Martens is the president and founder of Human Kinetics, the world's largest publisher of information about the sport sciences, physical education, sports, and fitness. Martens founded the American Sport Education Program (ASEP) which is the leading provider of youth, high school, and elite-level sport education programs in the United States. He is a member of the National Senior Softball Hall of Fame as player-coach for the Florida Legends. He competed in baseball as a Hornet.