There’s a very simple trick. Basically, pick up a bunch of free weights with one hand until you find the heaviest one that you can still, well, pick up and do a move with. (So, one bicep curl, if that’s what you’re going for. Or one chest press. Whatever move you’re hoping to practice with). “Once you know the heaviest one you can lift, you’ll want to choose a weight for your actual reps that’s around 60 to 70 percent of the weight of your heavy one,” says Justice.
So let’s say that you can lift a 15-pound weight in one arm at least once without falling over, but a 20-pounder is too much. Do the math: 65 percent of 15 pounds is 9.75 pounds, so try doing your reps with 10-pound free weights. Worried that’s too heavy or will get you jacked and bulky? Seriously, don’t. Read this inspiring piece on The Beauty of Lifting Heavy Weights, and then check out 4 Myths About Strength-Training, Busted! and 7 Tips for Strength-Training Newbies. Feel better? Great. Now back to the question at hand.
Keep in mind that how many reps you do can be just as important as how heavy your weights are, explains Justice. If you want to increase strength, go for 1-8 reps per set. If you want to build muscle, go for 5-12 reps per set. And if you want to improve muscle endurance, go for 12-20 reps per set.
"I’m an advocate of the 8-15 rep sets, as they help develop a combo of strength and muscular endurance," says Justice. "When you go over the 20-rep range, you hit a point of diminished returns"—especially if your form is flailing.