Jose is an 37 year old man with a long history of marathon r…

Jose is an 37 year old man with a long history of marathon running and low weight training. At a recent family pool party, Jose’s sister noticed he struggled to lift up his left arm during each stroke. Jose is a radio announcer for a weather station and kept dropping his pointer when reporting on the weather. With these occurrences, Jose decided to see his doctor before making changes to his weight training with the intent to strength his upper body. Jose’s physician ordered the following tests: EMG, nerve conduction studies and serum laboratory testing. What is the most probable diagnosis of Jose’s condition?