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 seeks to change that. The workout runs participants around the court urgently enough to keep their heart rates in the cardio-training zone (65 percent to 85 percent of maximum heart rate) while helping improve tennis skills. It's billed as similar to an interval workout.

(There are 15 sites in Hawai'i, according to its Web site, www.cardiotennis.com)

What happens during Cardio Tennis? On an indoor court at Sport Fit Bowie in Bowie, Md., instructor Kevin McClure starts five participants with warm-up footwork moves, followed by a repeating drill in which participants 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.

The class then picks up racquets and hits real balls that McClure feeds from the other side of the net. Miscues aside, this drill elevates heart rates and provides court sense and stroke work for everyone in the group, despite our varying skill levels.

Plus, there's the boombox. Like most get-fired-up-to-sweat fitness classes, Cardio Tennis has an audio component — the joy of smacking balls while singing along to, say, "Johnny B. Goode" (and in the case of McClure's class, 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. All do three bouts of this drill.

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.

One 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.