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Of the weight of a human in decent shape-all too few of us-muscle makes up fully 40 percent. Not blood, bone, brain, or liver contributes as much; only together do they add up to our weight of muscle. Moreover, all that muscle does just one task: It makes the chemical fuel that originally came from our food produce force and motion. It does neither more nor less than what we ask of the combustion engines of our cars and airplanes. As in the engines of our technology, its imperfect efficiency makes it get warm, a nuisance when we work hard in warm weather but nice enough as we jump around or shiver to offset the cold.
In power efficiency-how much work it can do for a given amount of fuel-muscle differs little from those combustion engines. In weight efficiency-how much work a given weight of muscle can do in a given time-it compares well with automobile engines but suffers badly when put up against a good jet turbine. Still, this is one remarkable device. Nature perfected it around a billion years ago (give or take a few hundred million), launching multicellular animals on our glorious trajectory. It powers ant and elephant alike, so alike that only a trained eye can see the subtle differences between their muscles when bitsare viewed under a microscope. Flies fly with it; clams clam up with it.
For over a million years, we humans (or our putative ancestors) have taken bits and pieces of nonliving material-sticks and stones, old bones and pieces of metal-and fashioned devices that apply our old engine to new tasks. Stone axes, spears, bows and arrows, fire drills, knives, saws, rasps, atlatls, and boomerangs; rowboats and bicycles; hand-cranked centrifuges and meat grinders and can openers: None does more than harness our various muscles. For ten thousand years or so we've pressed nonhuman muscles into our service as sources of power. We discovered that bovids, equids, camelids, and others could be persuaded to trade mechanical work for vegetation, mostly vegetation we found indigestible.
When the day's work is done, what then? We can eat the engine, either casually or ceremonially. Livers, kidneys, and eggs yield only a few calories next to that muscle. Most cultures account animal muscle as the best of foods, and rich ones rear animals solely to put muscle on the table. We even eat our own muscle-beyond episodes of cannibalism. In extreme starvation, when our bodies run out of fat, muscle gets mobilized by our metabolic machinery.
The urbane urbanite uses verbal tricks (making pig into pork, calf into veal, cow into beef) to disguise that link between barnyard and kitchen, at least for English speakers and their mammals. In odd contrast, we take our fowl at face value even when we're affluent enough to put two chickens in the fleshpot. Farm families, though, know quite a lot about that which they eat. My son, at age eight, remarked on the bad taste shown by the North Carolina State Fair, where, in full view of the pigs, hung a large poster illustrating the various cuts of pork. Some farmers at least distinguish between animals personal and animals victual: "Don't name it if you mean to eat it."
A decade or so ago I taught a course for adult nonscientists that looked at various mechanical aspects of existence. In one session I took apart the hind leg of a lamb, purchased at the nearest supermarket, to show how its diverse bones, muscles, tendons, and joints enabled the lamb to gambol about. Midway through the demonstration, one student, a person of some substance in our local computer industry, blossomed out with that look of epiphany that brings joy to any teacher. I paused for some comment on the subtle biomechanical role of the kneecap. Instead what I heard was "Muscle-you mean that that's what meat is?"
And that's what this book is about. Muscle, the prime mover of human history, aka meat: how it works, how we work it, and how the nature of this engine has shaped our history, our cultures, and our technologies. We're animals. Whether we're more than animals turns on one's theology, but we're at least animals. It would be strange, indeed, if our animal nature made no difference to history, culture, technology. Nothing is more animal than movement-animation, in a word-and underlying our every movement are our muscles.
Let's talk not about some substance called muscle but about a muscle in particular, even if we pick a generic and paradigmatic example. Removing the rest of the animal, then, what have we left? A flabby, spindle-shaped bit of flesh, thick in the middle and tapering off into the shiny tendons at either end, as in Figure 1.1. If this happens in an undergraduate physiology lab, the muscle is most likely the great calf muscle of a frog, its gastrocnemius (yes, the c is pronounced, as a k). Extending the foot rapidly and forcefully matters for both jumping and froggy swimming, so frogs have fine calves. Since some of us occasionally eat the leg muscles of frogs, the truly compulsive reader should be able to get a look at this particular bit of anatomy with no more than a trip to the gourmet meat market.
