EMPLOYEE WAS ASSISTING WITH CHANGING A VALVE THAT WEIGHED 70 LBS. THE EE WAS HELPING LIFT & SUPPORT THE VALVE WHEN HE OVEREXERTED CAUSING A MINOR MUSCLE STRAIN IN HIS LT. SHOULDER. BECAUSE OF PERSISTENT PAIN, HE REQUESTED MEDICAL TREATMENT ON 8/18/04 WHEN HE WAS DIAGNOSED & PROVIDED MEDICATION & PLACED ON RESTRICTIVE DUTY.