How to describe the phenomenon that occurs at the top of the Art Museum steps at 6 a.m. every Monday through Friday - regardless of sun, snow or a summon-Noah downpour?

Is this courage? Masochism? Pathological disregard for the basic human instinct to seek shelter in a storm?

"A strange group of people who work out like crazy," says Juan Norris.

Close enough.

Norris, a 29-year-old boxer from West Philadelphia, has noticed this group in passing before during his morning run past Eakins Oval. But late last week, recovering from bruised ribs from a recent match, he slows to a walk and has a chance to get a better look.

What he sees are seven people, ages 23 to 60, doing old-fashioned calisthenics, in obeisance to a guy with his hands parked in the pockets of his camouflage pants.

The group - which can range from two to more than 30, depending on the weather - belong to an exercise club called Platoon. The Main Line company promises those who sign up for the $80-a-month privilege a spectacular view, fresh air and unrelenting motivation for an hour nearly every (mercilessly early) morning.

As the gray sky, like a wet mitten, pats the city's head, they gather at the top of the fabled steps.

"On your butts!" barks Joe Petrusky, a 27-year-old personal trainer and last week's platoon leader. Down his minions go onto the cold pavement to stretch their hamstrings. "One leg out! Now the other."

Playing an almost convincing drill sergeant, he surveys his troops, looking as though he might, at the slightest provocation, pop his neck veins bellowing into any slacker's ear. His commands smack the air, shattering into steam. But he's really a sweetie. Kneeling beside a semi-folded bald man, he asks, tenderly, "How are you doing, Lou? OK?"

Nodding, Lou Rossman, an endodontist from Bala Cynwyd with a practice at 16th and Walnut, reaches a little farther toward his toes.

Two years ago, Rossman was working out in his gym when he realized he had lost his motivation. A former runner, now 60 and with bad knees, he said he missed being outdoors. Through the wife of a friend of his son's, he heard about Platoon and joined.

"Look at this," he says, looking out over the Parkway lined with hazy-haloed streetlights. "The city is in the palm of your hand."