The secret behind Ronaldo's keenness to paint his toenails black. It is believed that the coating adds a protective layer that prevents bacteria from entering the skin.
The coating increases the difficulty of nails cracking or splitting during exercise or violent collisions, which prompts mixed martial arts fighters to follow the same approach. Some top boxers also did the same, most notably former world champion Mike Tyson and current boxer Francis Ngannou. Some believe it helps prevent fungal infections in sweaty shoes.