Q&A: What kind of muscle is the diaphragm?

Last Tuesday, we had the first Human Anatomy Class of the degree in Physiotherapy in Barcelona (for more information, visit: http://physiotherapybarcelona.net/). To introduce the muscular system, I explained the three types of muscles we have in our body:

• Skeletal Muscle: Most of the muscles in our body are of this kind. When we look these muscle fibers under a microscope we can see that they have "stripes" on them, due to the arrangement of filaments of actin and myosin (muscular proteins), so it is a type of striated muscle. The skeletal muscle cells have multiple nuclei and this type of muscle provides a potent contraction.
• Cardiac Muscle: It is the muscle that we can only find in the walls of the heart. It is also striated, but the disposition of its cells is more loose and they have only one nucleus. This type of muscle is very resistant to fatigue.
• Smooth Muscle: It is formed by spindle-shaped cells that have only one nucleus and allows a slow and suistainable contraction. 

Both cardiac and smooth muscle are involuntarily contracted, we can only give conscient orders to contract the skeletal muscle type. 

So, one of my students had the following question: 

What kind of muscle is the diaphragm?

I didn't explain anything about the diaphragm, just gave them the idea that it is the muscle that separates the thoracic and the abdominal cavity. My student thought that the contraction of the diaphragm is involuntary, so he did not really understood that this muscle could be skeletal. I don't know if I made myself really clear, because it was at the end of the day after a pretty hard day, and my english at that moment was terminal, but I tried to explain that this is striated muscle and that we can contract it voluntarily...In fact, if this muscle is striated it has to be skeletal muscle, but it is true that we could say that this muscle is under "involuntary" control most of the time. I think that you will find this article useful to answer this question: Development of the diaphragm, a skeletal muscle essential for mammalian respiration.