Ultrasound can reliably measure arm muscle size in active women, and the same person can get consistent results when repeating the measurement—making it a trustworthy way to track muscle growth from workouts.

When untrained young men lift weights until they can't anymore, both heavy and light weights make their muscles grow about the same amount (6% bigger) if they do the same total work. This goes against the common belief that heavier weights are always better for muscle growth.

When researchers measured the thickness of the upper arm at two specific spots, both spots got thicker after 12 weeks of arm workouts.

Measuring muscle thickness at a few spots on the body may not correctly show how much muscle has grown overall, because the measurements might be inaccurate or have not been properly tested for reliability.