Biomechanics

The science of biomechanics applies mechanical principles to the study of organisms. Biomechanics uses mathematical models and computer simulations to study living organisms, in addition to direct biological measurements.

Biomechanics helps us understand limitations on the size of organisms, problems with scaling, energy efficiency, the advantages of internal versus external skeletons, and other concepts. Biomechanics can even help biologists understand animal behavior, such as how a whale can remain submerged for extended periods of time.

For example, the largest single-celled organisms are protists about the size of the period at the end of this sentence. There are larger cells that are part of multicellular organisms, but no single-celled organisms. So why are there no large, single-celled animals? The primary restriction is surface-to-volume ratio. A cubical cell 100 μm on a side has a volume and mass 1,000 times as great as the volume and mass of a cell 10 μm on a side. This larger mass requires roughly 1,000 times as much oxygen, food, and water. It also produces 1,000 times as much waste that must be excreted.

Where does the cell exchange all this material? Exchange takes place through the cell membrane. But the cell membrane of the larger cell is only 100 times as large as the smaller cell, so 1,000 times as much material must pass through a membrane only 100 times as large. If the cell membrane is wrinkled and folded its area is increased, but the cell will ultimately reach a point where it will be unable to feed or breathe through the membrane. This places a practical limit on the maximum size a single-celled organism can attain. Large organisms must be multicellular and have a complex system of specialized cells that can transport food, oxygen, and waste.

If you compare a house cat (Felis sylvestris) and a Bengal tiger (Felis tigris), it is obvious that multicellularity is not a sufficient solution to the problems of scaling up an organism to larger size. Weight is proportional to volume, so weight increases with the cube of height. Muscle and bone strength isproportional to cross-sectional area and increases with the square of height. This means the tiger requires much thicker legs than the house cat, relative to its overall size, to support its larger mass and move quickly.

A detailed biomechanical study of the effects of scale that considers factors such as weight, air resistance, muscle strength, heat loss, and bone stress can explain some surprising observations. For example, an impala, a domestic cat, a domestic dog, and a domestic horse can all jump to roughly the same height above the ground. Biomechanics helps us understand why. Biomechanics can also explain why large whales (air-breathing organisms) can remain submerged for a long time compared to small dolphins and seals. Underwater, body size is advantageous. In contrast, large hawks can only hover for a short time, whereas hummingbirds, kestrels, and kingfishers can hover for extended periods. In the air, large size is a disadvantage.

One of the most productive applications of biomechanics has been in the field of athletic competition. Coaches study the principles of biomechanics to learn how to improve the performance of the athletes they train. Ideas of conservation of angular momentum from physics can help coaches teach athletes how to improve their ability to throw a discus or put the shot. Energy conservation helps marathon runners learn how to train more effectively and run more efficiently.

The biomechanics of running, especially amateur running, has been an area of intense research and interest. Some sports doctors videotape their patients to study abnormalities in their gait that have the potential to cause injury. Doctors can then prescribe shoe inserts or other shoe modifications to help prevent injury. They may also recommend a change in running style or training regimen based on a runner's idiosyncrasies. For example, a doctor might notice that the runner is swinging his or her arms across the body. This causes an excessive rotation of the pelvis, which can lead to hip pain. If this is the case, the doctor may train the runner to move his or her arms parallel to the direction of motion.

Another important area of research in biomechanics is automobile safety design. Most people have seen films of crash-test dummies. Crash-test dummies are designed to simulate humans. Their joints move the same way that human joints move. By analyzing how car accidents affect the dummies, engineers can design safer automobiles.

More recently, biomechanics is moving toward computer models that can be used. The advent of fast, powerful computers and improved mathematical models make it possible to analyze the effects of a crash on humans with greater accuracy and less expense than is possible through mechanical simulations such as dummies.

Cavanagh, Peter R. "Biomechanics: A Bridge Builder among the Sport Sciences." Medicine and Science in Sports and Exercise 22, no. 5 (1990):546-557.

Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishing, 1989.

Hubbard, Mont. "Computer Simulation in Sport and Industry." Journal of Biomechanics 26 (1993):53-61.

Huxley, Julian S. Problems of Relative Growth. New York: Dial Press, 1932.

Purves, William K., and Gordon H. Orians. Life: The Science of Biology. Sunderland, MA: Sinauer Associates Inc., 1987.

Shorten, Martyn R. "The Energetics of Running and Running Shoes." Journal of Bio-mechanics 26 (1993):41-51.

Thompson, D'Arcy W. On Growth and Form. Cambridge, U.K.: Cambridge University Press, 1942.

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