Julie Ennis did not look like she was having fun. As she pushed at the weight machines, it was clear from her grimaces and groans that her muscles were working a lot harder than they felt like working.

Standing by with a clipboard and offering encouragement was Brandon Jonker, a personal trainer at Discover Strength in Minneapolis. "You are absolutely crushing this right now," he said as he guided the 39-year-old Maple Grove mother of four through the torture -- er, workout.

Aided by Jonker's supportive patter, countdowns and the occasional helpful push, Ennis worked each weight until she appeared to reach the point that fitness authorities typically advise people to aim for: the point at which Ennis would feel that if she summoned all her strength, she couldn't possibly do one more rep.

Then Jonker would have her do a few more reps. Sloooowly.

"With every workout, we overload in weight or in reps, every time we do each exercise," Jonker explained. "It's really taking her past the point of fatigue."

It was a tough workout but at 30 minutes a quick one, and with another appealing tradeoff: At Discover Strength, Ennis is told she only needs to do one or two of these arduous sessions a week to stay fit.

Strenuous but short and infrequent workouts are the key to high-intensity resistance training, a technique gradually gaining popularity across the country, including at Discover Strength, a local chain with facilities in Plymouth and Chanhassen and, most recently, downtown Minneapolis. Proponents say high-intensity training can deliver all the benefits of other forms of exercise without requiring hours each week of plugging away on a treadmill or mat.

"We need much less exercise than we think we do -- it's about intensity rather than duration," said Luke Carlson, Discover Strength's founder and CEO, who has also co-authored a training guide ("The Female Athlete: Train for Success," Wish Publishing, 2004) and served on the strength and conditioning staffs of two Twin Cities high schools and the Minnesota Vikings.

All the fitness you need in half an hour a week? That might sound appealing to busy people (i.e., just about everyone), though Carlson said it tends to meet resistance, so to speak, from fitness buffs accustomed to logging daily gym time. (The other main concern comes from women afraid of winding up looking like Arnold Schwarzenegger; to which Jonker replies that muscular size is limited by genetics in both sexes, and that few men and virtually no women are capable of building Arnold-style bodies without "some sort of supplement," if then.)

Contrary to conventional wisdom, lengthy and frequent workouts are unnecessary and can even be detrimental, said Fred Hahn, co-author of "The Slow Burn Fitness Revolution" (Crown Archetype, 2002), which describes the science behind the concept. That's because "you don't gain benefit from exercise when you exercise, you gain benefit from exercise when you rest and recover."

Working a muscle to exhaustion (often called "muscle failure," though Hahn prefers the upbeat "muscle success") in as little as 60 to 90 seconds of slow but well-executed reps causes what Hahn calls microscopic "alterations" in the muscle (actually microscopic tears, which sounds icky but is just the way muscles work). As the body repairs these microscopic alterations between workouts, the muscle gets stronger and adjusts to increasingly greater effort.

Also controversially, proponents say research shows that intensive strength training may be at least as effective as aerobic exercise at delivering health benefits, including revved-up metabolism and weight loss, reduced risk of chronic disease, increased muscle mass and bone density. Although for years most people have assumed that aerobic exercise is the way to improve cardiovascular and respiratory fitness, Hahn, who owns a training studio in New York called Serious Strength, said the same can be accomplished through strength training, which "delivers all the health and fitness goods that exercise can bestow, if done properly," he said.

For Ennis, high-intensity training has worked so well, and is so easy to fit into her schedule, that she has made it the core of her fitness plan. The tall, slender marketing specialist for Target also runs and cross-country skis. But she wasn't doing any strength training until about six years ago, when she discovered Discover Strength.

"I was just hooked," she said. "To me, it's so effective and it fit into my lifestyle. I was getting results I would never get on my own."

Katy Read • 612-673-4583