EE was operating the nut runner & a co-worker was using a 8 lb. sledgehammer on the stem of the valve. The co-worker struck the stem and the sledge hammer bounced off & struck EE on the top of his right hand. EE's hand was caught between the sledge hammer head & the nut runners trigger. EE was treated for a contusion. EE placed on restricted duty on 08/12/12.