The number of repetitions, or reps, you perform of weight training exercises matters a lot. In general, if your goal is to build the largest, strongest muscles that your genetic makeup allows, perform relatively few repetitions, about four to six (perhaps even as few as one or two).
This refers to lifting a heavy enough weight so that by the end of the last repetition, you can’t do another one with good form.
If you’re seeking a more moderate increase in strength and size — for example, if your goal is to improve your health or shape your muscles — aim for 8 to 12 repetitions to failure. This will help improve muscle endurance, or your ability to continuously work a muscle over a long period of time.
The American College of Sports Medicine (ACSM) recommends varying repetitions for different people.
High intensity: Perform 6 to 12 reps using heavy weights. Higher-intensity training poses a greater risk of injury. This approach to training is suitable for athletes and experienced exercisers.
Moderate intensity: Perform 8 to 12 repetitions using moderate weights because this is the ideal number to strike a balance between building muscular strength and endurance and has a lower risk of injury.
Low intensity: For older adults, the ACSM recommends doing between 10 to 15 repetitions using light weights at an even lower intensity.
Why does performing 6 reps result in more strength than doing 15 reps? Because the number of reps you perform links to the amount of weight you lift. So, when you perform 6 reps, you use a much heavier weight than when you perform 15 reps of the same exercise. Always use a weight that’s heavy enough to make that last repetition a real challenge, if not an outright struggle.
Weight training isn’t an exact science so don’t take these rep numbers too literally. It’s not as if performing six repetitions transforms you into a bodybuilder, whereas performing ten reps makes you look like Angelina Jolie. Everyone’s body responds a bit differently to weight training. Genetic factors play a significant role in determining the ultimate size that your muscles can develop.
Bodybuilders (who aim for massive size) and power lifters (who aim to lift the heaviest weight possible) often train by hoisting so much poundage that they can perform only one or two reps. You may not want to lift hundreds of pounds of weight over your head, so your goals are best served by doing 6 to 15 repetitions.
Doing more than 15 reps is generally not effective for building strength, but it can improve muscular endurance.
To focus on increasing muscular endurance, you want to do at least 12 reps or more, but only two to three sets. To increase muscle size, you want to do 6 to 12 reps, but more sets — anywhere from three to six. To increase muscular strength, you want to do fewer reps, no more than six, and anywhere from two to six sets each.