Kickboxing in flip-flops. Doing Pilates on a gravel surface. Common sense tells us these aren't smart workout habits. But not all exercise mistakes are so obvious; many are unknowingly committed by the gym set, especially those trying to cram their 45-minute circuit training into 30 minutes or less.

But what you save in time could end up costing you in other ways, including long-term damage to muscles, bones and ligaments. Benjamin Shaffer, an orthopedist with Washington Orthopedics and Sports Medicine, said overworking the body and ignoring its need to recuperate often lead to painful problems. Here are six common workout mistakes:

SKIPPING THE WARM-UP

Going from normal activity straight into high-impact exercising puts tremendous stress on the body — stress that could ultimately slow you down. "When your muscles are cold and stiff, you're more prone to injury," said Shaffer, team physician for the Washington Nationals and Capitals. Warming up raises core body temperature and fills muscles with blood so they are more amenable to stress. Those worried about time needn't fret: It takes only five minutes. Shaffer recommends stationary bike work, a brisk walk or jumping jacks.

FAILING TO STRETCH

Stretching is NOT the same as warming up, Shaffer said, even though it does help prep muscles for the stress of exercising. Three to five minutes of stretching provides greater flexibility and range of motion. And it's particularly important for people hitting middle age. "You're just not as elastic as you once were," Shaffer said.

Properly executed stretches should be gentle, easy and sustained. Avoid bouncing, Shaffer said. Also avoid stretching injured muscles, advised Jim Bradley, clinical professor in orthopedics at the University of Pittsburgh and team doctor for the Pittsburgh Steelers: "It decreases the chance for the muscle to repair itself."

CONDENSING YOUR ROUTINE

Shaffer calls them weekend warriors: people who cram their entire routine into the weekend and skip working out during the week. But Shaffer said, "You can't do all your athletic endeavors in two days of intensity and expect it to make up for the rest of the week." Bradley said it is "like asking a Chevy to be a Porsche." These exercisers have a tendency to overdo it, which strains the body and can result in painful musculoskeletal damage.

BELIEVING IN 'NO PAIN, NO GAIN'

It is normal to experience some muscle soreness during and after a routine. "You should feel tension and stress," Shaffer said. "But if what you're doing is really painful, you're not doing yourself any favors."

Working out to the point of exhaustion causes muscle fatigue and can lead to burnout. Exercising through the pain means ignoring your body's signs that it is sustaining unnecessary damage.

A related mistake is returning to a workout routine prematurely after an injury. Not allowing a sprained knee to fully recuperate can compound the problem. "Let the ligament heal fully before you exercise again," Shaffer said. "It's important that you be symptom-free, have normal motion, no swelling and normal strength."

USING THE WRONG EQUIPMENT

Equipment fit and quality is important to preventing injuries. "It's an easy mistake to make," Shaffer said. "It can seem like a real hassle to have to go around the house and collect all the right equipment." But it can mean the difference between injury and safety. Tennis elbow, a common injury, often results from using a racket with a handle that is too big for the player's hand.

FORGETTING TO MIX IT UP

"No pro athletes do the same exercise every day," Shaffer said. And neither should you. Muscles break down during a workout and can take 36 to 48 hours to heal, so don't always subject them to the same work. Instead, switch up your routine. If you're a runner, maybe hit the pool or bench press between runs. Bradley adds that it is important to know what exercises are most appropriate for your body type. A 6-foot, 280-pound man would not make a good long-distance runner, Bradley said, but he could be a tremendous golfer. Know what your body is capable of and feed it a fresh variety of workout routines.