Employee state he was loosening air line fittings from pipe fittings with two wrenches. Upon squeezing he wrenches to loosen the fittings he heard a tearing sound and burn in his left shoulder. Strength was weakened in left arm, thought it would be better. By Friday evening, bruise had formed on left bicep and pain persisted. DID NOT REPORT INCIDENT UNTIL 9/2/13 late evening.