The rectangle is the original shape for trampolines and therefore rectangular trampolines have been around since the very beginning.
George Niesen invented the trampoline in its modern form in 1934 having been inspired by a performance by trapeze artists he saw in his youth. He stretched a piece of canvas over a rectangular frame and attached it with the use of inner tyre tubes and then later metal springs as we do today.
In 1937 after graduating from the University of Iowa in Business Studies he formed his own rebound trampolining act called "The Three Leonardos" and performed in Mexico. It was also around this time he trademarked the word trampoline which is Spanish for "diving board" minus the e.
Rectangular trampolines would later go on to be used for training for the US airforce pilots to improve their fitness and acclimatise themselves to being in the air during the Second World War They have also been used to prepare NASA astronauts for going into space.
Eventually because of their fantastic expressive bounce, rectangular trampolines were used during the Sydney 2000 Olympic games when trampolining was introduced to the world as a sport. They continue to be used in competitions and meets to this day.
