Altosaar graduated from a technical school in 1925, and competed for Tallinna Voitleja in 1922–27. He held national titles in volleyball (1927, 1929, 1931, 1933 and 1935–39), basketball (1927, 1930–31 and 1941) and association football (1930). In the latter sport he was playing for JK Tallinna Kalev when that club won the 1930 Estonian Football Championship. Altosaar played 16 international basketball matches for Estonia, and captained the national team at EuroBasket 1939, where it placed fifth. After 1934 he worked as an accountant and police officer. Following the Soviet annexation of Estonia, he was arrested in 1941 on charge of anti-Soviet activity and, on a decision of the NKVD, shotin a Gulag camp.