Inside a darkly lit workout studio, a young woman wearing hot pink boxing gloves and a T-shirt proclaiming: "University of Pink" whales away on a bag that hangs from the ceiling like a giant slab of meat.

Whap! Whap! For 30 seconds or so, she punches the heavy bag with all her might while her partner holds it in place.

Pow! Pow! Turning, she lifts her right leg and unfurls a flurry of kicks.

Cassandra Bethke claims she's not the aggressive type. But the bag boxing class she takes every Sunday morning at the Firm in Minneapolis, has become a two-year addiction. She will gladly wake up early, even skip church, to get her punches in.

"It releases my tension," said the 24-year-old Plymouth woman, resting against a wall after a recent class. "It's so fun and high-energy, and it not only pushes you physically but mentally, as well."

For more than a decade, boxing classes of various forms have been fixtures at hundreds of fitness centers nationwide.

But bag boxing, taught by instructor Lori Fhima, is a hybrid of sorts. It's akin to a spinning session in the way participants can push themselves as hard as they want, but with gloves instead of a stationary bike. It's an offshoot of cardio boxing -- which combines boxing, martial arts and aerobics -- with big bags added to kick the workout up a notch.

The class even incorporates elements of yoga, requiring a similar need for intense focus and centeredness.

"When you walk out of that class, you're like the peaceful warrior," Fhima said. "You don't feel aggressive in any way. You've gotten all of your aggression out."

Fhima traces her fascination with boxing to a two-year stay in Spain in the early 1990s. While living there, she was hired to teach aerobics at a boxing gym, but so few people showed up that she passed the time by working out with Golden Gloves boxers. She learned about movement and form, and loved the way boxing improved her agility, stamina, coordination and balance.

After returning to the United States, Fhima took some Thai boxing classes with Kelly Miyamoto, owner of the Firm, who already had been working with a Muay Thai instructor. Eventually, the two developed a cardio kickboxing class that married their cardio aerobic backgrounds with their newfound passion for boxing. In 1995, the Firm added "bag boxing" to the mix.

For more than a decade, Miyamoto says, boxing was her gym's biggest draw, with nine classes weekly that sometimes drew upward of 50 people. The classes waned after Miyamoto retired from teaching and Fhima took time off to have children. (Spinning classes filled much of the void.)

A couple of years ago, Fhima revived the bag boxing class, and Miyamoto also plans to bring back cardio kickboxing soon.

An equal mix of men and women showed up to a recent class, where nearly a dozen bags, averaging 65 pounds, hung from chains attached to the ceiling. Participants warmed up with a series of light two-handed jabs as peppy music pulsated around them.

For the next 90 minutes, Fhima sent her class through a series of jabs, uppercuts and body-shot combinations and threw in a few kickboxing moves as well. Boxers took turns holding the bags for their partners, getting about 45 seconds of rest each time. The session closed with planks, pushups and stretching.

Fhima kept a close eye on everyone, especially the least experienced boxers. Moving from bag to bag, she dispensed pointers, illustrated proper technique and form, spouted encouragement. Always hold elbows close, she urged. Keep the body and torso in line to save the joints. Keep the feet moving. "I recognize people by their shoes," she joked.

Ellen Rolfes, 22, a first-timer from St. Paul, said she appreciated learning that the source of her power came from her core, not her hips. Punching a big bag was even more fun than she expected it would be. "It gives you something physical in front of yourself to put all of your energy into," she said.

Jaque Bethke, who often joins her daughter Cassandra on Sundays, said she loves the zen-like focus on technique, and the freedom to tailor the speed and intensity to her ability level.

She never thought she would be donning boxing gloves at 50 -- especially because she has lost part of her right lung to non-Hodgkin's lymphoma.

The first time she hit a bag, "I felt like, 'This is kind of cool,'" Bethke recalled. "Then you hit it harder and harder and harder. You know, it's exhilarating. ... It's such a positive way to get rid of that negative energy."

Pam Schmid is a Twin Cities freelance writer.