by E. Paul Zehr
Muscle fibers are composed of different proteins. Some of these proteins have a direct role in producing muscle force and are called “contractile proteins.” Others have an indirect role in regulating contraction. The contractile proteins are actin and myosin molecules. Motor units come in “sizes,” which means that the number of muscle fibers controlled by each motoneuron differs. But the number is variable in a logical way and can range from about ten muscle fibers for one motoneuron up to thousands of muscle fibers. Keep in mind that when a motoneuron is commanded to be active, all the innervated muscle fibers must also be active. Your motor units (or at least the muscle fibers innervated by them) are real team players! This means that a small motor unit with ten fibers will produce less force than a large unit with a hundred fibers. Not surprisingly then, we find that the motor units controlling the movement of your eye (which has a very small mass) are the smaller units and those controlling your much larger (and heavier) leg muscles are the larger units.
If Tony needs to push more forcefully with a handheld laser—that is, he wants to change force production in his muscles—his motor commands will make more motor units become active (called “recruitment”) or the motor units that are already going will be active at a higher frequency. Both of these things happen more or less at the same time, except motor units are recruited and then activated at a higher rate. Then more units are recruited and made to discharge higher and so on. Meanwhile, the force of contraction will be steadily increased. This is something you wouldn’t notice at all, but it is a great example of matching between the output of the nervous system and the mechanical ability of your muscles to produce force.
Tony Stark’s motor units come in two basic types depending on how fast they contract (or “twitch”) and how fatigable they are (how long they can keep contracting before having to stop). These two main types are called, rather unimaginatively, type I and type II or “slow twitch” and “fast twitch.” The fastest twitch and most forceful units are the type II. The slightly less strong and slower twitch units are the type I. When Tony is manipulating that laser, he would be bringing into activity more and more units at different frequencies. If he had to hold it for a prolonged time, he would begin to experience fatigue. His muscles would start to ache from the pain detected by the metabolic processes occurring and his arm might begin to shake or appear to vibrate a bit. This is called “physiological tremor” and wouldn’t be particularly helpful if accuracy were needed. So, he would have to rest a bit between his efforts.
Your muscles are a really efficient kind of biological motor. If we were to compare them with real technological motors, it would probably be most useful to compare power output based on weight. According to Steven Vogel and his work on human muscle, your skeletal muscle produces about 200 watts per kilogram (90 watts per pound). The steam pump, debuting during the industrial revolution in 1712 in the hands of Thomas Newcomen and refined by James Watt in 1775, produced just over half of that at about 50 watts. That doesn’t quite match 200 or 500 watts per pound for a car or motorcycle engine, but it still is pretty good. By the way, a jet aircraft turbine clocks in at approximately 2,500 watts per pound. All in all, for a squishy bit of biological material, our muscles do pretty well.
Probably the main thing you think about concerning your muscles is how much force they produce. You might be surprised to learn that there are a number of oddities that occur during muscle activation and force production that introduce a few wrinkles into what we are able to do and how we (unconsciously) do it. Imagine a forklift tractor with the loader on the front. The hydraulic piston that helps raise and lower the load on the tractor behaves in the same way each time it is used. This is a highly linear system, and it is tempting to think of muscles as working in a similar way. But, muscles operate in a nonlinear system and have some peculiarities. The force that your muscles produce depends on the length of the muscle fibers and the speed at which the muscle is contracting. The details of the relationship can be a bit complex but, generally, the faster a contraction is occurring, the less force that can be produced. During a slower contraction, more force can be produced. This is called the “force-velocity relation.” You can flip this around into a load-velocity relation and think instead about how fast you could contract arm muscles to move your arm when holding a light weight versus holding a heavy weight. The lower the weight, the faster you can move, since, as we’ve noted, less force is needed to move a lighter weight. All of this applies to what are called “shortening contractions,” which are when muscles are active and are shortening (also called “concentric actions”).
If you are sitting down right now, you could do a shortening contraction with your knee extensor muscles (your “quads,” or quadriceps, on the front of your upper leg) by raising your leg until your foot is parallel with your hip. If you hold it out at complete knee extension, you are performing a constant length, or “isometric,” contraction. When you slowly bring your foot back down and let your knee extensors relax, you are performing a lengthening, or “eccentric,” contraction. A lengthening contraction produces the highest forces, and this force goes up the faster you move.
During everyday tasks, your muscles are constantly doing both shortening and lengthening contractions. For example, when you go up stairs, the knee extensor muscles are doing a lot of shortening actions. When you come down, they perform lengthening contractions. But your muscles are about 30% more efficient going downstairs than they are going upstairs. The actual force that moves those bones comes from the contractile parts of the muscle fibers as well as the connective tissue that holds the fibers and the muscle as a whole and keeps everything connected to the tendons. All of those pieces together have some elasticity, and it is because muscles use this elasticity when they contract while stretched that makes them more efficient. Elasticity in muscle and tendon has important implications for moving Tony Stark and his Iron Man suit around.
How Can Iron Man’s Motors Mimic Tony’s Muscles?
