One of the boxing benefits for males includes the building up of shoulder muscles. It’s no surprise that the deltoids get a great workout during boxing. After all, they are the muscles that attach your arms to your body. Just look at the physique of any boxer and their shoulders are often a stand out feature. But it isn’t merely because the muscles are contracting, rather it’s the type of contraction that counts. Boxing requires both type 1 and type 2 muscular contractions aka slow and fast-twitch contractions. Your shoulders are always being used at low intensities to keep your arms up to protect your face but when you throw fast, explosive punches, it requires a greater utilisation of the fast-twitch muscle fibres. Also, the delts are forced to work in a variety of planes of motion which helps to give you a well-rounded shoulder workout targeting all three heads of the deltoids. If you currently train your shoulders in the gym with weights, boxing might be the plateau-breaker you’re looking for give you that lean rounded look.
Source: manofmany.com