You reference that they can be strenuous. My bet is that you often choose to push to a strenuous feeling often during the training period of each of these. My further bet is what you call "strenuous", I'm asserting that those unfamiliar would call "painful."

To be clear, I'm not claiming that you are constantly hurting yourself. I am claiming that if you weren't familiar with the feeling, you'd call it a pain. I'm thinking back to the original Matrix, "why do my eyes hurt?"