On paper, staying fit isn’t exactly difficult. All you need to do is stay active and keep your diet under control; simple, right?

Well, not so much. Work out the wrong way, you’ll end up with an injury. Eat the wrong way, and your body won’t work efficiently. Unfortunately, there’s a ton of bad fitness advice out there, often originating from “health” websites that draw from nothing but hearsay.

Therefore, when people show up to their first personal training sessions, they’re often surprised to learn that they’ve been following terrible health plans for years. Personal trainers are happy to correct this, of course—after all, their whole lives are based around the fitness of their clients, so dispelling a health myth can feel pretty rewarding.

It’s science, so stop worrying.

Sometimes, however, we hold onto those myths. That’s when your friendly trainer might get a little frustrated, since the bad advice can stop you from getting healthy. In some cases, it can even lead to an injury.

Personal trainers are constantly correcting their clients on myths like…

1. “It’s not working if it doesn’t hurt.”

Pain is your body’s way of saying “something’s wrong.” While some cardio exercises can hurt, you should never feel like you’re risking an injury. If muscles start feeling tight or uncomfortable, you’re likely doing something wrong. If you feel joint pain or musculoskeletal pain, stop exercising right away.

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“A little bit of a burn that goes away when your muscles stop working is often just a result of the exercise, so it’s OK to continue,” exercise physiologist Carly Ryan tells ABC. “But if it continues and you’re getting, say, a sharp pain in your knees or you feel a painful twinge in your hamstrings that affects your ability to keep moving, then it’s most likely pain because you’ve overdone it, so you need to stop.”

Likewise, there are plenty of great simple exercises that don’t hurt at all. Walking, for example, is ideal for burning calories and losing weight, even if you rarely break a sweat. With that said, aerobic training will be slightly uncomfortable, but we’ve got good news: Eventually, it’ll hurt less. A 2014 study found that aerobic training increases pain tolerance significantly, but only for healthy individuals who engaged in “moderate- to vigorous-intensity” exercise.

2. “Lifting weights will make your body bulky.”

When it comes to growing muscle, you just have to lift weights and you’ll get bigger…right? Many people seem to think so—in fact, a lot of people, particularly women, avoid heavy lifting for the fear of looking too bulky. But in our video below, our experts explain how, like most things involving fitness, it’s far more complex:

3. Pretty much anything involving carbs.

Everybody has something to say about carbs. “Load up,” some say. “Cut them out,” they say the next day.

You really don’t want to load up on carbs unless you’re running a marathon or working hard for more than two hours. Carb loading is useful for long-distance runners, but for most people, carbs are just extra calories.

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There are 75 calories in a cup of pasta, and that’s before you add olive oil, sauce, or anything else. If you’re planning a huge workout, those calories will help to maximize your energy source for the big event…but otherwise, your body will just save the extra calories for storage. That’s how we build fat.

With that said, carbohydrates aren’t necessarily bad for you—they’re just not health food, per se.

The ‘bad carbs’ are out there, but a simple word of advice: moderation.

“This entire [low- or no-carb] ketogenic cycle fad going around over the past year is nothing new,” says James Shapiro, an independent personal trainer in New York. “Ketones exists in your body, and that research has been out there for a while, but this doesn’t [mean that] carbohydrates are a toxin to your body.”

“You can achieve similar body composition goals with a carbohydrate-focused dietary plan as a ketogenic diet,” Shapiro says. “There are bodybuilders who are vegetarian, and there are bodybuilders who consume large portions of animal protein.”

“Complex and low-glycemic carbs are proper choice for strength training, and many would argue that they should be the backbone of your daily nutrition uptake. The ‘bad carbs’ are out there, but a simple word of advice: moderation.”

4. “You should be working out every day of the week.”

While you can do light exercises every day of the week, you should give each muscle group a full day of rest every three to five days. Otherwise, you won’t gain muscle as quickly.

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In fact, numerous studies have shown that recovery days are a critical component of a well-balanced training program. Recovery allows your physiological functions to return to a normal state. Your body also needs time to restock its stores of energy, including blood glucose and muscle glycogen, and some scientists believe that sustaining exercise without occasionally resting could lead to losses in bone density.