A main theme of this book is examining the amplification of human performance by the use of technology. Generally when we think of robotics and control issues inherent in the Iron Man suit of armor, this means thinking about the state of the technology in powered devices. Let’s consider another state-of-the-art example that has to do with something more day to day: sports biomechanics and performance using modified prosthetics. The term “prosthesis” simply refers to an artificial extension of a part of the body, usually a part that is missing or has been lost due to disease or injury. Unfortunately, and rather grimly, there is a strong link between research into amplifying the performance of amputees using prosthetics and assistance for soldiers. Soldiers in battle are and have always been at risk for losing limbs during combat. There has historically been a big technological advance in prosthetics development following large military operations in many cultures. For example, there was a huge upswing in the need and interest in prosthetics during the U.S. Civil War. In recent times, this has been greatly exacerbated by the role that “improvised explosive devices,” or IEDs, have played in the wars in Iraq and Afghanistan. Just like the horrible toll that landmines have and continue to take, these devices lead to many limb amputations.
As seems to be the case with so many things, the concept of prosthetic limbs can also be traced back to Egypt, where an artificial toe was created and used around 1500 BC. The first known and effective prosthetic hand that is relevant here is that of a German knight named Götz von Berlichingen about AD 1500. His hand was injured by cannon fire during battle and he had a mechanical one created. His use of the artificial hand gave him the nickname of the “Iron Hand” and was written about in the Goethe play of the same name. I cannot explain how amazing it was to discover a knight called “Iron Hand” when researching for this book about Iron Man. It could not have been better. As you can see in figure 2.4, the hand actually had a mechanism that allowed the fingers to flex, so it was really quite advanced for its time. If we zip forward
to 1800 and the Napoleonic wars, we come across an above the knee prosthetic leg called the “Anglesea leg.” It was used by Lord Uxbridge, who was a cavalry officer for the duke of Wellington (who, by the way, was also known as the “Iron Duke”!). This leg had an advanced mechanical design that allowed the foot to automatically rotate upward while the knee bent during the swing phase (when the foot is off the ground) of walking. This is just like the way the foot moves during walking across the ground normally, and this movement allowed for clearance between the foot and the ground to prevent tripping. This basic design persists into modern day in many prosthetic limbs.
Figure 2.4. Iron hand prosthetic of Gottfried “Götz” von Berlichingen (1480–1562). This mechanical prosthetic hand began use in 1504 when von Berlichingen lost his arm in military action. It could be used for a range of activities from wielding a sword to holding a pen.
Does Iron Man Have a “Spring” in His Step?
Let’s now also consider a sports example, namely, the use of specialized prosthetic legs by world-class sprinter and double-amputee Oscar Pistorius. There are two major bones in the lower leg called the tibia (your shin bone) and the fibula (the one that runs along the side of your leg—you can feel the bottom of it as the outside of your ankle). Both bones are needed to provide stability for the foot and to form the ankle joint. Oscar was born without fibulas in both legs, and there was really no chance that he would have been able to stand and walk. So, when he was still an infant (about 11 months of age), his legs were surgically amputated. This may sound dramatic and clearly caused a permanent loss. Removing the lower part of his legs allowed him to use prosthetics that could carry his weight and could be used for walking and, essential for what he does now, running. Very fast. Oscar is now the most dominant double-amputee sprinter and has won several gold medals at the 2008 Paralympics in the 100, 200, and 400 meter distances. His prosthetic legs are termed “blades” (you can see why in the image of him running shown in figure 2.5), but the technical name of these carbon-fiber prosthetics is the “Ossur Flex-Foot Cheetah.” The “flex-foot” part of the name is key to how the legs help with performance. Another component that helps Oscar’s performance is that the blades are so lightweight. In fact, the prosthetics work so well that, when combined with the superb sprinting physiology and mechanics of Oscar Pistorius, they created a controversy in track and field in 2007 when he first competed against runners with intact limbs. He did so well that claims were made that his “blades” gave him an unfair performance advantage over runners who had to use their own legs!
As a result of this, the International Amateur Athletic Association (or IAAF) as the governing body for track and field banned the use of performance-enhancing technology, included the use of the “blades” and ended any idea of Oscar’s competing in the 2008 Olympics. In fact, IAAF Rule #144.2 prohibits using “any technical device incorporating springs … that provides the user with an advantage over another athlete not using such a device.” As posed by Brendan Burkett, a professor of biomechanics at the University of the Sunshine Coast in Australia, “Does the technology create an unfair advantage for the Paralympian when competing against able-bodied Olympic athletes?” Burkett raised a number of very interesting “thought problems” when trying to work out the issue of technology in worldwide sports competitions like the Olympic and Paralympic games. If technology can play such an important role in human performance, what about the problem of equal access to all competitors? He points out the issue of access by highlighting the stunning fact that the winner of the 1960 Olympic Marathon was an Ethiopian named Abebe Bikila—who ran barefoot. With access to technological advances like running shoes, Bikila’s time would have certainly been even better than the winning margin he did produce. There are numerous more recent examples, such as the use of specialized fibers in tracksuits or swimming suits that can change the drag and friction experienced while running and swimming.
