ARMS AND LEGS ARE REAL HARD HAVE COURAGE FRIEND. Especially knees and hands, holy SHIT those are a pain. If you're lookin for resources, I heartily recommend Andrew Loomis' book "Figure Drawing for All It's Worth"! The PDF is free online. He's an artist from the 40s and 50s but he knows his stuff and he's helped me a lot.

This guy's arms look pretty good so far tho! His right arm has one too many bumps for the triceps, but on his left arm, the deltoid and bicep look the right size and shape. Good shoulder width too-- it always surprises me how much farther the shoulders go beyond the sides of the chest. One thing to also keep in mind is that the elbows are pretty much at the same level as the navel.