Weakness with left elbow flexion · left anterior shoulder pain · Dx?

A 41-year-old, right-hand dominant man sought care at our facility one day after trying to pull his boat out of the water. He'd tried to lift the boat with his hands while his forearms were fully supinated and his elbows were flexed to about 90 degrees. He then felt a sharp burning sensation in his left anterior shoulder and was unable to lift the boat. The patient denied feeling a popping sensation at the time of the injury. He had mild pain at night, but was able to sleep. He said that he had mild diminished strength with elbow flexion, but denied having any numbness, tingling, or discoloration of his skin.