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.

In people who haven't trained before, doing biceps curls and triceps pushdowns with either controlled form or momentum leads to modest growth in the muscles that bend the elbow and greater growth in the muscles that straighten the elbow after eight weeks.

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.