Weightlifting is like the Swiss army knife of fitness activities: It can accomplish so much, from strengthening muscles and increasing bone density to revving up metabolism and more. It also can be dangerous if you’re lifting heavy weights, so you may want to have someone spot you. But don't just flag down the first hard body you see.

“With improper spotting, the lifter risks torn ligaments, tendons, muscles or meniscus, herniated disks, even death,” warns strength coach Eric Emig of Evolution Fitness in St. Louis.

FInd when you really need a spot, and use these guidelines for asking the person to do the job.

Related: Don't Be a Dumbbell: Avoid These 11 Common Gym Mistakes

When do you need a spotter?

In general, it’s often a good idea to have a spotter when you increase the amount of weight you’re using for a particular exercise or when you add more reps. “A spotter also can be required whenever a person lifting can’t safely return a weight to the rack or ground if he gets stuck,” says Emig.

There also are certain standard exercises that are safest if a spotter is on hand. The bench press is one. “If you get stuck with a heavy barbell on your chest there’s little you can do [to get out from under it],” Emig says. “Trying to roll 300-plus pounds down your stomach and hips is extremely painful and dangerous.”

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The barbell shoulder press, in which you lift a barbell straight overhead, is another. You may be able to handle the lift, but your rotator cuff (the muscles and tendons that protect the shoulder joint) could give out when it comes to stabilizing it. A spotter also can guide a back-loaded squat (a squat while holding a heavily weighted barbell along the tops of the shoulders) and then aid the squatter in putting the weight back on the rack.

What a good spotter does — and doesn’t — do

A good spotter communicates. He'll ask how many reps you expect to do and where you’re likely to get suck. He’ll also ask you to specify how you want to be spotted — what he should pay most attention to. “For example, during a dumbbell press some people like their elbows spotted, while others like their wrists spotted,” says Emig.

A good spotter is touchy. If you're doing a chest press, for example, she’ll keep both hands on the weight in case you start to lose control of it. One hand will hold the barbell underhand, the other will hold it overhand. She’ll also help keep the weight moving. “If a lifter stops moving for more than a second or two, there’s a higher risk of injury.” Although you don’t want to lift a weight too quickly, you do need to heft it at a rate that allows you to use a bit of momentum.

But not too touchy. This is especially important during the beginning part of the lift, when you’re trying to get a feel for what you’re hefting. Too much aid from a spotter can give you a false sense of security.

A good spotter knows his own limits. He won’t agree to spot someone who’s attempting to lift weights he (the spotter) isn’t able to handle. (It's also important for the person lifting to not attempt to handle more weight than he knows he can. Otherwise he could be putting the spotter in a tough, er, spot.)

Related: How to Use Kettlebells Safely

How to spot a good spotter

You can't judge a reliable spotter by his bulging muscles alone.

He's a pro. If a personal trainer is monitoring the weight room, he should be your first go-to.

She knows what she's doing. The fact that someone looks good when she lifts doesn’t automatically mean she'll make a good spotter, Emig says. But when a fellow exerciser clearly knows what she’s doing, that’s a good sign she’ll be a competent spotter.

He appears to be your fitness equal. Look for someone who's obviously as strong as you, or stronger, Emig recommends.

She asks the right questions when you ask her for a spot. See above. If the person you approach to spot you agrees without talking to you first about what kind of help you're looking for, thank her and find someone else.

What if there's no one available to spot you? Either skip the heavy lifting that day or use a squat rack with "spotter arms." “For a bench press, slide the bench under the rack and set the arms at a level that allows the barbell to slightly compress your chest,” Emig says. “You can do the same thing for shoulder presses by setting the arms just below chin level. If you lose control of the barbell, the rack will stop it from crushing you."

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Amy Roberts is a certified personal trainer. She writes about fitness, health and a variety of other topics for many well-known publications.