MONDAY, April 4, 2005 (HealthDay News) -- If you can't finish the marathon or can't even start one, try looking at your family tree.
Researchers say animal studies suggest your tendency toward the playing field or the couch could be in your genes.
In a series of experiments that lasted through 15 generations, rats who were bred for performance outran and outperformed their couch-potato counterparts by several degrees.
"This underscores the fact that the genetic basis of our endurance ability is pretty high, surprisingly high," said Richard Howlett, a faculty member in the department of medicine at the University of California, San Diego.
"They [the rats] were destined to be better performers," added Dr. Todd Schlifstein, a sports rehabilitation physician at New York University Medical Center in New York City.
Howlett is co-author of three papers being presented April 5 at the annual Congress of the International Union of Physiological Sciences, in San Diego. The papers are part of a much bigger project involving numerous collaborators.
The project initially involved taking an equal number of male and female lab rats that had been inbred. "The idea is that they are genetically similar so if you take two from the same populations, the differences you see aren't genetic," Howlett explained.
Even in this initial population, some rats ran well on a treadmill while some didn't.
Next, the researchers took the 10 or 12 best males and females and bred them. They also took the 10 or 12 worst-performing from each gender and bred them. None of the rats were trained to run on a treadmill.
After seven generations, untrained rats from the "athletic" line had greater endurance and 12 percent greater oxygen consumption than the "couch potato" rats. The differences were mainly due to differences in skeletal muscle, the researchers said.
"Most of the divergence between the two groups [in oxygen utilization and greater endurance] was at the level of the muscle," explained Howett. "Muscle could take up and use more oxygen but the body didn't necessarily deliver more."
The researchers had presented those findings earlier.
Now, Howlett and his colleagues are presenting results from generation 15, where the differences in performance between the two lines were even greater. There was a 44 percent greater difference in oxygen consumption and endurance between generation 15 and generation 7.
And the biomechanical changes in performance were seen elsewhere in the body. "We saw more changes in cardiovascular and pulmonary function. The structure and function of the heart and the structure and function of the lung had improved," Howlett said. "They were breathing more per minute and pumping more blood and delivering more oxygen to the muscle. The muscles were still different, but everything else has caught up."
The "elite athlete" rodents had hearts that were 16 percent larger and a 33 percent greater blood flow. Lungs were also 17 percent larger and could deliver oxygen more efficiently.
"In terms of performance, it's stark," Howlett added. "These aren't tiny little changes. In terms of running to exhaustion, it's separated by three- or four-fold. Speed is two to two-and-a-half times as much."
What does all this mean for two-legged humans?
While the research suggests that athletic prowess may lie in your genes, Howlettt said, "Unfortunately, for us, we can't pick our parents."
"People who are elite are elite for a reason, although diet and mental things may help you boost performance," he added. "It's not set in stone, but there are still going to be limits."
And translating rat findings into human results can be tricky, Schlifstein pointed out.
Still, it could lead to a lot of other interesting research, including determining which genetic mechanisms may be responsible for the elite rats' enhanced performance. In fact, rats in the two groups are still being bred to take the studies further.
And the research may one day lead to therapies to improve muscular, cardiovascular and respiratory problems in people, the researchers said.
The American Council on Exercise has more on exercise -- even for couch potatoes.