I'm pretty sure that you are talking about mass.
The answer to this question is lactic acid. The reason why sports psychologists would need to look for a buildup of this chemical is because it is the chemical that produces pain or "feeling the burn" in an exercise when someone pushes their limit. Also, after so long, a person with a lactic acid buildup needs to take a break because high levels of lactic acid is dangerous for the bloodstream.
Is effect because you have to find in map]
