Nope...not the muscles, the bones.

I just watched an interesting video on the NorCal Cycling YouTube channel tonight. The presenter tested the aerodynamic differences between the newest "narrow bar from shifter to shifter with a flared drop...37cm" to his standard 42cm aero bars. Testing was done on identical bikes, wheels, same watts on the same course and riding in the same position on the shifters on both bikes. The narrow handlebar was four seconds faster on the almost 5 mile course. So a very marginal difference. He did say it felt good and may be more useful in a crit compared to a wider bar so there is that.

I use Roval Rapide 38cm bars on my 52cm Aethos frame and I quite like the bars. My shoulder measurement is near 40cm so no noticeable impact and to be honest I don't really notice if there is a difference between the 38cm and 40cm bar I was using...Two centimeters is barely an inch.