The behaviour falls in line with their other behaviour.  Go for low effort and don't think of the consequences.  Locking the knees requires not effort from your muscles anymore (teehee look how smart), they blatantly disregard the consequences ( http://www.dailymail.co.uk/video/news/video-1165684/Horrible-moment-woman-breaks-leg-whilst-doing-leg-press.html ) which are that your knees bend over the wrong way and you get to sit in a wheelchair for a few years/forever. (Teehee rolling is more efficient and I can suck more benefits this way)