That would be … Wayne Miller.
Jeff T. Hennessy:
… The 2 ½ twisting backward 1 ½ somersault performed on the springboard was the diving skill that was the birthplace of the triple twisting double backward somersaults. While evaluating Wayne Miller performing a 2 ½ twisting backward 1 ½ somersault, I could see if added a barani to the end of the dive …, he would have a new trampoline skill. …
After a considerable number of well-executed performances, of this new skill from the springboard, he moved to the trampoline. The trampoline bed was a one inch web bed. This took place during 1963-64. …
Trampoline Pundit – The Miller: Triple Twisting Double Back Somersault
See more historical photos on TrampolinePundit’s Flickr.
related – responses to that post from Wayne Miller, Ron Munn, and more

