The ‘rate limiting’ factor re shoulder width is the length of one’s clavicles. Advanced swimmers often have very long clavicles, but this is because of selection bias; ie, they’re advanced swimmers in part because they have long clavicles, not vice versa.
At 21, it’s safe to say your clavicles are finished growing. So, as a charter member of the Narrow Clavicle Club, allow me to welcome you as a member. All is not lost, however. Three basic strategies can be employed to create the illusion of a wide shoulder structure:
- A tremendous amount of ‘meat’ can be added to the medial deltoids, so working them should be a priority. In that regard, review this thread:
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The bigger your traps get, the narrower your shoulders will look. So avoid overdoing trap work.
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The smaller your waist, the wider your shoulders will look in proportion. So lose fat (if you carry a significant amount), and work on making your waist/abs tight.
There have been many top-tier BBers who had to overcome narrow clavicles (Casey Viator comes to mind; in the modern era, Phil Heath is not sporting what I would label an impressive set of clavicles). You can do it too. Best of luck.