@goodhunter most sumo athlete has strong knees, considering the mermaid in this universe convert their fins into feet while maintaining their strength and stamina, I wouldn’t be too surprised. I think the potion they use has been researched over and over with mermaids going to land to get herbs that possibly doesn’t exist in sea.
As a reference, usually fish doesn’t stop unless they are extremely tired. Shark are literally will die if they stop, obviously if you convert them into human, they probably be the one that has the most stamina out of animal kingdom. (Not counting insects)