I think you could blend out the hard edge on his butt to merge the shadow with the background.

the drawing is beautiful and fun, but the rendering is interfering with your representation of your anatomy knowledge. For example, how do you see the sartorius working on the viewer left leg? because you've done such a good job rendering it, it's hard to see what you are thinking or "have learned". from what I can see here- 1) the "ledge" above the knee on the viewer left leg as it transitions into the quad is more extreme than what's seen in the photo. 2) the transition from the bicep into the brachioradialis muscles in the viewer right arm looks segmented. Maybe the bicep is popping forward too much with it's lightness and needs to be a tad darker- maybe the brachioradialis muscles need to come a little higher up the arm. 3) the first phalange in the big toe on the viewer left foot needs to be longer (toes are too short) . But all those observations are helping you to copy an image and not really understand anatomy, but i hope you find it helpful.