Why can you see the diaphragm on an x-ray, but you can't see other skeletal muscles?

Contrast: We see things on x-ray when structures with different x-ray densities are next to each other. So, the diaphragm is a water density that is next to the lung which is air density. With a good quality image we can make out general muscle contours and sometimes more if there is a lot of fat next to them. Great question! Do you see the air in the stomach in the picture?