Terry Schlenz loves tennis so much, he wants to play it until he can’t move his arms anymore.

That day, he said on Saturday after playing a doubles match at the second annual Napa Valley Invitational Wheelchair Tennis Tournament at Napa Valley College, could come as soon as this winter.

The Granite Bay resident can’t move his legs as it is, thanks to amyotrophic lateral sclerosis. Often referred to as Lou Gehrig’s Disease, ALS is the progressive degeneration of the motor neurons that send impulses from the brain to the spinal cord and from the spinal cord to the muscles throughout the body. This causes the muscles to deteriorate.

Schlenz, 58, was diagnosed last summer and immediately confined to a wheelchair. He picked up wheelchair tennis in October, starting out in his regular chair before buying an expensive athletics chair that has angled wheels for easier turning and extra wheels in the back to prevent tipping.

“I’d played tennis off and on since I was 16, but I’ve played more in the last year than in my entire life before that,” he said. “You appreciate things more when you know you only have a limited time to do them. I don’t know when it will all come to an end. I’m losing all the strength in my body and by winter I may not be able to push the chair or hold onto the racket any more.”

Schlenz spoke matter-of-factly, not like one might expect a person for whom practicing will do no good.

“I’m not improving any more. I’m getting worse,” he said. “I reached my peak about three months ago. I’m still very competitive, but I’m more doing it because I can and I love it and I’m just trying to enjoy as much of life as I got left.”

Schlenz embraced life even before his diagnosis, delving in water and snow skiing, tennis, racquetball, wind surfing, bowling and golf. When he became disabled, it wasn’t long before he was doing marathons in Sacramento and Los Angeles on a hand bike — before his arms became to weak for it.

He said wheelchair tennis is more intense than any other sport he’s done.

“There are so many things people who are in wheelchairs can’t do, but this they can do, so they really go after it,” he said.

Tournament director Jeff Stuhr said that 23 players signed up to play, most from Northern California but none from Napa County. One drove all the way from Florida, only to have to withdraw due to a shoulder injury. The tournament began with Up-Down doubles matches on Friday, featuring able-bodied and wheelchair players on the same teams. It wraps up today with semifinals and finals, beginning at 9 a.m.

About 20 people volunteered to chase down ball at the nets, many of them high schoolers. Among them were Napa High School senior-to-be Nathan Miller, the Indians’ No. 1 player, and 200O NHS graduate Heath Rosa.

Miller — who played in the Union City Junior Open on Tuesday and lost his second 18-and-under match to the eventual champion — said Napa High teammate Sean Day also volunteered on Saturday.

“They’re really good players and they can move around the court pretty well,” Miller said of the wheelchair players. “(Their strategy) seems to be a lot about placement.”

Rosa heard about the volunteering opportunity from a tennis class at NVC.

“I watched this tournament last year and got interested in it, so I thought I’d come out and be a bigger part of it this year,” he said. “It’s awesome watching the quickness they have to get to the ball, and some of their serves blow me away. They move into the right positions and they can read the spin because they can’t accelerate or decelerate really fast.

“To have the determination and love of the game to come out in a wheelchair and do it is awesome.”

Kentfield’s Dylan Young came to play doubles with returning player Johnny Rios of Cupertino. Both have been in wheelchairs since birth.

Young said he played tennis in his youth then resumed it about two years ago.

“I didn’t practice much, so I got frustrated and took a 12- or 13-year hiatus,” he said. “Now I’m more patient. I practice a skill rather than just try to be faster and stronger.”

Young said he’s also played wheelchair rugby, on basketball courts, and said it wasn’t as bruising as able-bodied rugby.

“It’s mostly rough on the chairs, like bumper cars, and it’s really high-scoring,” he said.

Sharon Kelleher and Don Beardon were the only Open-Division players on hand Saturday, so the San Jose residents played each other. Kelleher won the first set 6-4 but lost the next two by the same score.

“I beat him in Manteca two years ago,” she said of Beardon.

Kelleher will be the director of another wheelchair tennis tournament, the Golden State Challenge, Aug. 30-31 at Stanford University.

Kelleher was ranked No. 1 in the country when she competed in the Sydney Paralympics in 2000, and then top-four when she played in the 2004 Athens Games. She lost in the quarterfinals to the same Japanese opponent both years, in both singles and doubles.

“It was a great experience,” said Kelleher, who has also played in Brazil, New Zealand, Japan and Thailand.

She’s been playing wheelchair tennis for 22 years, since a car accident during her senior year at La Canada High in Southern California left her paralyzed.

“I used to play soccer and when I couldn’t do that anymore, having a new sport really helped me start to think about the future and have new goals,” she said. “I totally stopped thinking about what I couldn’t do anymore. It made me feel a lot better about myself, to know I still had athletic skills and could still be successful at something. I work with (wheelchair-bound) kids and sports are really eye-opening for them, to realize they can do a lot more than they though they could.”

She’s been married four years to her able-bodied husband, Patrick, who sings in a choir with the wife of her tennis coach.

“One of the things that attracted me to her,” he said, “was that she doesn’t take life sitting down.”