are the type II, and the slightly less strong and slower contracting units are the type I. If you have ever done something like tried to hold a bent arm chin-up for as long as you could, what was happening was that you had both kinds of motor units active during the contraction. Then, as you fatigued and slowly dropped to the ground, your type II units were dropping out but your slightly weaker type I units kept right on going! It is the type II units that are most affected by strength training.
Getting Warmer and Stronger
Now let’s tie our discussion of muscles into one of our general themes: stress. Let’s look at how, in the process of getting stronger and more powerful, biological tissue actually knows that it should respond to stress and how it detects mechanical stresses. First, we have lots of “sensors” in our muscles, tendons, and skin that inform the nervous system about things like muscle stretch and contact of the hands and feet with objects during our movements. In our explorations here we aren’t yet concerned with these kinds of sensors—called proprioceptors. We’ll talk about them more in Chapters 9 and 10. In tissue bio-mechanics, muscle, bone, and ligament have been described as mechanocytes (remember them from the last chapter?) since they can respond to mechanical stresses by altering gene expression. This can happen because although we obviously can’t change the genes we were born with, what happens to us in life can alter our genes.
What sets muscles apart from other mechanocytes is that their cells respond to the mechanical stresses that are generated by their own activity. So, we can think of the adaptation that muscle has to produce as being a kind of self-inflicted challenge to homeostasis. This contrasts with other cell types like osteocytes in bone, which respond to mechanical stresses that are induced by activity of other cells . . . like muscle. Anyway, when muscles are subjected to mechanical stresses that exceed their accustomed levels, a whole host of properties—such as speed of contraction and activity of enzymes—are adapted. The result of the adaptation is to minimize the overall effect of the same stress when applied again to the muscle. This process is really very much like the negative feedback system described before (see Figure 3.1). This concept of adaptation to applied stress was originally presented by Hippocrates (460-370 BC) when he wrote “that which is used develops, and that which is not used wastes away.” We might paraphrase that nowadays as “use it or lose it.” But right now we don’t have anything to lose and are instead trying to ask, how do we get it in the first place?
Let’s go from this general discussion of muscles to a young Bruce Wayne just beginning his long journey to become Batman. He wants to increase his strength, so he does something simple like buying a set of barbells and some free weights. Bruce could choose to purchase a set of weights and follow a program such as the one advertised in many comic books back in the days of his (and my) youth. The Charles Atlas ad showing a “weakling” getting sand kicked in his face was very prominent when I was first reading comics as a kid. Maybe you have seen similar ads in comics or on television. Bruce then performs many different exercises with the weights. He would have no reason to perform the exercise shown in the second panel of Figure 4.4. Holding a heavy weight overhead with one hand is really not a very useful part of Batman’s actual required training regimen. In fact this is contrary to the main point of specificity in training. This example is one panel from the story “The Legend of the Batman—Who He Is and How He Came to Be!” (Detective Comics #33, 1939; Batman #1, 1940). This story, while obviously very much ridiculously simplified, was the first comic book documentation of the physical training Bruce Wayne had to undergo to become Batman.
To take a more realistic example of what a person can do to train his or her muscles, concentrate on a very clear and simple bicep curl exercise which involves elbow flexion. To get the idea, stand up and straighten your arms with your palms facing forward. A bicep curl is when you flex your arm by raising your hand. Your body (and that of Bruce Wayne) is calibrated for the mass of your limbs and has been adjusting to your daily activities for your entire life. We could say that your muscles are adapted already to the level of mechanical stress that you subject them to with whatever movement and activity you already do. Now, if you want to make your muscles stronger, you have to provide a mechanical stress that exceeds this already adapted level.
Bruce could do lots of flexion movements with no weights each day if he wanted to. He would get tired, for sure, after doing hundreds of empty-handed bicep curls. However, he wouldn’t really get stronger. That is because a key principle for adaptation to increased training stress is something called overload. Overload means pretty much what the name implies. Muscle needs to experience a load over and above the previously adapted level in order to trigger a new adaptation.
Bruce, therefore, needs to put some weights on the barbell. This is often described as increasing the load or resistance against which muscles contract. It will result in his having to produce more force when he flexes his elbows. If we cast backward to Greece in the sixth century BC, we might find the story of wrestler Milo of Croton instructive. Legend has it that he trained by carrying a calf on his back each day until it grew into a fully grown cow. This is a very clever application of the overload principle, to be sure.
Figure 4.4. The first description of Batman’s training regime from “The Legend of the Batman—Who He Is and How He Came to Be!” (Batman #1, 1940). Left, the sole documentation of Bruce’s scientific training in which Bruce stares at a test tube of unknown material. Right, Bruce is shown lifting a barbell above his head, which would not have been a likely part of his training.
