When young guys do super intense bicep curls they're not used to, their muscle scans show changes that start right away, get worse over a week, and can still be seen two months later.

When young guys do a tough bicep workout for the second time 8 weeks later, their muscles show less swelling and recover faster, based on MRI scans — which means their bodies adapt after the first workout.

When young guys do a specific arm exercise, their muscle response right after is about the same the first and second time, even though their body adapts later on.

After a tough bicep workout, guys' arm muscles swell up a lot in just two days, then shrink below normal size and stay that way for over two months.

Whether you lift your arm weights slowly or with a little swing, your biceps and triceps both get noticeably bigger after eight weeks of training—your triceps grow even more than your biceps.

When people who don't normally lift weights do a tough arm exercise twice, four weeks apart, their biceps don't stretch as much the second time, even though they're trying just as hard.