Strength training, traditionally favored by body builders seeking to bulk up, has become the go-to regimen for athletes, weekend warriors and exercise enthusiasts determined to slim down.

Fitness experts say metabolic strength training, a high-intensity, full-body interval workout, can add definition to the shape of runners, cyclists and other cardio devotees willing to put some muscle into it.

Florida-based trainer Nick Tumminello believes strength training should be the primary form of exercise for everyone except beginners.

"If you're looking to lose fat, go with strength training," said Tumminello, author of "Strength Training for Fat Loss." "Watch your diet to reveal your shape, and strength train to improve that shape."

It is not your grandfather's body-building program.

While traditional strength training uses free weights or weight machine to build endurance and muscles, metabolic strength training combines high-intensity interval circuits with changing combinations and repetitions using free weights, kettlebells, barbells, dumbbells and resistance bands to increase the metabolic rate after and during the workout.

"The body-building model is great for maximizing body building, but for the average individual looking for fat loss, or to feel better, or to improve general fitness, this creates more of a metabolic disturbance," he said.

Because the combinations are intense, Tumminello recommends starting at once or twice a week and building up gradually to three or four times.

"You do need recovery time," he said, adding that too many beginners tend to take an all-or-nothing approach. "Try to set a realistic goal."

Jenn Burke, a San Francisco-based personal trainer and fitness manager at Crunch, a national chain of gyms, said that steady cardiovascular activity, such as running or cycling, is great for burning lots of calories at a time and increasing heart rate and lung capacity.

But unlike cardio activity, strength training will continue to burn calories up to 72 hours after the exercise is over through a phenomenon called after-burn.

Modern strength training, she said, is less about how much weight you can lift than it is about how to make the body more efficient, lean, toned and strong.

"Strength training is about the quality of life," Burke said. "You can be skinny but not have the ability to lift your suitcase."

Although research has shown people cannot spot reduce, such as targeting just the thighs or arms for slimming, Burke said it is possible to enhance an area of the body with strength training.

"Men are generally drawn to body building seeking size or presence," she said. "Women come for toned and sculpted arms, legs and butts."

Muscle strength, increased bone mineral density and increased strength in the connective tissues or tendons and ligaments are among the benefits of strength training, according to Jacque Ratliff, an exercise physiologist with the American Council on Exercise.

"If someone falls, the more strength in their muscles, bones, tendons, ligaments, the less likely they are to be injured," Ratliff said.

Strength training also promotes elasticity in muscle.

"Generally, people want to tone up, to become leaner so the muscles are more defined," she said. "Muscular tone from a biomechanical aspect is the ability of muscles and connective tissue to hold the body in position."

The more muscle mass we have, she added, the higher the metabolism at rest.

The American Council on Exercise recommends a minimum of one to two days of strength training per week, Ratliff said, but she cautions that metabolic strength training is a high intensity activity that needs to be monitored and programmed correctly to avoid injury.

"This is not something you would start someone on who is very new to exercising," she said.