Muscle acts simply. Appropriately stimulated by nerves, it draws its ends closer together, or at least it pulls on its ends, trying to bring them close. In the case of both the frog's calf and our own, the lower tendon runs over the heel, so that action makes the foot protrude. A frog jumps; a person's heel (and body) get pulled up, leaving only the toes for support. Still, no muscle can make ends meet. It ordinarily shortens by about 20 percent of its length or, under unusual conditions and at lower power and efficiency, by 30 or 40 percent. Some muscles of notably high effectiveness barely shorten at all; the flight muscles of many insects contract less than 5 percent. So it's a short-stroke engine that usually needs help-levers and such-for most tasks. Nor can muscle lengthen without assistance. So it needs yet more help in the form of springs, other muscles, or hydraulic devices if it's to make more than one contraction per lifetime. Not that such behavior characterizes muscle alone. Burning fuel drives a piston of an automobile only a relatively short distance in one direction, and the piston needs either a flywheel or the action of other pistons to complete its cycle, not to mention a transmission to convert its motion into something appropriate for driving a set of wheels.
Not even its associated levers and such suit muscle for all tasks. How hard can you hit? You can hit harder if you first grab a weighted stick, a hammer. How fast can you send off a projectile? You can propel it faster if you use a golf club, baseball bat, or spear-thrower. Nor did muscle-amplifying machinery originate with humans. Galileo first showed that, neglecting air resistance, all muscle-powered animals, whether fleas, grasshoppers, galagos, or gazelles, should be able to jump to the same height, roughly three feet. But the flea, being so small, suffers mightily from air resistance, which ought to reduce its height over 80 percent. Undeterred, it remains fully competitive in the high jump. It offsets air resistance by using a cunning bit of elastic and a trigger to store up and then suddenly to release a burst of muscular energy, doing just what the ancient mariner did with his crossbow and the vaulter does with the pole.
Even levers, elastic elements, and triggers can't circumvent every limitation of the basic machine. Consider what happens when you lift something: You do some work, an amount equal to the weight of what you lift times the height to which you lift it. If that's what the physical scientist means by work, then when you simply hold something aloft, you do no actual work. So that task should take no energy, if we define energy (as we do) as the capacity for doing work. Is something wrong here? Since you get tired doing it, you know that holding something up takes energy. Your muscles clearly work even though you do no work in a strictly physical sense. The problem, not merely a definitional one, comes down to a distinct drawback of a muscular engine. A rope can hold something aloft indefinitely without expending any energy. For all its billion years of experience and perfection, muscle hasn't figured out how to lock up at some shortened length so as to hold that length without further work. Indeed, the counterintuitive feel of the physicist's definition of work comes from just that need to use energy when your muscles do no more than hold a weight aloft.
Oh, yes, while we're on the queerness of this engine, here's another, one, though, of language and habit. We speak of muscle "contraction," but does muscle actually "contract"? A simple test shows that it doesn't. Immerse yourself in a pool or lake, retaining just enough air in your respiratory system so you barely float. Now squeeze as many of your muscles as you can. If they really contracted, you'd get a little smaller. Your mass being unchanged, you'd be denser. If you were denser, you'd sink. But you don't sink when you "contract" your muscles because they don't really contract. A muscle may pull its ends together, but in doing so, it gets thicker, enough so that its volume remains the same. For that matter, a "contracting" muscle may not even get shorter, as when you hold up a weight or push on a wall. It can even get longer, as when you try to stop something, such as a baseball, that's approaching you at high speed.
A few more items at the start to give the flavor of what will follow:
• Recently I spent a few days brainstorming with kindred eclectic souls for a company intent on producing some appliance that would transform humans (ideally a large and profitable number of humans) into efficient swimmers. We do swim, but by the standards of any salmon or seal even the most highly trained and talented among us is laughably bad. In cardiovascular equipment we rank among the better-endowed mammals, and we're not seriously short of muscle. The main troubles trace to our nonstreamlined shape and the location of all that muscle; fundamentally we're terrestrial walkers and runners, not aquatic swimmers. So what would be required to make proper swimmers out of us? What kind of snap-on or strap-on gear might harness our muscles to the task?
The specific questions may be of immediate and commercial interest, but they exemplify a problem far older than recorded history. We want to do more than directly grab, pull, bite, run, and lift. So we make tools, from levers and cranks to vaulting poles and micromanipulators, designing each to couple effectively to our muscular engines. We want to harness the power of domestic animals, so we make plows and windlasses, even treadmills and carriages. Again coupling proves critical, and according to well-regarded historians, improvements in coupling mark turning points in human history.
• Usually biology draws on physics, but on rare occasions biology gives something in return. Muscle has made at least two contributions to the physical sciences. Among the key people in the development of the principle of energy conservation in the mid-nineteenth century a pair came from biology. Julius Robert von Mayer was a physician, and Hermann von Helmholtz began as a physiologist. Muscle played at least an indirect role in getting Mayer on the right track, while for Helmholtz the relationship between muscle's heat production and metabolism was central. We can't any longer imagine doing physics-indeed science of almost any kind-without that energy conservation principle, what we call the first law of thermodynamics. The other occasion came earlier, in the late eighteenth century. Luigi Galvani found that he could use the twitch of an exposed frog muscle to detect electricity, as what we now term a galvanometer. Fortunately for the frogs, this shocking bioassay for electricity soon passed from the scene; voltage and other electrical phenomena, as emphasized by his contemporary Alessandro Volta, transcended the world of organisms. Incidentally, Galvani used just that calf muscle mentioned a few pages back. Some experimental systems long retain their utility!
