Mary Kate Frerichs of Timonium during the boot camp at MAC Hunt Valley Express.

Mary Kate Frerichs of Timonium during the boot camp at MAC Hunt Valley Express. (Lloyd Fox, Baltimore Sun / January 1, 2014)

It's a small room, but it has plenty of potential, tucked away beyond the lines of traditional treadmills and weight machines at the Maryland Athletic Club Express in Hunt Valley.

The room is filled with some items you expect to see at any gym — a rowing machine, a weighted ball, a punching bag — and some you might not: thick battle ropes and TRX suspension straps hanging from the ceiling.

Each of the students in personal trainer Doug Bopst's boot camp class last week used one of these pieces of equipment for a minute-long interval. After a quick and nonstop movement — lifting heavy battle ropes, throwing boxing punches or pulling themselves up by the TRX straps — they took a 10-20 second break. Then they rotated to a new piece of equipment. Bopst, 26, counted down from five, gave the go-ahead, and the cycle continued.

Within a few minutes of starting the workout, all four students in the class were sweating and breathing hard. This is high-intensity interval training. Heavy on the intensity.

"It's hard for me to motivate myself, but when we leave, [Doug] will be like 'great job today!'" said HIITer Mary Kate Frerichs, 21, a University of Richmond student. "Him pushing us and motivating us is the biggest thing that keeps us coming back."

Frerichs attended Bopst's boot camp class along with three other young women: her sister Colleen, 19, who attends Dickinson College, and her cousins Maggie, 21, who goes to High Point University, and Grace Geary, 18, who attends Notre Dame Preparatory School. The majority of the boot camp is made up of high-intensity interval training, a popular workout trend among young people.

The American College of Sports Medicine Health & Fitness Journal released a survey, completed by more than 3,800 fitness professionals worldwide, in its November/December issue forecasting the top fitness trends of 2014. High-intensity interval training took the No. 1 spot, its first appearance on the list, now published for the eighth year. Other trends making the list are also related to HIIT: body weight training (No. 2), personal training (No. 6), group personal training (No. 9).

HIIT is broken into brief intervals of high-intensity motion followed by brief periods of rest, said Billy DeLorbe, a personal trainer at the MAC in Timonium. The activity can be any type of exercise — sprinting, pull-ups, squats, tire flipping — as long as the heart rate changes between the intervals of motion and rest. Classes usually run about 30 to 45 minutes (with rest), but some programs are performed in less than 30 minutes.

DeLorbe said the period of rest doesn't necessitate ceasing all movement, just something less intense — like sprinting for a quarter of a mile and then jogging for another.

Reese Ashe, a personal trainer at Federal Hill Fitness, said HIIT started to grow in popularity around two to three years ago, after CrossFit became a success and served as a catalyst for various HIIT-focused gyms.

"I think [HIIT] will get bigger," Ashe said. "More classes will be designed, and more specialty gyms will be popping up."

Ashe, whose boot camp class is the most popular class at Federal Hill Fitness, said the average age of people participating in his HIIT class ranges from 21 to 28.

Tiffany Bryant, an exercise specialist at Towson University's Wellness Center, said she sees a lot of young people drawn to high-intensity workouts such as boot camps and CrossFit. One reason for its appeal to a younger crowd, she said, is its efficiency. Young people "like to work hard and then move on," Bryant said. "It's a get-in-and-get-out kind of thing."

Bryant said HIIT is a much faster trend than those of the past.

"Past fitness trends were more cardio-based," she said. "Running for longer periods of time, or lifting lighter weights with more reps. Now it's more heavy lifting and burning way more calories in a short amount of time."

And DeLorbe said HIIT combines exercises the way past trends did not.

"A big difference is in the past, your cardio and strength were separate," he said. "Now your cardio and strength aspects are blended together."

Tabata, Bryant said, is a popular form of HIIT. Like other types of HIIT, Tabata is broken into intervals, but it has a set ratio of 2:1 of activity to rest and always includes eight rounds.

Bryant said Tabata can be done with almost any exercise, or some combination of exercises — pull-ups, push-ups, burpees — but is typically done using body weight exercises or very light-weight exercises.

Bopst said he sees young people excel in HIIT classes and that HIIT often appeals to young athletes and former athletes because of its similarity to many sports.