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Think of it this way: Exercise breaks down muscle tissue, at which point your body needs to repair that tissue and build it up so that it’s more capable of handling the next workout. Without an occasional off day, you’re preventing your body from doing that work.

5. “Eating less will stimulate your metabolism.”

“People say, ‘Well, I’m just not going to eat,'” says fitness trainer Latreal Mitchell. “Well, that’s the worst thing to do. I always tell people to looks at their metabolism as a fireplace: If you’re not putting wood in the fireplace, it stops burning. If it’s not burning, your body is just going to store fuel.”

The more you eat, the more you get to your goal, provided that you’re eating the right things.

Researched performed by a group of Cambridge University researchers showed that our bodies really do have a “starvation mode.” When we’re not consuming enough calories, our brains stop activating the neurons that control appetite. That limits the number of calories our bodies burn, limiting weight loss.

In other words, we burn more calories if we’re consuming more calories—Mitchell’s furnace analogy is spot on.

“Get into a habit making sure you’re eating multiple times a day,” she says. “That’s a big myth, that not eating is going to get you to your goal. It’s the opposite. The more you eat, the more you get to your goal, provided that you’re eating the right things. It’s just about portion control.”

Try to avoid big meals, too; while eating small meals won’t necessarily boost your metabolism, large meals can have a negative effect, according to Duke researchers, and may lead to chronic health problems.

6. “Cardio will stop you from gaining muscle.”

If you’re performing intense cardio every day, there’s an element of truth here: You won’t be able to put on bulk very easily.

That doesn’t mean that you should avoid cardio if you’re trying to gain muscle. Light cardio workouts will improve circulation, shorten recovery times, and benefit your fitness program.

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“Let’s get this clear: Cardiovascular [exercise] does not kill muscle,” Shapiro says. ” If anything, adding in cardiovascular exercise can boost your strength training and increase lean body mass/reduce body fat percentage … The only way you would lose muscle is if you go into a energy caloric restriction and live on the cardio machine for more than 30 minutes a day.”

7. “Running will ruin your knees.”

Actually, runners have very strong knees, provided that they strengthen their hips.

No, seriously. Many knee injuries occur when a person has underdeveloped hamstrings and hip flexors. While “runner’s knee” is definitely something to watch out for, it’s avoidable with proper form, high-quality shoes, and a decent stretching program.

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Runners should try to vary their routes and avoid hard surfaces like concrete. They also shouldn’t run through injuries, as this can quickly lead to debilitating knee damage. Provided that they take these precautions, they’re not risking knee injuries with every run, and they’re actually strengthening those tissues substantially.

We should note that this is only true for runners in the long term; if you’re trying to lose weight, your best bet is to use a non-plyometric activity to get down to your goal, then gradually add running into your workouts.

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“Running is a plyometric,” Mitchell says. “You’re suspended in air at every point, so it’s a plyometric—you’re constantly coming down on your joints. So when you’re putting that type of stress on your bones and your joints, and you don’t have the right mechanics, you’re going to hurt yourself.”

8. “There’s only one way to do (insert exercise here).”

From squats and lunges to barbell workouts, there’s often more than one way to complete an exercise. Some people absolutely need to use these alternatives, particularly if they have injuries or disabilities. Does that mean that form isn’t important? Absolutely not—but different people benefit from different forms.

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A qualified personal trainer will be able to determine the best form for a particular exercise and provide a person with guidance. However, if you’re not a personal trainer, you generally shouldn’t try to correct someone’s form.

“Just kind of going online and looking at stuff doesn’t make you an expert,” Mitchell says. “When someone studies this all their life, and this is what they do for a living, they tend to have a little more experience in those areas.”

There are exceptions, of course—if you see someone flinging up a barbell, locking their legs, and not breathing, go ahead and step in—but don’t assume that every exercise has only one correct form.

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That’s really the most important takeaway: Personal trainers, when properly qualified, know what they’re doing. Given the glut of information available on health websites, weightlifting forums, and well-intentioned people at your gym, it’s easy to get off course. To stay healthy and get the body you want, you really need to listen to your trainer.

Finally, if you’ve got a question about fitness, don’t be afraid to ask them. Even if you think it’s stupid, it’s better to ask than end up with an injury.