ENDURANCE EXERCISE might be good for your piece of mind, but how about your muscles? Physiologist David Hood is studying the way muscle responds - at the genetic level - to endurance exercises like swimming, jogging and cycling.

The key to muscle endurance is tiny organs within muscle cells known as mitochondria, says Hood, a professor in York's kinesiology department. They work like batteries providing energy to the cell. The more workouts, the greater the mitochondria, the more mitochondria, the greater your endurance.

There appears to be no biologically-determined upper limit to the number of mitochondria a muscle cell can have (the heart, which works out 24 hours a day, has the highest mitochondrial content of any body tissue). "The number of mitochondria is really limited by our ability to train hard," Hood says.

But exactly how exercise increases the number of mitochondria is still unknown. Hood thinks the answer to the problem probably lies in the communication within the cell between mitochondria and genes. "We're really studying the problem at the level of gene transcription," he says.

Gene transcription is the process whereby a gene produces a desired protein. Within cells, proteins do the work, and genes can be thought of as "blueprints" for them. Somehow, exercise stimulates the mitochondria to send a signal that calls for the creation of more mitochondria. The signal seems to have something to do with calcium, which is essential for a muscle cell to contract, but which also appears to "turn on" the genes that are involved in building mitochondria themselves.

Hood's work could have medical applications down the road such as preventing the muscle-wasting that occurs when a broken leg or arm is in a cast, he says. It might also be useful to astronauts who lose muscle endurance after long periods in space.

Illustration: Celia Calle