EE started shift @ 6:30 am & was working w/co-worker. At about 6:50 am, EE carried a come-a-long weighing about 20#. Immediately afterwards, EE commented he may have pulled a muscle in his back. He continued working, incl. walking, carrying tools & picking up 80# angle iron (@ 8:30 am). When he returned to co-worker & handed him 15# bearing he said he felt sharp pain in his back