If you're already strong and you train your arms using only the part of the movement where your muscles are stretched (like holding the bottom of a bicep curl), you'll build just as much arm muscle as if you did the full movement — the stretch alone seems to be all you need.

If you're already fit and lift weights, doing partial reps that focus on the stretched part of the movement works just as well as full reps for building arm muscle over 8 weeks.

If you're trying to build upper-body muscle and you're already experienced with lifting, it's probably better to focus on the part of the exercise where your muscle is stretched, even if you don't do the full movement. Adding in the part where the muscle is more squished doesn't seem to help you grow more.

If you're already strength-trained, doing arm exercises with only part of the motion (like half-bicep curls) builds your arm muscles just as much as doing the full motion — and the data suggests there's probably no real difference between the two.

If you're already strength-trained and do upper-body workouts for 8 weeks, it doesn't matter much if you lift all the way up and down or just do part of the motion—your muscles grow and get stronger about the same either way.

If you're already strong and you train your back with partial pull-downs that stretch the muscle at the bottom, you'll get just as strong at doing full pull-downs as someone who does full movements the whole time — meaning stretching the muscle really hard might be more important than moving through the full range.