When you first start strength training, almost any weight you lift will bring some results.
But also use this time to learn proper form, the American College of Sports Medicine advises. As you progress, you can zero in on the best amount of weight as well as the number of repetitions and sets of repetitions you do.
A general starting guideline for beginners is to choose a weight that you can lift for one set of 12 to 15 repetitions. Note that one repetition or “rep” has three parts: lengthening the muscle (the lift), a momentary pause (the contraction) and shortening the muscle (the return to starting position). Then, over the course of a few weeks, build up to two and then three sets, at which point you’ll add weight and repeat the process.
To progress effectively, the American Council on Exercise says to aim for muscle fatigue, which is actually more important than the overall number of reps you do. Muscle fatigue occurs when the muscle being worked is no longer capable of performing any more reps. This is the sign that all the muscle fibers of that muscle have been engaged.
Lifting heavier weights will lead to muscle fatigue with fewer reps than lower weights, which brings us to the next point: Choosing whether you’re training for strength or for muscle definition.
If you’re training for strength, use weights that cause fatigue after a maximum of six repetitions. If you’re training for definition, do a higher number of reps with a lower weight and at a slower pace — you want to hold the contraction phase longer. You still want to achieve muscle fatigue, which in this case will typically occur between eight and 15 reps.
And always be sure to rest between sets. The heavier the weight you lift, the longer the resting period should be. Figure on one or two minutes of rest with a lighter weight, two or three minutes with a moderate weight, and three or more minutes with a heavy weight.
The American Council on Exercise has more tips on how many reps you should do during your weight workouts to help you reach your goals.