Fatigue–the self-inflicted, fitness-obsessed, cash-burning variety–is anathema to those who think the best reason to join a health club is the massage setting on the locker-room shower head. But for certified couch potatoes who need to be yelled into shape and disciplined athletes who want to get tougher, ““boot camp’’ is a faster, increasingly popular route. ““Gym rats are always looking for something new to stay motivated and work new muscles,’’ says Shape magazine consultant Stephanie Oakes. The International Health, Racquet & Sportsclub Association says boot camp is second only to yoga as the latest fitness fad.

““Personal trainers are there to push you,’’ says Kevin Philion, a fitness pro who shaves his head (himself) for each of the half-dozen annual boot camps at Williams Island, Fla.–including the one Zuniga recently attended. ““Boot-camp trainers are there to shove you.’’ Programs range from one-hour classes at private gyms to six-day group trips to the Everglades. Some instructors wear fatigues; others, spandex. But the basic, rapid-fire drills are the same: chin-ups, squat thrusts and rope climbs not seen since high-school PE class.

The trend started as a thinly veiled attempt to lure men into aerobics studios. The hook: machismo. ““I’m a jarhead, a manly man,’’ says Keith Byard, creator of boot camps for New York’s Reebok Club and Chelsea Piers and Los Angeles’s Sports Club. ““Guys won’t do butt tucks,’’ says Oakes. ““They will do jumping jacks.’’ What nobody had counted on was that women would do them, too.

Half of the 600 students in the Sergeant’s Program of Rockville, Md., are women in their 30s and 40s. And they know from jumping jacks. They also know showing up late at the park (rain or shine) means dropping to the mud for 20 push-ups. Founder Patrick Avon, like other trainers, claims his $70-a-month program is the original civilian boot camp. But he has lots of company now–some of it more Private Benjamin than G.I. Jane.

At New York’s World Gym, Jourdan Zayles makes her 25 grunts count off bicep curls to the tune of ““So Many Men, So Little Time.’’ Michael Lechonczack, yoga guru for Equinox Fitness’s new boot camp, says: ““You can have a mantra and be a marine.’’ And at Williams Island, gleaming weight rooms and suites (Sophia Loren has a condo there) make the place look more like a spa retreat than the army-inspired hell week that it is. Each grueling 9-to-5 day includes three meals and a massage. Says client Stephanie Curtis, 33: ““It forces you to make a full commitment to your body.’’ But unlike in the real military, if the yessir! shtik gets old, you can leave.