People with MS benefit from special exercises.(The Orange County Register)

Michelle Hamel misses many things about tennis. Sprinting to make a volley. Holding the body in perfect balance to serve.

Hamel, 40, of Fountain Valley, Calif., hopes she will be able to play again. But for now, she's grateful that she can be active in other ways. Hamel has multiple sclerosis, a chronic disease that attacks the nervous system.

There are 400,000 people nationwide with the disease.

Symptoms of MS vary. Some people experience one or several of the following: muscle weakness, poor coordination, unusual fatigue, muscle tightness, spasticity and paralysis.

Over 15 years, Hamel developed balance problems and needs the help of a walker or wheelchair. But Hamel attends strength-training and flexibility classes at local colleges.

"I refuse to be defined by my disability," she said. "I can do many things I've been doing. It just takes me longer and I may fall."

Hamel will attend Fitness for All, an all-day event Sunday in Huntington Beach, Calif., in which people with MS learn about how to stay active while living with the disease. Hamel attended last year and found it empowering.