Young guys who took a break from working out didn’t see much muscle growth in certain areas—like the middle of the outer thigh and upper arm—after nine weeks of doing the same strength exercises.

When young men who haven't trained before do arm exercises twice a week for about 2 months, their biceps and related muscles grow by about 6% on average.

When young men who don't work out start doing arm exercises twice a week for 10 weeks, their biceps get about 6% bigger.

If young guys who used to work out take a break and then do the same strength exercises over and over for about two months, some parts of their muscles might not grow much — especially on the side of the thigh and upper arm — which means doing the same routine might not help all muscles grow equally.

If young guys who haven’t worked out in a while switch up their strength exercises for the same muscles, they’ll likely build muscle more evenly across their thighs and arms over nine weeks compared to doing the same moves every time.

After 10 weeks of hard lifting, your arms only get a tiny bit bigger — less than the thickness of a penny — no matter which method you use.