INJURED EE & HIS PARTNER WERE REMOVING BOLTS FROM A FLOW METER ON AN OVERFLOW PUMP. AS EE WAS REMOVING LAST BOLT, HE WAS HOLDING THE HEAD OF THE BOLT W/HIS LT HAND & REMOVING THE NUT W/HIS RIG HT, HE FELT A SLIGHT POP IN HIS LT SHOULDER WHICH CAUSED HIS LT ARM & SHOULDER TO FEEL NUMB. EE WASN'T IN AN IMPROPER POSITION OR USING EXCESSIVE FORCE WITH HIS LEFT HAND.