He spent his youth as an avid sportsman, practising swimming, football, cycling, running, boxing, skating and water polo.

[5] Instead of searching for talents country wide, Stender tried to develop them in every athlete, and was said to be able "to teach a piece of iron to swim" (Dutch: een stuk ijzer kon leren zwemmen).

By 1955, he produced eight world record holders from girls living nearby in Hilversum,[2] a town of fewer than 100,000 people.

His pupils by then included Nel van Vliet, Mary Kok, Geertje Wielema, Hannie Termeulen, Lenie de Nijs, Judith de Nijs, Atie Voorbij, Greetje Kraan, Ineke Tigelaar, Rita Tigelaar, Rita Kroon and Herman Willemse.

[2][6] However, the next year brought him a great upset: none of them could compete in the 1956 Summer Olympics because of its boycott by the Dutch government.