Figure 2.5. South African Paralympic runner Oscar Pistorius using his lower leg lightweight carbon-fiber “flex-foot” prosthetics (A) while running in Iceland; running stride using “flex-foot” blades (B). Panel A courtesy Elvar Palsson; panel B adapted from Weyand et al. (2009a and 2009b).
When we come back to the example of the “blades” of Oscar Pistorius and the issue of using prosthetic limbs to enhance performance, for a minute it does seem a bit bizarre to question the performance of a person who has something that should limit running performance—that is, no lower legs. However, if we think a bit further maybe there is something to this. Can technology be used to not just restore or replace function that is normally part of the body, like having legs to walk and run with, but to actually enhance performance beyond the level of those with intact bodies?
To fully appreciate why this design can be so useful, let’s consider the way walking works. Walking and running are like falling forward and continually catching up to our falling bodies by taking our next steps with our legs. This is often visualized by the movement of the body’s center of mass (COM) across the ground. On average, your COM is approximately located in the middle of your abdomen behind your belly button. The movement of the body is often described as an inverted pendulum, because, once the next step is taken, the COM kind of vaults over the place on the ground. However, a key part is that there is a springiness to this vaulting. As your weight moves onto your leg, there is a compression due to the motions at the hip, ankle, but mainly the knee. There is a large elastic—or springy again—property to your muscles and tendons, which is what is tapped into during walking (and other behaviors). Because of this, a bio-mechanical model of the legs during walking often shows a spring where the knee is. Then the limb is described as having a certain stiffness or compliance. By changing muscle activity (or more generally by wearing different shoes), you can change this stiffness in a step-by-step way. A portion of this is carried out by your reflexes, particularly the stretch reflex (figure 2.6). Quite a lot of our efficient walking and hopping is due to the way our nervous system assists our movement using our stretch reflexes. Stretch reflexes occur when the lengthening (or stretching) of a muscle activates sensors in the muscle itself. Those receptors send signals back to the spinal cord, which then reinforces and supports the activity of that muscle.
Figure 2.6. Simple reflex pathway (“stretch reflex”) to support contraction of the lower leg muscles during running and walking. Courtesy David Collins.
A dynamic change occurs in your walking or running when you go from running barefoot on a soft sand beach to running on a hard pavement foot path. This can trip you up—both literally and figuratively—sometimes. Once I was running across an airport trying to make a connection due to a delayed flight. I was running on a very soft springy moving walkway and just kept right on running as I came off the other end and landed on the hard tiled surface of the normal airport walkway. The jarring that I experienced felt like it had loosened all my teeth. It only took one step and then my body—set by my brain—had adjusted. You may have experienced this kind of thing by actually running in the sand. You get a similar kind of mismatch if you are running or walking down a staircase—typically at night—and then misjudge where the last step is. That can be very jarring too, and it is caused by that mismatch in what is expected to occur and what actually happens.
Dan Ferris and his colleagues have been researching aspects of this very problem—the dynamic changes in the control that the body can produce—for quite some time by using exoskeletons fitting over the legs to modify natural movement and to examine how neural control adapts when this happens. Their work has shown that when we run or hop on different surfaces, the stiffness of our legs is adjusted to compensate for the surface. This means there is less disturbance to the motion of the whole body. Think about running on a sandy beach compared to running on a paved road. Or running as a human and running as Iron Man. The general idea is kind of like having an adaptable shock absorber that is tuned exactly to what kind of shock you will experience. Fi
gure 2.7 shows the lower limb working like a “shock” absorbing spring during walking and running. Panel A demonstrates how movement of the ankle and knee (and activation of muscles acting at those joints) produces a springlike effect that the body works on while walking and running. As you walk or run, remember that your COM is always rising and falling slightly. This is controlled by the “stiffness” of your legs and shown by the circle on the top of the spring in the figure. The stick diagrams (panel B) show the movements at the ankle, knee, and hip. The exoskeleton (panel C) is used to change the mechanical responses of the ankle joint. Ferris’s research has been tremendously successful in advancing our understanding of how the body works during walking, running, and hopping.
If you can appreciate the mechanics of walking in this way, it becomes pretty clear why there has been so much controversy over a device that changes the properties of the legs. Essentially the “blade runner” prosthetic used by Oscar Pistorius dramatically increases that springiness. It stands to reason that this would also make performance much better. However it was mostly an intellectual argument until recently.
Because of all the legal issues that arose from the track and field controversy, it was necessary to see if there was any science behind the contentions. Several movement scientists were called in to measure the physiological cost of walking and running and the bio-mechanics of using prosthetics. Peter Weyand and colleagues performed studies comparing the speeds, metabolic energy cost, and biomechanical characteristics of the double amputee running with the blades with those of elite, high performance track athletes. Incredibly, the use of the blades allowed for almost equivalent performance to elite level runners. This is stunning in terms of how technological advance in prosthetics can give rise to such a leveling of the playing field. However, it has raised some additional concerns about whether the use of such technology may actually allow for increased performance, that is, for performance better than that of people with intact limbs.