I have always had trouble drawing the arm muscles, but since I spent plenty of time on the arm muscles yesterday. I sort of found where I often do wrong; I often make the triceps + biceps too large in relation to the Deltoid. So from now on, I’ll have to make sure that I draw the deltoid bigger in relation to biceps/triceps. Also, the overall SHAPE of the biceps/triceps is an aera I need to practice more on. It’s mostly when I draw more muscular arms where I get trouble with the arm muscles, more skinny arms are no trouble - since I have skinny arms myself I can easily use myself as a quick reference.