He continued his winning streak in the 1971–74 competitions. In <answer>1975</answer>, Schwarzenegger was once again in top form, and won the title for the sixth consecutive time, beating Franco Columbu. After the <answer>1975</answer> Mr. Olympia contest, Schwarzenegger announced his retirement from professional bodybuilding.
What year did Schwarzenegger quit competing as a professional bodybuilder?
1975