Lets end a strength myth right now: Use low reps to bulk up and high reps to tone muscles. Sorry, your muscles are more complex than that. You can pack on mass through 20-rep sets and get lean on two reps at a time — if you plan your workout right. Here's a simple guide to matching sets and reps to your goals, whether you want to get stronger, lose your gut, or get huge.
For Fat Loss
When you want to burn calories, you don't need to go big on reps. You can achieve fat loss using heavy sets with just two or three repetitions just as effectively as lighter sets of 20 — if you keep your rest brief. Besides, occasional heavy loads help release more muscle-building, fat-burning hormones. For most traditional barbell and dumbbell exercises, set an upper limit of 10 reps for at least three sets, and keep rest intervals to 90 seconds or less. Better yet: pair non-competing lifts (like squats and pull-ups) into supersets to keep your heart rate up. As you lower your reps at heavier weights, add more sets. Overall, your goal is to increase the metabolic demand on your body. That comes from explosive movements, multi-joint exercises, and an elevated heart rate.
To Get Big
Volume — think of total weight moved over all your reps — is more important than anything for adding muscle. In addition to traditional low-rep plans, sets of 12 reps or more with 90 seconds of rest will bring on muscle exhaustion and opens the door for growth. That said, workouts including heavy sets (at least five) of six or fewer reps with three minutes of rest helps release testosterone and human growth hormone triggered by the neural output of those heavy lifts.
While you’ll definitely get strong from heavy lifting, only focusing on big efforts isn’t the best method to pack on size. Long sets are a more effective way to break down a muscle group and spur on growth and development. The good news is, you can keep lifting heavy if you add a burnout set at the end of your strength workouts. Shoot for at least 15 reps at about 60 percent of what you were lifting, and perform a single set of as many reps as possible until you hit technical failure.
To Build Strength
Your muscles are comprised of a mix of fast- and slow-twitch fibers, but certain muscles are more geared toward endurance — like the upper back, quads, and the lower leg's soleus and plantaris — due to a higher distribution of slow-twitch fibers. These predisposed slow-twitch muscles respond best to endurance-based work and longer sets. This can be taken a step further still by understanding that everyone has different levels of fast-twitch and slow-twitch fibers. Unfortunately, beyond those leg muscles, research is limited on what areas tend to have more slow- or fast-twitch fibers. So you'll have to treat yourself as a study-of-one and pay attention to what your body best responds.