At the 1937 International University Games in Paris Issak won gold in the javelin with a throw of 70.25 m, a meeting record that was only broken twenty years later. His best throw that year, 70.56 m, placed him sixth on the annual world list. Issak placed fifth at the 1938 European Championships, also in Paris, and recorded his personal best (72.07 m) in Tartu in July 1939. At that year's International University Games in Vienna he won the bronze medal with 66.79 m.