John (Jonus) Bertelsen (1917-1978) was Emdrup’s first playleader. His work helped define the entire adventure play movement. He was a member of the Danish Resistance Movement.fighting against the Nazi's, which meant that he was enormously respected by the children at Emdrup. It is noteworthy that Emdrup was started and developed whilst Denmark was under Nazi occupation.

John Bertelsen wrote a diary of his early days of Emdrup (printed in ‘Adventure Playgrounds’ by Arvid Bengtsson).

"The adventure playground is an attempt to give the city child a substitute for the play and development potential it has lost as the city has become a place where there is no space for the child's imagination and play. Access to all building sites is forbidden to unauthorized persons, there are no trees where the children can climb and play Tarzan. The railway station grounds and the common, where they used to be able to fight great battles and have strange adventures, do not exist any more. No! It is now not easy to be a child in the city when you feel the urge to be a caveman or a bushman".

Bertelsen, J. (1946)

John Bertelsen also coined the phrase "skrammologi" or "junkology" for what is now known as adventure play.