I remember reading Tarzan by Edgar Rice Burroughs. The protagonist is a an extremely strong man, capable of hand to paw combat with lions. The origin of his strength is attributed to living among gorillas all his young life.
What hand wavy arguments could I invoke for that incredible strength, speed and stamina of Tarzan, the ape-man? It doesn't look to me like any human being would win a wrestling match with a hungry lion, however hard they would train.
Tarzan is slightly more than 2 meters tall, he must have very strong bones and very efficient and powerful muscles. How did he get those, it's a mystery.