whenever i shade the torso, i always connect it somehow to the arm shading. i always make the shading heavier where the limbs connect. on my simpler skins, i normally make a singular line running up the arm that kind of tapers off (you can do that effect using lighter colors), sometimes i’ll pop in a pixel along the middle of the arm to emulate an elbow crease. i wouldn’t recommend doing complicated highlights until you have a specific shading style