The elbows should line up with the waistline, about 3 heads from the top. Use your stick figure proportions as a guide to find the correct arm lengths.
Draw a figure’s arm like this:-
At the end of each shoulder line, lightly pencil in two cylinders for the upper and lower arms. A circle between the two represents the elbow.
The length of the upper arm is about 1-1/2 heads, and the forearm is about 1 head long. End the first cylinder at the elbow, which is across from the waistline. The waistline is 3 heads down.
The second cylinder ends arounds the top of the thigh (or 4 heads down) and tapers down to where the wrist and hand begin.
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Turn the sides of the cylinders into gentle curves to represent the muscles.