We have fit people and deconditioned people who overdo it," said Geralyn Coopersmith, national director of the Equinox Fitness Training Institute.
"Exercise is like a drug, if you don't have enough, you get no benefits, if you have too much, you have problems," she said.
Shin splints, heel spurs, tendonitis are among the common overuse injuries that Coopersmith, who oversees the training of personal trainers for Equinox fitness centres, sees.
"Some days should be intense, some days not so intense," she said. "Exercise is a stressor. If it's too much, the body can break down."
Extreme fatigue, irritability, moodiness, an elevated resting heart rate, fever, and an inability to work your earlier level are among the signs that you've overdone it, she said.
California-based group fitness instructor Amy Dixon has broached the subject of overtraining with her clients, she said, but delicately, and only when they are ready to listen.
"I had a woman come in before my [indoor] cycling class," said Dixon, creator of the "Give Me 10" DVD series. "I'd see her on the treadmill for an hour, then she'd take my class, then after she would ride longer or go on the elliptical [trainer] for another 40 minutes."
Poke an exercise addiction, Dixon believes, and you'll often uncover another addiction.
"Maybe they're a binge eater, or they really party on the weekend," she said. "If you're working out morning and night, you're over-trained. Your body's getting beaten up."
For Dixon and her colleagues, overtraining is an occupational hazard.
"A lot of group fitness instructors and trainers fall into that category because it's our job," she said. "I know instructors who teach over 30 classes a week."
Connecticut-based exercise physiologist Tom Holland, who has coached people in everything from climbing mountains to running marathons, has actually dropped clients who wanted him to push them too hard.
"I have a lot of types that think they're Lance Armstrongs," said Holland, author of "Beat the Gym: Personal Trainer Secrets Without the Personal Trainer Price Tag," said, referring to the seven-time Tour de France winner.
He said a lot of his job involves telling clients what not to do.
"I try to keep them from getting hurt," he said. "I design programs on a case-by-case basis but there's always a rest day. When clients want to eliminate it I try to explain that you don't get healthier during the workouts, but during the rest days."
Jessica Matthews, an exercise physiologist with the American Council on Exercise, said she has referred several over-trained clients to psychologists.
"It's great to work with other professionals to help them [clients] recognise that they might have a problem," said Matthews, who is based in San Diego, California.
She said symptoms of overtraining can include constant headaches, sleeplessness and severe muscle soreness, as well as diminished performance.
"There are so many benefits to exercise, but if they're exercising excessively even the greatest benefits, like positive mood and better sleep, start to fall away," she said.
