“The fear of getting really big is not plausible for most people,” he said. Competitive distance runners and cyclists, who are naturally slender and light, “don’t have the muscle fiber number to get really big,” Dr. Kraemer said. “I can train them until the cows come home and they are not going to have big muscles.”
But for his legs, he does exercises like leg extensions using one leg at a time, to correct any muscle imbalances or weaknesses. Mr. Martin, who lives in Lawrenceville, N.J., said he got the idea from coaches and from his own reading.
“Cycling and running are one-leg-at-a-time activities,” he explained. And one-legged exercises “recruit more muscles that help the hips.”
“If your goal is to improve running performance, then weight training should probably mimic the running pattern,” he said. “If you do leg extensions, you can get stronger, but people don’t run like that.”
DR. O’CONNOR, for example, lifts weights for health, for enjoyment and for vanity’s sake (he does not want an emaciated upper body, he said), but stops lifting when he is training to run a marathon. Those muscles, he said, “are just dead weight you have to carry around.” He adds that a sport like rowing, swimming or running requires specific muscles and nerve-firing patterns that may best be developed by actually doing the sport