It’s music to the ears of the time-crunched and the gym-shy: Researchers have found that when it comes to shaping up and shedding the pounds, quick workouts are as effective as hours spent exercising. In a study conducted by sport scientists at the University of Glamorgan at South Wales, it was found that fitness enthusiasts who cut the time of their gym sessions by up to two-thirds achieved the same results as those who plugged away for longer.

As part of the study, regular exercisers were split into two groups and instructed to follow an upper-body weight-training program for two months. All subjects exercised three days a week, but while one group performed a single set of eight repetitions on the weights equipment, the other did three sets. At the end of the research, both groups displayed significant improvements in their muscular strength and a corresponding decrease in body fat.

“The results prove that it is counterproductive to spend hours at the gym, and that a shorter workout can achieve exactly the same results,” said Dr. Julien Baker, who led the study. “Although our research focused on upper body-strength training, these results may also be true for other types of exercise.”

Dr. Baker’s findings add credence to the fitness industry’s predictions that “convenience workouts” are the way forward: In January, the American Council on Exercise suggested that “abbreviated fitness programs” will be the exercise phenomenon of 2005. Key to their effectiveness, said Vicky Mahoney, a personal trainer at the Holmes Place Academy, a British gym chain, is that you raise the intensity of your workouts to put in more effort over a shorter length of time. “It’s not how long you slog away, but what you do with the limited time you have that counts,” she said. “A lot of people waste time at the gym by standing around or chatting. If you really get down to it you can get a lot done in less than half an hour.”

So what should you do if you have only 15 minutes or so to devote to the sweat and grind of daily exercise?

Guidelines from the American College of Sports Medicine suggest that you can indeed halve the duration of your daily jog or power walk to no ill effect. They say that just 30 minutes of high intensity exercise at 80% of your maximum aerobic capacity – a level at which you would be puffing and sweating – is as good as an hour at a workload of 60%

“If you have only 15 minutes to run, then you need to push your body outside its comfort zone with some interval training,” said Louise Sutton, head of Health and Exercise Science at Leeds Metropolitan University. “Try to incorporate 30-second bursts of fast running to raise your heart rate to at least 75% of its maximum, followed by a 30-second jog to recover, and repeat that throughout the run.

“Always do a steady jog or walk to warm up and cool down. An alternative is to fast hill-run for 15 minutes, which is a great way to work the bottom and thigh muscles,” she added.

Fitness experts such as Jorge Cruise, author of “Eight Minutes in the Morning” (Rodale Books), and Brad Pitt’s personal trainer, Ken Hutchins, are forging a trend for weight-training sessions that last as little as eight minutes.

They claim that by lifting heavier weights – a higher percentage of the maximum you can lift – and by slowing the pace of each exercise, you will achieve muscle tone and fat loss as effectively as if you spend an hour at the gym. Instead of the usual five seconds that it takes to lift and to lower a weight, Messrs. Hutchins and Cruise recommend that, for a more intense workout, you take 10 seconds to raise and 10 more to lower. “Within two to three minutes of exercising a muscle this slowly, it reaches a threshold,” Mr. Hutchins said. “The body then gets a signal to make that muscle grow stronger.”