The twelve riders take their marks, balancing on bikes of carbon fiber, steel, and titanium as the opening notes of The Who's ''Baba O'Riley" fill the air.
At a signal from their leader, they're off, pedaling furiously for 90 minutes, changing gears in unison, glancing from side to side to see how their neighbors are holding up.
This is not a race; technically, it's not even a ride, since these athletes aren't going anywhere. They're lined up on the third floor of International Bicycle Centers on Brighton Avenue, speedy racing bikes held in place by resistance units that simulate hills and headwinds without the cold, darkness, and danger of riding outdoors in winter.
While this might sound less than exciting, participants say that the group sessions are far more enjoyable than going nowhere solo.
''I don't do it at home; it's hard to motivate myself without my teammates and my friends around," says Laurie Damianos, 41, a triathlete and one of several members of the Boston Triathlon Team in attendance. ''I have five really close friends here tonight, and I keep getting them to come every year."
''As adult athletes, you connect with people," agrees teammate Josh Fisher, 25. ''We're not in college anymore, so other than people we know at work, this is one of the best ways to meet people."
For most of the session, though, it's not about socializing. It's about pushing the pedals. The pace is set by trainer and coach Ali Winslow, 31, who calls out instructions over a microphone connected to a stereo system, directing the riders to pedal all-out for two minutes, rest for two more, adjust the resistance on their trainers up or down, or pedal with one leg only for a minute at a time. It's an odd scene for the uninitiated, but one that makes perfect sense for the students and for the shop.
''It's just a great way to incorporate cycling into the off-season, which is always tough for a bike shop to figure out," says Craig Gaulzetti, one of the managers at International who watches over the classes and takes care of any mechanical issues. The fee for the classes ($180 for six weeks of twice-a-week sessions) goes to the teachers, who are not store staff.
The mood and the pace are set by musical selections from U2 and AC/DC, as well as the relentless chirp of an electronic metronome, which Winslow frequently adjusts up or down to get the riders to pedal faster or slower. Like the play list of songs, Winslow says that each of the twice-weekly sessions is customized toward a particular goal -- in this case, shaking some rust off the legs.
''This is the 'acclimation period'; a lot of these people haven't been on their bikes yet this season," says Winslow, who also works as a trainer at The Sports Club/LA in downtown Boston. ''We work on lower heart rates, we work on form, and after that we'll move into just workouts, and it actually becomes a lot tougher. A lot of people want to do a half-Ironman in June, and if you don't start training now, you're in trouble."
But not everyone here is thinking ahead to the race season, even as they cool down from a workout geared to get them race-ready.
''I wouldn't call myself a triathlete. I have finished three triathlons, but I would not call myself a triathlete," says Andrea Franz, 42, laughing as she draws the distinction between Boston Triathlon Team members and people like her. ''I consider myself a recreational person, but I like to be able to keep up with people. I don't like to be in the back of the pack."
Here, they're all in the middle of the pack, in two neat rows, sweating together as some dream about not being left behind, and others dream of shaving a few hours off their Ironman time.
''You're physically spent, but you're mentally charged. I'm raring to go," says Mike Fogassey, 50, as he and the other students break down their equipment and get ready to head back out into the night. ''When I go home, I'll probably just sit in bed about an hour, just stare up, because I can't even sleep."
Will Kilburn can be reached at firstname.lastname@example.org.