Tennis can be a great workout if you have skilled, competitively matched partners playing singles. For others, the game is more stop than go, sprinkled with cuss-muttering frustration: hit, run, whack ball into net, stop. Hit, return, re-return(!), stop, curse, hail taxi to retrieve errant shot, etc. Entertaining, yes, even social. But not a heart-rate booster.

Cardio Tennis, a national program being offered at a variety of venues in the Washington area, seeks to change that. The workout runs participants around the court urgently enough to keep their heart rates in the cardio-training zone (65 to 85 percent of maximum heart rate) while helping improve tennis skills. It's billed as similar to an interval workout. Armed with a racquet, a watch and my well-honed ability to count heart beats via my carotid artery, I showed up to give it a try.

On an indoor court at Sport Fit Bowie in Bowie, instructor Kevin McClure starts me and four other participants with warm-up footwork moves followed by a repeating drill in which we hustle from one spot to another, squaring up to imaginary balls and swinging imaginary racquets, culminating in a phantom overhead slam down a nonexistent opponent's throat. No unforced errors!

We then pick up our racquets and hit real balls that Kevin feeds us from the other side of the net. Miscues aside, this drill elevates our heart rates (mine to around 60 percent of maximum) and provides court sense and stroke work for everyone in the group, despite our varying skill levels: I am advanced-intermediate (around a 4 on the 0-to-7 scale used to sort tennis players); sisters Alicia Bousman and Kim Brimigian are beginners; Nancy Roderick is about a 3 player and Jonathan Ross, Sport Fit Bowie's director of personal training, is about my level.

Oh, did I mention the boombox? Like most get-fired-up-to-sweat fitness classes, Cardio Tennis has an audio component, allowing a musical throwback like me the joy of smacking balls while singing along to "Johnny B. Goode" (and drawing glares from players on nearby courts).

The next exercise involves a sequence of about 15 rapid-fire down-the-sideline shots with, after each shot, a footwork-enhancing shuffle around a cone set up on the baseline. We each do three bouts of this drill. This pushes my heart rate above 75 percent of max. But since there is room for only two players to do this at once (one on each sideline), my pulse dips below the cardio zone between my turns.

That respite is brief. The class moves on to a brutal set of crunches, performed while playing catch with a medicine ball; then a circus-like volley-and-overhead drill with three players dancing around the same side of the net; and a three-on-three game that simulates live court play.

My only nitpick: In feeding balls to multiple people simultaneously and emphasizing motion, McClure is unable to really examine stroke form the way he might during a one-on-one lesson (though he does shout occasional corrective instruction). In fairness, Cardio Tennis is marketed foremost as a great workout, not a clinic. And Brimigian, who attends two classes weekly, reports that her skills have steadily improved.

"I initially joined just for the cardio," she said, "but now I'm really into the tennis."

The hour-long class I took costs $14 for non-members of Sport Fit Bowie. Details on Cardio Tennis and class locations at www.cardiotennis.com.

Come to the net for the Moving Crew's online fitness volley, today at 11 a.m. at www.washingtonpost.com.

-- John Briley

Cardio Tennis adds high-intensity balance, agility and speed drills to conventional stroke work.