The System of Flexing and Extending Muscles Makes Movement of Appendages

The system of flexing and extending muscles makes movement of appendages easy to understand. But how are some actions possible (sticking out your tongue, beating of your heart) when these structures have no bone structure with opposing flexors and extensors in them?

A) One set of muscle cells must therefore be able to both contract and expand.
B) These structures must be empty chambers, and fluids forced into them expand them.
C) Since muscle cells can only contract and relax, muscle cells running diagonally or crosswise amidst the longitudinal muscles must contract and force the long relaxed muscle cells to elongate.
D) This cannot be explained at present.