Standing 2.05 metres (6 feet 9 inches) tall, Karppinen was able to make long strokes with minimal strain. His easy strokes and great strength made him a strong finisher, and he often stayed at the back of the field before putting on a fierce sprint in the last 500 metres of a race. Throughout his career Karppinen battled Kolbe for world and Olympic titles. Kolbe dominated the world single sculls competition, winning five titles to Karppinen's two. At the 1976 Olympic Games in Montreal, however, Karppinen beat Kolbe by more than two seconds to take the gold medal.

Karppinen defended his Olympic title at the 1980 Moscow Games in Moscow (in which Kolbe did not participate) and again beat Kolbe for his third gold medal at the 1984 Olympics in Los Angeles. At the end of his career, Karppinen placed out of the medal count in the 1988 and 1992 Olympic Games, finishing 7th and 10th respectively. In addition to his two world championship victories, he also won three silver medals (1977, 1981, 1986) and a bronze (1987).