The plank is recognized as a legit exercise position. One should always consult their doctor before starting any exercise program, especially if you are older. A lot of yoga moves are not valid in terms of exercise physiology and you can hurt yourself. I know a lot of yoga teachers (I lived in the Bay area for a loooong time) that have hurt their backs and can't do it at all anymore. The group camaraderie is definitely good for the soul :)