KANSAS CITY, Mo. — Maybe the biggest barrier to working out is time. Barrier, challenge, excuse?

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.

Perhaps a different exercise would be a better use of time. Or a certain exercise ultimately yields more injury and pain than progress. You might be surprised by their picks.

But first, let’s address the flagrant waste of workout time, and the chief culprit here is — you guessed it — the cellphone.

"It drives me bananas," says John Benz, co-owner of CrossFit on 18th in Kansas City, Mo. "If you can talk on the phone or text, your workout isn’t intense enough."

Recently, a person was observed just barely moving her legs on an elliptical machine because she couldn’t coordinate legs and texting thumbs.

"She wasn’t breathing hard, but she was definitely communicating with someone," Benz says.

Unless you’re taking a call from a patient or a babysitter or some other emergency — "Somebody better be dying on the other end" — leave the cellphone alone, he says. And if you’re using your phone during a class or while working with a trainer, that’s just rude.

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

INSTEAD OF CRUNCHES, DO PLANKS

Corey Scott knows why people do crunches, those truncated sit-ups meant to target abdominal muscles. They want a "six-pack," a washboard stomach, that shrink-wrapped look.

But the real way to get the shrink-wrapped look is to shrink the wrap. That requires improving nutrition and cutting calories.

"We all have the same musculature," says Scott, owner of Corey Scott Personal Training Studios in Prairie Village, Kan. "It’s just that we have to reduce down to it. The six-pack starts in the kitchen. Or better yet, the grocery store."

If your goal is to strengthen your "core," which means the torso muscles, including stomach, back and hips, your impulse is right on, Scott says. But crunches aren’t good for that, either.

Few people keep perfect form during crunches, he says. And it gets worse as they try to increase repetitions. Most notably 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 an exercise or stability ball.

INSTEAD OF ROTE CARDIO ROUTINES, DO INTERVAL TRAINING

Cynthia Kernodle gives certain people credit — at least they’re not sitting at home on the couch — but still 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."

Several problems: The body gets accustomed to long stretches of routine exercise, and fitness doesn’t improve; you increase your risk for repetitive motion injuries; and workouts lack mental focus. They’re boring.

Two solutions, says Kernodle, owner of Choices Personal Training:

If you perform your cardio at the gym, spend shorter amounts of time on each of several machines: elliptical, treadmill, bike.

And switch to interval training, which means alternating periods of high-intensity and low-intensity exercise. The latter are also called rest intervals (but that doesn’t mean to stop).

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.

"Even doing that for 30 seconds and then backing off for recovery is going to make changes," Kernodle said. "Otherwise your muscles just become immune to your exercise, and they quit changing."

INSTEAD OF BICEP CURLS, DO PULL-UPS OR ASSISTED PULL-UPS

No matter how toned or bulky you’d like your biceps to be, John Benz has a message for the bicep-curl fans among us: "Way too much time is dedicated to that tiny muscle."

Benz realizes that the bicep curl, lifting a hand-held weight by bending the elbow, 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, Benz says. Or, at first, stand on a chair or stool and approximate a free-hanging pull-up.

And if you’re too old school to completely abandon bicep curls?

"Less than 3 minutes," Benz says. Hit them fast, intensely, and move on."

By now you’re picking up on a general theme here: For fitness and function, choose exercises that work large and multiple muscle groups rather than those that attempt to target specific muscles.

INSTEAD OF LEG HOLDS, DO ALTERNATING LEG EXERCISES

Our legs contain the body’s biggest muscles. They’re heavy. That’s one reason leg-hold exercises can cause injury, Kernodle says.

For leg holds, 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.

This is meant as an abdominal exercise, with the legs serving as dead weight. But remember the crunch? Similar issue.

"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. Others risk injury unless they maintain perfect form, with the full spine against the floor.

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.

INSTEAD OF OVERHEAD SHOULDER LIFTS, DO SCAPTIONS

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 be wary of injury, Scott says. Especially 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.

"I was in Hawaii at a gym hotel," Scott says, "and I saw a guy doing upright rows. So I told him I was a trainer, and that what he was doing could cause impingement of the shoulder joint. He looked at me, said, ‘hmm,’ and went right back to what he was doing."