A trainer responsible for transforming the bodies of Michael B. Jordan and Keke Palmer shares the 2 biggest fitness mistakes to avoid

• Celeb trainer Corey Calliet said common fitness mistakes can stall your progress in the gym.
• Not eating enough and doing too much cardio can prevent you from building lean muscle, he said.

If you want to build a strong, athletic physique, stop doing hours of cardio and cutting your calories too low, according to celebrity personal trainer Corey Calliet.

Calliet, a fitness transformation specialist, working with Keke Palmer on her Met Gala diet and exercise routine. The actress lifted weights and ate more in preparation to hit the red carpet in a strapless, form-fitting dress after she gave birth to her first child in February.

He told Insider that many people struggle to improve their fitness because they don't have a plan.

"People are afraid to go to the gym. They don't have direction when it comes to what they want to do," he said. "You have to know where you're going before you start."

Calliet said lack of organization leads to the two most common fitness mistakes — doing too much cardio and undereating, both of which can prevent muscle growth, strength gains, and overall progress.

Instead, he recommends lifting weights to burn fat and build muscle, and eating enough to fuel your workouts.

Overdoing cardio can stall your progress

Calliet, who also trained Michael B. Jordan for "Creed 3", said cardio can be a helpful tool to burn calories and keep your heart rate up, but too much can backfire.

"Don't overdo it," he said.

Evidence suggests that exercise alone may not be helpful for weight loss in the long-term, because your body can adapt. However, lifting weights can help increase your muscle mass and boosts your metabolism, which in turn can help burn fat and keep a lean physique over time, Calliet said.

He recommends combination of the two, and studies have found strength training with cardio exercise is effective to build lean muscle and improve overall health.

To get the best results from lifting weights, Calliet said it's important to focus on the muscles you're working and lean into the feeling of tension or discomfort to create a mind-body connection.

"It's where the pain lies. When it starts to hurt, the person wants to stop but you need to go past that," he said.

Exercises that create more tension, like eccentric movements, are ideal for building muscle.

Slow, controlled exercise can help you tap into the "good" pain of muscle fatigue, but you should avoid sharp pain that can be a sign of injury, trainers previously told Insider.

Not eating enough will prevent you from building muscle

Even the best gym routine won't lead to results unless you also prioritize good nutrition, according to Calliet.

"You can work out all day long but if nutrition is not aligned, you won't get there," he said.

Calliet said many people, especially women, trying to change their bodies by keeping a strict diet and severely cutting calories, with little success.

That's because it takes energy, and food, to build muscle. Calliet said Palmer initially was not eating enough to build muscle, and helped her get more carbs, fats, and protein was key to her "impressive" results.

"The main thing was eating more food because you have to eat to lose weight, and you have to lift weights to burn fat," he said.

Even if you do everything right, however, building muscle can take time. It's important to be patient, trust the process, and stick to a good program, said Calliet.

"You need purposeful training. It's like an art," he said. "That's how a transformation really happens."