Blender texture issue after extruding. What to do?

Question

Blender

Hello dear users. I am a noob in 3D modeling, I do it as a hobby, in fact, I have a ready-made body model with superimposed textures (I didn’t create it), when I tried to add abdominal muscles to the model using extrusion in places where faces were added after extruding on In the textures, light areas appeared that seem to outline the muscles along the contour, nothing was broken in the layout of the texture, light stripes appear even on rendering. How to fix it? Making muscles in sculpting is not an option, there are too few polygons, you can’t add them because the model has shape keys and rigs are applied. Thanks in advance.