Muscle, genes andathletic performance

Genes
by Jesper L. Andersen, Peter Schjerling and Bengt Saltin

Muscle,

and Athletic Performance

The cellular biology of muscle helps to explain why a particular athlete wins and suggests what future athletes might do to better their odds

Copyright 2000 Scientific American, Inc.

n your marks!” A hush falls as 60,000 pairs of eyes are fixed on eight of the fastest men on earth. Thedate is August 22, 1999, and the runners are crouched at the starting line of the 100-meter final at the track-and-field world championships in Seville, Spain. “Get set!” The crack of the gun echoes in the warm evening air, and the crowd roars as the competitors leap from their blocks. Just 9.80 seconds later the winner streaks past the finish line. On this particular day, it is Maurice Greene, a25-year-old athlete from Los Angeles. Why, we might ask, is Maurice Greene, and not Bruny Surin of Canada, who finished second, the fastest man on earth? After all, both men have trained incessantly for this moment for years, maintaining an ascetic regimen based on exercise, rest, a strict diet and little else. The answer, of course, is a complex one, touching on myriad small details such as theathletes’ mental outlook on race day and even the design of their running shoes. But in a sprint, dependent as it is on raw power, one of the biggest single contributors to victory is physiological: the muscle fibers in Greene’s legs, particularly his thighs, are able to generate slightly more power for the brief duration of the sprint than can those of his competitors. Recent findings in ourlaboratories and elsewhere have expanded our knowledge of how human muscle adapts to exercise or the lack of it and the extent to which an individual’s muscle can alter itself to meet different challenges— such as the long struggle of a marathon or the explosive burst of a sprint. The information helps us understand why an athlete like Greene triumphs and also gives us insights into the range of capabilitiesof ordinary people. It even sheds light on the perennial issue of whether elite runners, swimmers, cyclists and cross-country skiers are born different from the rest of us or whether proper training and determination could turn almost anyone into a champion. Skeletal muscle is the most abundant tissue in the human body and also one of the most adaptable. Vigorous training with weights can doubleor triple a muscle’s size, whereas disuse, as in space travel, can shrink it by 20 percent in two weeks. The many biomechanical and biochemical phenomena behind these adaptations are enormously complex,
Muscle, Genes and Athletic Performance

O

but decades of research have built up a reasonably complete picture of how muscles respond to athletic training. What most people think of as amuscle is actually a bundle of cells, also known as fibers, kept together by collagen tissue [see illustration on pages 50 and 51]. A single fiber of skeletal muscle consists of a membrane, many scattered nuclei that contain the genes and lie just under the membrane along the length of the fiber, and thousands of inner strands called myofibrils that constitute the cytoplasm of the cell. The largest andlongest human muscle fibers are up to 30 centimeters long and 0.05 to 0.15 millimeter wide and contain several thousand nuclei. Filling the inside of a muscle fiber, the myofibrils are the same length as the fiber and are the part that causes the cell to contract forcefully in response to nerve impulses. Motor nerve cells, or neurons, extend from the spinal cord to a group of fibers, making up a motorunit. In leg muscles, a motor neuron controls, or “innervates,” several hundred to 1,000 or more muscle fibers. Where
100 80
Percent of Total Muscle

tion a sarcomere shortens like a collapsing telescope, as the actin filaments at each end of a central myosin filament slide toward the myosin’s center. One component of the myosin molecule, the so-called heavy chain, determines the functional...