A prolongation or process extending from a muscle cell. A muscle cell is a mature contractile cell, commonly known as a myocyte. This cell has as part of its cytoplasm myofibrils organized in various patterns. [ http://purl.obolibrary.org/obo/CL_0000187 http://www.ncbi.nlm.nih.gov/pubmed/15930100 GOC:kmv http://www.ncbi.nlm.nih.gov/pubmed/22464329 ]