I attended a seminar in Columbia, South Carolina back in the 1980's conducted by Nishiyama Sensei. He brought with him, a Godan that trained with him back in his Los Angeles dojo, and used him to "demonstrate different positions" of kicks, punches, etc.
The most impressive thing that happened, was when Sensei was discussing side kicks. This guy whacked his leg out in a perfect head-high side kick, and simply held it there. As Nishiyama Sensei talked, he kept his leg out... still in perfect position for several minutes. Then, it got interesting...
He didn't let his leg drop, but Sensei walked around him turning his foot in different positions as he lectured, and discussed what dynamics were effected by allowing the foot to "drift" off line, or turn incorrectly. Eventually, when that part of the discussion was over, he allowed the man to drop his leg... no less than 8 to 10 minutes from when he had first put it into that perfect side kick.
During this entire discussion, the man was never off balance or hopping around, never appeared to be the slightest bit tired, and never allowed his leg to drop even a little. When he put his leg down, he and Nishiyama Sensei both stepped behind and did the smoothest turns to start discussing punches that you've ever seen.
In 40+ years of classes and seminars, I don't remember seeing anyone else with the kind of body control that was exhibited at that seminar. It was (to quote Rocky's trainer) ... "a t'ing of beauty".