Are you making mistakes that are sabotaging your workouts? I think you might be shocked when you find out the answer. Below, I’ve shared the four most common mistakes I see people make at the gym.

Mistake No. 1: You only lift light weights. Somewhere along the line, a myth began circulating that lifting heavy weights will make you big and bulky. Most people believe they have to lift light weights to get the toned look they’re after, but that couldn’t be further from the truth!

To make things worse, certain group fitness classes seem to be perpetuating this idea. Every time I go to the gym, I peek in at the group fitness classes, and it’s always the same thing: a huge group of people doing 100 squats and lunges with little to no weight. What a waste of time! Your muscles need to be stressed in order to grow lean and strong. If you can do 100 repetitions of an exercise, then it’s not hard enough, and you need to up the weight.

You might be wondering how to know what weight is heavy enough for you. If you want to get toned, stay active and be healthy, you should aim for a weight you can lift six to 10 times. To be clear, this doesn’t mean you stop the exercise somewhere between six and 10 reps. It means the weight should be light enough to complete at least six reps. However, if you can do more than ten reps, the weight is too light. Once you find the right weight you can do one to three sets of each exercise.

A special note for the ladies out there: Lifting heavy weight will not make you bulky! Women’s bodies do not produce enough testosterone to grow massive amounts of muscle. What lifting heavier weights willdo is define, tone and shape your muscles. Plus, it will also make your bones stronger and healthier. So lower the reps, and up your weight!

Mistake No. 2: You watch TV, read a magazine or use your smartphone while working out. Doing any of these things will make it impossible to focus on your workout. I can’t tell you how many times I’ve been on the treadmill doing intense intervals while the person beside me is strolling along reading a magazine. If you’re going to read a magazine, sit on your couch, or go to a coffee shop or library. When you go to the gym, it’s time to focus on your workout. Here’s the bottom line: If you are able to watch TV, read a magazine or play on your phone, then you are not working out hard enough. Put all of that stuff down, and pick up the pace!

Mistake No. 3: You go to the gym without a clear plan. One very simple thing you can do is write down your workout plan before you go to the gym. For example, you can write, “Today I will do pushups, lunges, squats and pull-ups.” Be specific – write down both the exercises and the sets and reps you plan to do.

Having a clear plan will make your time at the gym more efficient. You won’t accidentally skip muscle groups, and you won’t end up wandering around aimlessly, wondering what exercise you should do next.

Mistake No. 4: You do too much cardio and skip weights. Doing too much cardio is a surefire way to break down your body and set yourself up for injury. For example, marathon runners often have injuries and compromised immune systems because of the high amount of training they do. Over time, the constant pounding puts a lot of stress and strain on their muscles, bones and joints.

With that said, high volume, long-duration cardio isn’t that great for your body, whether it’s through running, biking or the elliptical. We have been taught to believe that cardio is more important than weight training, especially if you are trying to lose weight, but this is simply not true.

Instead of long, boring cardio sessions, try interval training. So rather than doing five or six sessions of long-duration cardio per week, do two shorter sessions of interval training. The idea behind interval training is hard work coupled with recovery. Mixing up walking with running is an example. You can do a 30-second hard-work interval with a one-minute recovery if you are a beginner, or a one-minute hard-work interval with a 30-second recovery if you are more advanced.

Because interval training is much harder than continuous cardio, you should only do it for about 20 minutes. Don’t worry about decreasing your time. Chances are, you will burn more calories and fat during a 20-minute interval training session than you would during a much longer cardio session.

Those are the four big mistakes I continue to see that I needed to get off my chest. If you avoid these four mistakes, you will stop sabotaging your workouts for good!