By Dorene Internicola NEW YORK (Reuters) – 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.