The energy molecule that breaks apart in muscle cells to yield energy is adenosine triphosphate (ATP).

Here's why:

* ATP is the universal energy currency of cells. It stores chemical energy in its phosphate bonds.

* Muscle contraction requires energy. ATP provides that energy.

* When ATP breaks down, it loses a phosphate group, becoming adenosine diphosphate (ADP). This process releases energy that the muscle can use to contract.

Let me know if you have any other questions about muscle cells and energy!