there seems to be a lot of unnecessary dithering on his face and jacket, and the shading could use some work -- it doesn't suggest that the garment has a shape so much as it suggests you made random lines with a pen, especially on the shorts.
his ankles are positioned too high on his legs for the angle of the sprite (I'm assuming it's fullview, facing the player,) and his arms are too short. on a human being, the fingertips should be roughly at mid-thigh.
as for the musculature of the arms, take a look at some references and rely more on shading than just the presence of random lines. showing how light and shadow show up on a shape often define the shape better than just outlining the major features. remember, shading isn't just a diagonal gradient away from the chosen light source -- it follows the contours of the object.
if you're new to spriting, I'd suggest working on a smaller scale first. smaller sprites are good for learning a few basic principles, which can then be applied to larger sprites. starting with a big sprite from the getgo is a little like trying to run before you're 100% on walking, though.