Fusion index is a method for determining muscle cell myotubule formation. Myotubules are multinucleated cells which occur from cells fusing together and are the precursor to muscle fibers. The fusion index is calculated by staining the fixed cell and determining the number of nuclei in a myotubule compared to total number of nuclei in the sample. This can be used to determine if the pharmacological based treatment being investigated alters cell fusion events that lead to myotube formation.