When Jimmy Connors was 21, he trained hard and won tennis matches. At age 39, he still trains hard -- and that's one reason why he is in the U.S. Open semifinals, say exercise specialists.
Indeed, keeping his muscles and reflexes in top shape helps Connors compensate for the physiological effects that everyone experiences as middle age sets in, they say.
"The normal decline in power and speed is not as great for someone like Connors only because he has kept up his training," said Arlette Perry, director of the Human Performance Laboratory at the University of Miami.
"If you can keep the intensity of the training program up close to what it was in your 20s or early 30s, you don't lose too much," Perry said.
Connors, who met Jim Courier, 21, in a semifinal match yesterday, has been able to train consistently over many years because he has rarely had to back off because of injury, noted James Graves, a researcher at the University of Florida's Center for Exercise Science.
"If he had let himself get out of condition, then he would have a lot of trouble not only recovering from the matches, but also maintaining the stamina needed to carry him through five sets," Graves said.
The same can be said for Martina Navratilova, 34, who has turned back the clock in this year's Open and Friday defeated top-seeded Steffi Graf to reach her eighth Open final and first since 1987.
Still, athletes do face undeniable physiological changes between ages 30 and 40, said James Skinner, director of the Exercise and Sports Research Institute at Arizona State University.
"The aging process occurs all your life, but it's masked by growth and development until about the age of 25. Then growth stops, and around age 30 you begin to notice the aging that's been going on all along," Skinner said. "You're losing cells, you're losing function in every system of the body."
Among the results:
Reaction time and speed are the first skills to decline, because the body loses "fast-twitch" muscle fibers responsible for quick movement.
Later, strength falls as other types of muscle fibers also atrophy.
Flexibility decreases because the body's connective tissues contain less water.
Joints take a harder pounding as cartilage is worn away or compressed over time.
Agility and coordination also drop off, partly because muscle fibers no longer respond as efficiently to the chemical signal that causes muscle cells to contract.
This nerve-cell chemical, acetylcholine, works by attaching in a lock-and-key manner to "receptors" on muscle-cell walls; but with aging the receptors decrease in number, said Joe Signorile, of UM's Human Performance Laboratory.
The middle-aged body needs more recovery time after heavy exercise -- at least 40 hours -- to replace energy stores and repair microscopic damage. An older player uses up more stored energy, move for move, than does a younger one, Skinner said.
"His maximal aerobic capacity is dropping off, so he's working at closer to his maximum capacity than a younger person would be who was similarly trained," Skinner said.
Connors, who already has played two four-hour matches at the Open, could hurt his play in another long match, the exercise experts said. A younger opponent probably will have recovered better than Connors, they said.