ZZ Top's Billy Gibbons is singing the chorus to "My Head's in Mississippi" while the band's driving boogie sounds fill the room where 14 of us furiously pedal our stationary bikes.

As the song winds down, John Hines, our spin instructor, shouts, "Now we're coming to a big hill; it's gonna take you about four-and-half minutes to get to the top. Tighten up the resistance and go until you reach 85 percent, then hold it there."

John turns up the volume and the elephantine beat of Joe Walsh's "Rocky Mountain Way" fills our ears as we stand on our pedals, quadriceps straining to keep a smooth, quick cadence.

I glance at the heart-rate monitor on my wrist. Numbers slowly climb: 146-147-149-150. Sweat drips off my forehead; my legs feel like I'm pushing through deep snow.

Everyone's straining. Some are pedaling a fast cadence, others slower, but no one's loafing.

The song ends. John tells us to loosen the resistance, and every hand reaches down to twist back the tension knob. Another mid-tempo rocker, a Bob Dylan tune, comes on as we try to relax and get our heart rates down a dozen or 15 beats.

After a few minutes of recovery, John will call for another hill or maybe tell we're going to run a 100 or 110 revolutions-per-minute cadence for five minutes.

An hour after climbing on the bikes, we're done. Everybody's drunk at least a half-liter of water and sweated off at least that much.

Puddles of sweat lay at both sides of my bike. My legs are tight, almost wobbly, but the endorphin rush is as good as any I get from a hard running workout. And I won't have the stiffness tomorrow that would be inevitable if I ran that hard.

Spinning students at Cadillac's Pine Grove Athletic Club range from teens to octogenarians. Regulars include two-time Michigan Runner of the Year Laurie Decker and her husband, Kevin, who supplement their running with twice-a-week sessions on the bike.

Although the instructor dictates the workout agenda, the spinner, by tightening or loosening the tension knob, can control how hard it is to spin the pedals. In effect, everyone can tailor their own workout to their own level of fitness or desire.

To simulate a hill workout, tighten the resistance lever way down, stand over pedals, then start pedaling. It's as tough as running uphill and you avoid the pounding that comes from downhills. If you feel like taking it easy, just keep the knob loose. Make a few dramatic facial expressions and everyone thinks you are killing yourself while your legs pedal effortlessly.

Spin classes are available in many health clubs; often non-members can participate by paying an extra fee. Heart monitors, which can be bought quite cheaply, are necessary, as readings are used to gauge effort expended. Taking a stress test to determine your heart-rate zones enables those with monitors to collect more-accurate data to use in workouts.

Although I've referred to them as stationary bikes, we are riding spin bikes, which are more like high-performance road bikes than the exercise bikes most people have stashed in their basement.

When I first heard about spinning classes, I figured pedaling a stationary bike for an hour would be monotonous. Each session, I soon discovered, was different, as John, Gus Mejyes and Stacey Feister, our instructors, led us through simulated hill workouts, track-style intervals and sprints, endurance runs and assorted other drills.

Although I've long been of the opinion that nothing improves your running like running, spinning may be the exception. A good instructor will put you through a workout that tests your cardiovascular conditioning as well as putting stress on your leg muscles.

Sure, I still run outside almost every day, but icy or snow-covered roads sometimes make it impractical to attack hills or do fast running. A session or two each week on a spin bike adds the quality I miss on my outside runs. Last winter when our snow vanished for a couple weeks, hard workouts on the indoor bike kept my legs in shape for the rigors of snowshoe running.

Give it a spin and see. MR