The key to building lean muscle is strength training. If you've been lifting weights without seeing a transformation to your body composition [1], you probably aren't lifting the right amount of weight [2], or doing enough sets, reps, or a combination of the two. POPSUGAR spoke to LA-based celebrity trainer and Revenge Body coach Latreal Mitchell [3] to find out exactly how many reps and sets are necessary in order to burn fat and build lean muscle mass [3].

Mitchell advises splitting your workouts up into heavy, medium, and light days. Her reasoning — "Muscle confusion is everything. The body never hits a plateau when we constantly switch up the weight and rep schemes." Here's a sample of what your programming should look like:

• Heavy weight day: Do three sets, decreasing the amount of reps in each set. Start with 10 reps, decrease to eight reps in your second set, and your final set should be only six reps. Be sure to take one and a half to two minutes rest in between each set.
• Medium weight day: Do three sets of 12 reps, taking 45 seconds to one minute of rest between sets.
• Light weight day: Do three sets of 15 reps with 45 seconds to one minute of rest in between each set.

As a personal trainer I often see people lifting weights that are too light for them (we've got goals to reach!), so I'm going to let you in on a little secret you can use to figure out how heavy you should be lifting.

• Heavy weight: Start around 75 to 85 percent of your one-rep max. For example, if your max on squats is 125 pounds, try a weight that falls anywhere between 90 and 110 pounds.
• Medium/light weight: These should be around 50 to 70 percent of your one-rep max, with the light weight being closer to 50 percent.

These are just ranges, so be sure to test out various weights to find what works best for you. Don't forget that compromised form can lead to injury and is a good indicator that you may need to lighten the weight!

"Everyone should be lifting heavy at least once a week no matter what your goal is," Mitchell told us. If you're a beginner, Mitchell advises alternating between medium and light weight days in order to perfect your form. Once your form is spot-on, begin incorporating heavy weight days into your workout routine.