A TRA man reported on 1/3/14 feeling a slight pull in his lower back and right hamstring muscle after using a maul to open an underflow valve under #30 precipitator. He reported pull as minor, and said he was in no discomfort. He worked Sat.& Sun. night. On 1/8/14, he complained of pain & was sent to Medical, seen by physician for hamstring pain with ""avoid maul use"" restriction.