Editor’s note: We are pleased to present a series adapted from biologist Michael Denton’s book, Fire-Maker: How Humans Were Designed to Harness Fire and Transform Our Planet, from Discovery Institute Press. Find the whole series here. Dr. Denton’s forthcoming book, The Miracle of the Cell, will be published in September.

Standing up and maintaining our upright posture is only possible because we possess muscles of sufficient power to resist the downward pull of gravity. Even a trained athlete finds it hard to lift much more than his own weight above his head. An ant, however, can easily lift many times its own weight, without training and seemingly without effort, and carry it back over all manner of obstacles to the nest. How is it that an ant appears proportionately so much stronger than a trained human weight lifter? In his book Scaling, Knut Schmidt-Nielsen explains why: “When we see an ant carrying in its jaws a seed that weighs more than the animal itself, we gain the impression that its muscles must be inordinately strong. However, measurements of insect muscles show that they are not stronger. In fact, they exert the same force per unit cross-sectional area as vertebrate muscles.”¹

A Matter of Scaling

The answer is related to size. With decreasing size of an animal, its volume, or mass: 

decreases in proportion to the third power of L [its length measurement], but the cross-sectional area of muscles (which determine the force they can exert) decreases only as the square of L. Thus, the force exerted by muscles, relative to mass, increases in proportion to the decrease in L. This is the reason that the ant appears to have muscles of unmatched strength.²

Again, it’s a matter of scaling! As size increases, the mass or weight of an organism increases by the cube of its length, while muscle power only increases by cross-sectional area, i.e., the square of its length! This means that the power of muscles imposes yet another limit on our size as an upright bipedal organism in addition to the fact that the strength of bones varies, as J. B. S. Haldane points out, as their cross-sectional area (L²) while the kinetic force (breaking force) imposes another limit on possible height. 

If the force exerted by muscles were not very close to what it is, large terrestrial organisms — our size and bigger — that must resist the downward pull of gravity and must face the L²/L³ “scaling challenge” would be impossible. It is worth noting in this context that although the size range of organisms is enormous, we are just about as big as most organisms get,³ and certainly we are just about as big as an upright bipedal organism could be. As Steven Vogel points out, “Only a little more than an order of magnitude separates us from the largest living things, but six to seven orders lie between us and the smallest.”⁴ (Emphasis added.)

Tomorrow, “Appreciating the Design of Human Muscles.”

Notes

1. Knut Schmidt-Nielsen, Scaling: Why is Animal Size So Important? (Cambridge: Cambridge University Press, 1984), 210-211. 
2. Ibid., 210-211.
3. Vogel, Comparative Biomechanics, 18. 
4. Ibid.