For guys who lift weights, doing arm curls with the arm stretched out doesn’t build more muscle than doing them with the arm bent, even when using light weights and blood flow restriction — at least over 3 weeks.

When people do light weight training with blood flow restriction on their biceps, the muscle grows evenly no matter if they do the exercise with their arm bent a lot or a little — growth looks the same along the whole muscle.

Doing light weightlifting with blood flow restriction four times a week for three weeks can make arm muscles grow a little in young, trained guys — about 4% to 8% bigger.

For young guys who lift weights, squeezing the biceps at the top or bottom of the move doesn’t make a difference in muscle growth when using light weights with blood flow restriction.

Doing bicep curls only halfway up (keeping the arm stretched) might make the lower part of your bicep grow a tiny bit more than doing full curls, but the difference is very small.

If you're new to lifting and do cable curls the same way each time, it doesn't really matter whether your arms are stretched far back or in a neutral position — your biceps will grow about the same amount after 10 weeks.