In his youth, Assmy planned to be a civilian pilot and joined the German Air Force before World War II as a start for an aviation career. In those pre-war years he had also participated in other sports, including soccer, handball, swimming. In 1938, he crashed on a training flight in a Henschel Hs 123 fighter plane at Württemberg, and lost his vision in both eyes at the age of 23.[2]
Around 1971, Assmy realized that blindness need not keep him out of active sport and resumed training, first in gymnastics, then in track and field.[3]: 15
During competition he was guided by a sighted runner with a short rope tethered to their wrists, running in the outer lane to avoid getting in the way of other competitors.[4]: 10
His most notable achievements in athletics were at the World Masters Athletics Outdoor Championships, from 1977 to 1991. In his first 3 WMA Championships (1977, 1979, 1981), he was guided by his son-in-law Klaus Hinrichsen.[2][5] Then, after his daughter and son-in-law split up, he switched to his son as the guide for 1983 and 1985,[6] and then to his grandson in 1991.[7]
Some claimed his guide had "pulled" or "tugged" Assmy in some events, giving him an unfair advantage.[8]: 178 [9][6] In a response to such criticisms, Assmy defended his running technique in a letter published in the June 1985 National Masters News newsletter.[10]