Muscles are formed of bands, sheets, or columns of elongated cells (or fibres) containing interlocking parallel arrays of the proteins actin and myosin. Projections on the myosin molecules respond to chemical signals by forming and reforming chemical bonds to the actin, so that the filaments move past each other and interlock more deeply. This converts chemical energy into the mechanical force of contraction, and also generates heat
late Middle English: from French, from Latin musculus, diminutive of mus 'mouse' (some muscles being thought to be mouse-like in form)
Do not confuse muscle with mussel. Muscle means 'the tissue that moves a body part' (tone up your thigh muscles), whereas mussel means 'a shellfish' (fish soup with mussels and clams).