NEW YORK - Breaking up your fitness routine is hard to do. But it's crucial for avoiding the bigger heartache of overuse injury, fitness experts say.

"People tend to do the same thing over and over again, without varying it, without taking adequate rest, without building slowly, and they end up with an overuse injury," said Geralyn Coopersmith, national manager for the Equinox Fitness Training Institute.

"Tendonitis, bursitis, fasciitis, these kinds of inflammations are pretty much guaranteed if you don't' vary your training," said Coopersmith, who oversees the training of 1400 personal trainers in 48 Equinox clubs nationwide.

Yet she concedes that even clients who complain of nagging aches and pains are loath to change their routine.

"People get terrified. They'll say, 'The treadmill made me lose weight.' Well, exercise made you lose weight. The treadmill was the modality. That doesn't' mean it's the only way or the best way," she explained.

"Most people don't cross train enough," she said. "Maybe they've been doing yoga for years so their flexibility is great but ask them to hold a plank position and there's no core strength at all."

For Adrian Shepard, fitness director for the recreation department at Butler University, over-exercisers can suffer more than pain or poor performance.

"Overall it's a tricky thing to notice," he said. "Some signs, like sleeplessness, apathy, depression and difficulty concentrating, may be associated with other conditions. You really have to focus on the big picture."

Shepard says some young adults he works with at the Butler campus in Indianapolis, Indiana, are especially vulnerable.

"Most at risk are those training for an event or sport," he said, "or those with a preoccupation with being thinner."

He stresses the importance of consulting a fitness professional.

"If you're feeling pain while doing an exercise the form might be wrong or you might be injured," he said.

Dr. Heather Gillespie, of the American College of Sports Medicine, sees everyone from elite athletes to weekend warriors in her sports medicine practice at UCLA.

She said the consequences of over-exercising can include stress fractures as well as overuse injuries.

"Some runners just run, no cross training, and they may do the same thing every week," she explained.

For them she recommends strength training.

"Some sort of weight lifting routine focusing on their weaknesses," she said. "A lot of knee pain comes from weak gluteus muscles."

She suggests varying routines and taking time to recover.

"Rest and recovery is very important. It's recommended that you take at least one day off," she said, particularly after an injury has been diagnosed.

"For stress fractures, rest is essential," she added. "But just because you can't run doesn't mean you can't run in the pool. I consider that active rehabilitation."

She acknowledges the difficulty of prying even an injured addict away from his activity.

"Exercise is a great form of stress release," she said. "And injury can cause depression. I've had a lot of people break down in my office because they can't run and they're in tears."

Coopersmith urges all who work out to seek professional advice.

"We tell people that exercise is a drug, and like a drug you need a prescription. So much of it is good, and then so much of it is an overdose."