The biggest barrier to working out is time. So fitness trainers hate to see anyone frittering away precious workout periods or filling them with less-than-effective exercises. Actually, it makes them crazy.

The biggest barrier to working out is time. So fitness trainers hate to see anyone frittering away precious workout periods or filling them with less-than-effective exercises. Actually, it makes them crazy.

We asked a few trainers to point out things they see in the workout world that they really wish they didn't.

The chief culprit of wasted workout time is the cellphone.

"It drives me bananas," says John Benz, co-owner of a CrossFit gym in Kansas City, Mo. "If you can talk on the phone or text, your workout isn't intense enough."

Bottom line: If you aren't actually working out during your workout, you can't claim a workout.

Corey Scott knows why people do crunches, those truncated sit-ups meant to target abdominal muscles. They want a washboard stomach.

But if your goal is to strengthen your core, says Scott, owner of Corey Scott Personal Training Studios in Prairie Village, Kan., crunches aren't the best exercise.

Few people keep perfect form during crunches, he says. And it gets worse as they increase repetitions. They will arch their backs, a strain that can lead to injury.

A simple and effective core exercise is the plank, Scott says. It involves a host of abdominal, back and stabilizer muscles.

Lie on the floor face down and raise your body, balancing on your forearms and toes. Hold for 20 seconds or more, lower your body to the floor and repeat several times. Be sure to keep your rear from poking up or sagging.

For a more advanced plank, place your forearms on a stability ball.

For Cynthia Kernodle, it's disturbing: "I see the same people on a piece of cardio equipment like an elliptical machine or stationary bike, doing the same thing every time, at the same level.

"It's better than being sedentary, but they're not going to change their bodies or increase their aerobic fitness."

If you perform your cardio at the gym, spend shorter amounts of time on each of several machines: elliptical, treadmill, bike, says Kernodle, owner of Choices Personal Training.

And switch to interval training, which means alternating periods of high-intensity and low-intensity exercise.

Intensity is pushing yourself hard to maximum effort, which is a different level for different people. Think of sprinting, running as hard as you can, followed by jogging. Or increase the slope or resistance on a cardio machine for a time, then lower it.

John Benz has a message for the bicep-curl fans: "Way too much time is dedicated to that tiny muscle."

Benz realizes that the bicep curl is a hallowed weight-training maneuver. But the time would be better spent doing pull-ups, he says.

Gripping a bar and lifting your body weight will give you great biceps plus recruit an array of muscles in the back and elsewhere. It's also more aerobic and will improve your grip, forearms and shoulder stability.

Can't do one? Ask a friend to grasp your ankles with both hands and provide just as much support as is needed for you to lift your body and get your head above the bar.

Leg-hold exercises can cause injury, Kernodle says. Most people lie on their backs and with their legs straight, raise them to a right angle with the floor, then lower them to about 10 inches from the ground, hovering there as long as possible.

"You see people's backs arching away from the mat or the bench," she says. "Really their lower back is taking the strain, and the abdominals aren't working."

Anyone with chronic lower-back pain shouldn't do leg holds, Kernodle says.

Here are a couple of alternatives:

Lying on the floor face-up, bend the right leg at the knee and keep the right foot on the floor. Extend the left leg and raise it off the floor, hold for several seconds, and return the left leg to the floor. Make sure you don't push your right foot against the floor, which recruits the right hamstring rather than the abdominals. Then switch legs.

Now, lying face-up, arms at your sides, position your legs as though sitting in a chair, thighs at a right angle to the ground and knees bent. Lower one leg to the ground and return to the starting position. Do the same with the other leg.

If you find yourself arching your back, place your hands, face down to the floor, under your rear.

Scott was at a fitness convention when a physical therapist asked a group of trainers whether they instruct their clients to do shoulder presses and other overhead weight-lifting routines for shoulder strength. Most hands shot up.

"Stop it," the therapist said. "I'm tired of trying to fix all these patients with messed up shoulders."

Pushing heavy weights above the head is a staple of gym work, but as people age, such weight training can damage shoulder joints and tendons. A common term is "shoulder impingement syndrome."

Try scaptions as an alternative, he says.

With light to moderate weights in each hand, place your arms about even with your front pockets. Now raise the weights to just below shoulder height, then lower them. Your arms are angled rather than straight at your sides or straight in front.