How (living) things move

kw: book reviews, nonfiction, biomechanics

We take motion for granted. For most of our lives, from about the age of one until we are in our dotage, we walk, trot, jog, run, and jump; we also creep, crawl, sidle, and shimmy. Most of us dance in one way or another. Familiar pets also move about, as do animals in general. That is what defines animals: the ability to move with intention and comparative rapidity.

How many of us give any thought to how we move? Eadweard Muybridge was one who did more; he set up groups of cameras, either in a line or in a cluster, and set them to trigger in sequence while someone, or himself, walked or ran or performed some motion of interest.

This sequence of photos, from three different angles, of a woman performing an underhand serve in lawn tennis, was captured in 1887.

The word "biomechanics" has been around since the 1860's, possibly coined by Sir Norman Lockyer for a series of articles in Nature. But it remained a niche discipline for a century, until the late 1960's and the great expansion of cross-discipline work among scientists, the "breaking down of the stovepipes", so that physicists and mechanical engineers and biologists could collaborate more freely.

For the book Exploring Biomechanics: Animals in Motion, author R. McNeill Alexander could have taken various approaches. He chose a functional arrangement: beginning with a discussion of muscles and their mechanisms—and noting that non-muscular mechanisms of motion would be discussed as he came to them—he gathered his material into chapters on

• Running and walking
• Jumping, climbing and crawling
• Gliding and soaring
• Powered flight (such as flapping)
• Floating (in water)
• Swimming
• Non-muscle motions, primarily of microbes and protozoans

I was particularly intrigued by "Floating". I had not thought of that as a biomechanical area. But it turns out there are several ways that sea creatures keep from sinking into the abyss, other than to just keep swimming (Of course, one way is to live in water shallow enough that one can safely rest on the bottom). I knew that bony fishes usually have gas-filled swim bladders, and I also knew that deep-sea squid such as the giant squid have lots of ammonia in their tissues to reduce their density (and that's why if you have a grilled giant squid steak at the Explorer's Club it will taste like soap!). There are several other passive flotation methods. None is perfect, but each serves a purpose for a certain group of animals. The most extreme to my mind is the semi-cohesive, fatty "jelly" of a jellyfish. I didn't realize that 95-99+% of a jellyfish is nonliving, low-density jelly, surrounded by a layer of living tissue only one or two cells thick!

A lot of biomechanics has to do with ratios. The ratio of limb length to body size tells a lot about lifestyle: short legs on a long body (think dachshund or mole) are good for tunnel running or tunnel digging; long legs and a flexible backbone are for speed (such as the ongoing contest between antelope and cheetah); long, slender wings are good for soaring but not so good for speed runs; the short, wide wings of a hawk give it speed and maneuverability; and on and on.

In an epilog "What we want to know next" the author emphasizes that most past study of animal flight was based on our understanding of airplanes. Helicopters and gyrocopters are a better model, because in birds, bats, and insects, the flight surfaces are dynamic elements, not static. The details of their dynamic motion allow greater efficiency than any static wing can hope to accomplish. But dynamic aerodynamics is an infant field. In fact, as much as we may know already, we are nearer the threshold of biomechanical studies than we are to the finish line.