You might wonder why muscle (or other tissue) needs an overload to respond with an adaptation. If you are going to do some exercise to increase strength, why not just have your body start adapting right away to provide responses for increased strength? The bottom line is that our physiological systems are essentially lazy. Many physiologists would actually say that a bit differently and call it “efficiency,” but it really comes down to the same thing—your systems will generate large and meaningful adaptations only if they have to. You and I operate in the same way. Often when we have to do something, we may try to do the minimum needed first. If that works then everything is fine. If it doesn’t, we then ratchet up our efforts to the next minimally more difficult or least costly next step. We will continue doing only what is necessary until our objective has been achieved.
Your cellular responses are similar. Cells are more thrifty than lazy, because every cellular adaptation has a corresponding physiological cost that has to be paid. So, a minimum level of training stress for a certain duration of time needs to be experienced to trigger physiological adaptation. Also, we have a bit of a buffer in our responses, as most systems tend to generate a safety margin when they adapt. This is to ensure that the applied stress plus a little bit more can be handled without damage to the body.
To return to our example of the young Batman early on in his training for increased strength, we can now appreciate that to keep gaining in strength he will have to incrementally keep increasing the load or resistance against which he is forcing his muscles to contract. This process will continue on and on until Batman stops at his desired level or reaches the adaptive physiological limit.
So, Batman Lifts Some Weights, Then What?
> When Batman does strength training, stresses on his muscle result in his muscle fibers getting stronger because of increased contractile proteins inside them. Muscle cells are really quite good at this and have some very specialized features. For example, unlike most other cells, they have many nuclei. So, more actin and myosin proteins need to be synthesized and inserted. This process is called muscle hypertrophy. Hypertrophy sounds like a bad thing, but it just means that muscle cells are increasing in size.
Think back to the interaction of the contractile proteins actin and myosin being likened to pulling a rope. Imagine more hands on the rope and increasing the strands on the rope and you will get the general idea that more force can be generated and larger forces tolerated with hypertrophy. If the muscle has been severely damaged, such as can occur with extremely high loads during exercises that cause muscles to lengthen, it may be that a normally dormant undifferentiated muscle cell comes into play. This has been called the satellite cell and was discovered almost 50 years ago by Alexander Mauro. Mauro discovered a kind of stem cell in skeletal muscle that he suggested “might be pertinent to the vexing problem of skeletal muscle regeneration.” This was quite an understatement, as it is now known that satellite cells can be fully activated to regenerate damaged muscle tissue and participate in the process of hypertrophy. However, the extent to which satellite cell populations are “activated” in response to exercise training is still mostly uncertain.
A puzzling issue about the response of skeletal muscle to exercise stress has been this: exactly what is the necessary stimulus for hypertrophy? To answer this question, let’s think back to Bruce’s preparation to become Batman for a minute (see Figure 4.4). Bruce knew that he would have to both understand the science of his body and work hard physically if he wanted to achieve his full potential. We cannot grow stronger unless specific chemical and genetic changes take place. We all know that we are born with certain genes, but remember from Chapter 2 that our cells are constantly being created anew and that RNA is always in action in our bodies.
Having said this, let’s look at what happens during hypertrophy. It was well established that stretching muscle (such as occurs during contraction) can lead to hypertrophy. It turns out that stretch-activated ion channels (that relate to movement of calcium ions) in muscle cells appear to be involved in this action. So, strange as it may seem, you get stronger because the cells in your muscles increase in size from the chemical activity of calcium. Another reason that we should all drink our milk!
Another element of hypertrophy is that messenger RNA in muscle cells increases on a very short time scale. In fact, the elevation of transcriptional activity (creation of new RNA) occurs within hours of the training stress and can remain elevated for days. When this process is completed, the same stress that induced the adaptation will no longer cause the same relative stress to the muscle fibers.
So, when Batman subjects his skeletal muscle to overload stress while doing his weight-lifting exercises, his muscles undergo a sequence of molecular-level events that lead to increased creation (anabolism) of contractile proteins. This process also increases the size of his muscles. In this way the stress from the adaptation is spread over a larger surface area of muscle protein and, just like the concept of pressure, leads to a smaller stress on the muscle. Because the increase in contractile protein is an expression of the capacity to produce forces, muscle strength is often expressed relative to its cross-sectional area. It is important to note that the cross-sectional area of muscle fiber has limits beyond which no further increases in size will occur.
