Nice work Marco. You've made the triceps look very clear, and for the purpose of showing that area, it's good study. My feedback is more of a personal opinion and has more to do with the drawing itself, rather than the anatomy, so keep that in mind: I feel that the shift from the rendered muscles to the non-rendered area on the right side of the arm (picture wise) is a little sudden. This has nothing to do with the study, but I think making the transition from rendered to non-rendered could've been a bit more gradual. It's a bit better on the left side of the drawing, all the way down. I hope you find that useful :)