Call me crazy but it's his upper arm.
Diameter of football = 22cm
Average upper arm length for male = 37cm
Every chance it's going to be near both the shoulder and the elbow.
Arguing semantics....he tried for a header, missed and it hit his arm, which was in a natural position for jumping. Refs obviously think that the ball hitting his arm was preventing a goal scoring chance, or that perhaps people jump like penguins with their arms down by their sides. Actually FIFA should make this a new rule in football, for comedy value.