Last, during Batman’s training a host of hormone and signaling chemical cascades are triggered. These include many growth factors that trigger the anabolic muscle-building phase of training response. These chemical actions along with the direct mechanical stresses of strength training lead to increased muscle hypertrophy. It should be clear from this that, while training does eventually make Batman stronger, the immediate effect is to make him weaker! The training stress is induced during the training, whereas the adaptation occurs later. It is therefore important for Batman to not train on days when he will be saving Gotham from evil hordes.
Now, in Batman’s case, he doesn’t really want to be at the limit of his strength adaptations. He does need to be near his power limit, though. Let’s now look more closely at the difference between strength and power.
Are the Strong Always More Powerful?
Strength and power are often incorrectly thought of as identical. We might see an extremely strong athlete and assume that the athlete also possesses great power and vice versa. Strength should be understood as the capacity for gross muscular effort. Having great strength means having the ability to generate large forces regardless of the speed of contraction. Think of the task of lifting an extremely heavily weighted barbell during an Olympic dead lift (where you hold the bar and stand up). That is a feat of great strength. Power, on the other hand, should be thought of as effort at high speed. In this case, let’s use another Olympic example and imagine a 100-meter sprinter exploding out of the blocks and streaking down the track toward the finish line. That is an example of great power. It is possible to be strong and yet not that powerful, but to be powerful requires a certain base of strength.
The real reason that Batman wants to get strong is not merely to lift up heavy objects. That is not a well-established virtue for winning fights against criminals! He isn’t going to be a very effective crime-fighter if all he can do is perform feats of strength. Batman needs to be strong but also quick and agile so that he can perform powerful martial arts techniques and acrobatics. The proper performance of fighting techniques and acrobatics necessitates very high velocity movements. So, power is vitally important to Batman’s training and practice and is more important to excellent performance than is strength. However, it is absolutely necessary to have a certain strength base before, or developed along with, power training.
Batman understands the subtle difference between strength and power. In the story “Daughter of the Demon” (Batman #232, 1971), Batman is fighting one of R’as al Ghul’s main henchman, who is a huge man and very strong. Using typical bad guy banter, the henchman says to Batman, “I shall dance on your corpse!” Batman responds by saying, “Not likely! Oh sure, you’re large and powerful . . . size and strength don’t count for much! Skill is the item. Agility helps too! And those things you lack! You’re clumsy!” Batman correctly tells him that skill and agility are needed for fighting, which is just what we have been discussing.
Strength and Power Training
Although there is a major distinction between the concepts of strength and power, the principles underlying training for each and physiological adaptations for each are very similar. Let’s discuss, then, some of the basic principles underlying any resistance-training program that would lead to increased strength and power. Two foundational principles are overload and specificity. The overload principle can easily be understood with reference to the stimulus-response model proposed earlier. In order to bring about an adaptation, a certain stimulus must be applied. In the case of strength training and the overload principle, one must apply a stimulus to the muscles that is greater than that normally experienced. The overload principle can be understood as simply applying a stimulus to the body and gradually increasing this stimulus as the individual responds to the stimulus by getting stronger.
The principle of specificity must also be considered under the umbrella of the stimulus-response model. Any stimulus that is applied to the body will cause some response. This response is simply a protective one in that the body is attempting to minimize the stress that is being experienced and to regain homeostasis. It should come as no surprise that the response the body exhibits is quite specific to the stimulus applied. This specificity of response applies to movement pattern, velocity of movement, type of muscular contraction (lengthening or shortening), and frequency of contractions or movements. Therefore, to bring about a training
adaptation that will be useful to Batman’s job performance, the training stimulus must be as close as possible to the movement pattern, velocity, and type of contraction that he uses.
Based on the principle of specificity, Batman should try to train with the same movement patterns for which he wants his strength and power gains to be used. It is extremely difficult to exactly replicate fighting movements with conventional strength training equipment. However, concentrating on a rough approximation of martial arts movement patterns would be his focus. Training exercises could be done with fixed-weight machines or free weights. For example, squats and calf raises would be done with free weights to simulate springing and lunging, while leg extensions and curls could be employed to develop kicking and jumping power.
The emphasis in training for Batman’s strength should be on 12 to 15 repetitions to failure, also known as repetition maximum (RM), or the amount of weight a person can lift in one repetition for a given exercise. It is vitally important that the execution of the movement be performed as quickly as possible. In total, the actual number of repetitions of any particular strength training movement is therefore only going to be about 30 to 60 on any given training day. Studies show that this level, which means using a weight that is about 60% of the maximum load that could be lifted once, is good for making muscle stronger.