This exercise comes in handy when drawing people. It helps a lot when trying to understand how stuff moves and looks when you see it from the inside.
I know it's not really a tutorial but I didn't find a better category.
Feel free to study and/or reference from it, but I would advise you to practice from medical diagrams, as this is not 100% accurate, but works well for the purpose of practicing drawing people.
Even though the names are in Spanish it should still be helpful for you
Muy bueno. Hay mínimos detalles que no sean exactos, el resto está perfecto. Te felicito no ya solo por la exactitud, si no por los trazos y las proporciones, que son inmejorables. Un gran trabajo