Question about running and bowel movement?

I have been running since 1977 (34 years)..Before every single race I have ever participated in, I have had to experience bowel movements (usually 3 or 4) up to the time of the race.. They always start about 40 minutes before the race. I always need to use the porta john on multiple occasions. This NEVER has happened on training runs. I am told that the term is called chicken sh*t. I believe it's my body telling me to eliminate every possible ounce for optimum performance. Anyone else have this happen as often as me? I am so used to it that I accept it and never worry about it.