Is a myth 99% of the time. The reality is that you can't apply the joint lock whatsoever on a person of similar build and strength if the person resists. You just get stuck. Believe me, I have challenged people on it, including instructors giving out the lecture, and others have too. They can't apply it to me and I can't apply it to them. Simple as that. So where does this notion stem from that you have to cooperate? It's just not true. With that in mind, how do you know where your level of Aikido is if it's never tested? What are the criterias? Is it something to do with center of gravity and balance, flow? I have no idea since it's a cooperation between individuals.