As for tickle fights, for most of my tickle spots I don't feel weaker but I was always very restrained in how I defended myself, since all of my tickle fight partners were women. I was kinda afraid that I might accidentally hurt them. Some of them realised that and used it to their advantage. But then there's the neck. I turn into jelly when my neck is tickled. I try to curl up into a ball and just swat at the ler's hands weakly and helplessly.
As for tickle sessions, they are a workout. I start strong and get weaker the longer I get tickled. And my muscles feel sore the next day from all the flailing. Unless, the ler hits the neck, of course. If she does, I'm Mr. Jelly but this time I have nowhere to go.