Randall Parker cites Toshihiko Komatsu‘s work coming out of Osaka University, which demonstrates that some people have extra muscles:
However, in Komatsu’s research, 14 to 20 percent of people were found to have three muscles and 1 to 4 percent were found to have four muscles in their biceps.People with more than the usual two muscles also tended to have more muscles than normal in other parts of their body, such as their elbows or fingers.