For that matter, the muscle needn't be exposed. I used to ask students in a biology lab to move small pairs of electrodes around on their forearms, giving themselves a mild electric shock at each location and noting the particular finger movements that resulted. With patience (and a little fortitude) each could thus map the locations and determine the actions of the main muscles of the forearm. A look at the anatomical drawings of the great Vesalius (1514-1564) then confirmed the result. But that stimulating exercise required a less sensitive (sorry, again) era.
• How hard can a muscle pull? Or, to put it another way, with how much force would we have to stretch a muscle just to counteract its contractile force? That depends a little bit on which muscle we look at. A force of 30 is typical, although the range extends from about 15 to 140 pounds per square inch (100 to 1,000 kilopascals, or kPa, in standard scientific notation). As rope muscle is wimpy. A steel cable with a cross section of a square inch can withstand a pull of over 50,000 pounds (350 megapascals, or MPa), around a thousand times more. But the cable of course can't contract and act as an engine. Of more relevance here, we can compare the tensile strength of muscles with that of tendons, the things to which muscles ordinarily attach. Tendons will withstand pulls of 15,000 pounds per square inch (100 MPa) before they break, over a hundred times what a muscle can manage. Figure 1.2 puts these comparisons in graphic form. Appropriately enough, that calf muscle we considered earlier has a belly that's a few hundred times thicker (in cross section) than the tendons on its ends. So those spindle-shaped ends reflect underlying mechanical behavior, as does so much of our anatomy. As Hamlet put it, "There's a divinity that shapes our ends...."
• How fast can a muscle shorten? That depends on the particular muscle, on both its behavior and its size. Size comes into the picture in a way that's perhaps clearest if one views a muscle as a long series of people linked hand to hand (perhaps on frictionless ice skates). If all of them simultaneously flex their arms, the line will get shorter. If another series of people, twice as long as the first, flex their arms in just the same way, that longer line will shorten twice as fast. So we need to consider an intrinsic speed that adjusts for length. A muscle, then, shortens by about 10 percent of its length in something between about a hundredth and a tenth of a second. To put it another way, a muscle that's a foot long shortens at between ten and a hundred feet per second, or between seven and seventy miles per hour. As a general rule, the faster it shortens, the less forcefully it pulls.
• Work gets done when a muscle shortens against some force that would rather it didn't do so. How much work can a muscle do? That depends on how long we give it to work. So the question is better posed in terms of the power a muscle can deliver, with power defined as the rate at which work gets done. The best muscular systems can sustain power outputs of around 90 watts per pound (200 W/kg). That suggests that a 150-pound mammal (one of us, perhaps) with 40 percent muscle ought to put out over 5,000 watts of power or about 7.5 horsepower! Well, we can't and don't. We just can't supply fuel and oxygen or remove wastes and heat at anything close to the requisite rate. How much power you can put out turns out to depend strongly on how long you're asked to sustain it, even more than on your age and physical condition, neither of these being negligible, by the way. The longer the run, the slower the average speeds. The issue becomes critical when you design human-powered aircraft, which have high minimum power requirements.
As terrestrial mammals go, we humans go decently enough, up there with dogs and horses among the sustained runners. What distances people, dogs, and horses from less competent cats and cows isn't so much our muscles, though, as our cardiovascular equipment. Oxygen supply defines what an animal can do, and we do pretty well on that score. Still, runners of any species finish behind fliers-birds, bats, and insects-and sustained swimmers: billfish, seals, whales. The difference may, once again, come back to muscle. The complexity of the motions involved in running with legs may preclude equally effective use of large masses of muscle.
Excerpted from Prime Mover by STEVEN VOGEL Copyright © 2001 by Steven Vogel
Excerpted by permission. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
|2||How Muscle Works||10|
|3||And How We Found Out||30|
|4||Flying High, Making Noise, and Clamming Up||53|
|5||Knowing What We're Doing||74|
|6||Connecting Up Muscles||95|
|7||Using Hand Tools||126|
|9||More Tough Tasks||184|
|10||Bringing Animals to Bear ...||209|
|11||Bos versus Equus||234|
|12||Killing Tools: The Big Picture||256|
|13||Wielding the Weapons||281|
|14||Muscle as Meat||299|
|15||Pulling Things Together||315|