Endurance superstars may be 'born to run.'

Endurance superstars may be 'born to run'

A study of muscle chemistry in world-class long-distance runners suggests these athletes' ability to "reach levels of physical endurance unattainable by all but a very few persons" lies largely in their genes, researchers report in the December PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (Vol. 85, No. 23).

Scientists have known that different people have different proportions of endurance, or "slow-twitch," muscle fibers relative to "fast-twitch," short-energy-burst fibers. They have also documented that training can alter the proportion of these two types. They have debated, however, how large a role genes play, says study leader Jane H. Park of Vanderbilt University in Nashville, Tenn.

"I think this is one of the first times people have said that genes are important [in determining human endurance performance]," Park told SCIENCE NEWS. One research team had suggested a genetic component when it found an "unexpectedly high" number of slow-twitch fibers in the untrained muscle of certain endurance athletes. However, "there are no earlier measurements of metabolites in untrained muscle of athletes to our knowledge," Park and her colleagues write.

They found that the untrained wrist muscles of the four long-distance runners studied maintained force better during prolonged exercise -- involving repeated wrist flexion against a resisting bar -- than did those of five healthy but relatively sedentary sex-matched individuals of comparable age. By measuring muscle metabolites -- chemicals involved in muscle contraction -- the researchers determined that the athletes' muscles sustained their chemical energy reserves much better than did those of the non-athletes. And indicators of muscle acidity showed that the athletes, unlike the controls, did not supplement their energy supplies through anaerobic muscle metabolism, which causes muscle fatigue and lactic acid buildup, according to Park.

Instead of examining muscle composition by surgically removing tiny bits of muscle tissue, Park and her co-workers placed the subjects' arms in the bore of a superconducting magnet and used magnetic resonance spectroscopy to monitor how the phosphorus in various muscle metabolites reacted to the magnet. This noninvasive method enabled them to collect a large quantity of data on a minute-to-minute basis. It also allowed them to examine a larger, and thus more representative, mass of tissue than possible with biopsies, they say.

The researchers found that, both at rest and during exercise, the athletes' untrained wrist muscles maintained higher levels of adenosine triphosphate, which supplies muscles' chemical energy, and phosphocreatine, the "storehouse" of adenosine triphosphate. The athletes "start out with a bigger storehouse [of energy] and maintain a bigger storehouse all the way through the exercise," Park says.

Muscle acidity increased in the non-athletes at the start of exercise, indicating the use of lactic acid-producing anaerobing metabolism, which quickly peters out. Acid buildup caused the non-athletes' performance to decline by tiring their muscles, which had too few slow-twitch fibers to sustain the previous level of force, Park says. No such acid increase occurred in the athletes' muscles.

Although athletic training is known to increase heart and lung capacity, the experimental exercise protocol did not significantly increase participants' pulse and respiratory rates, say the researchers. They note, however, that "additional systemic effects of training cannot be completely excluded."