Fasciculations refer to involuntary, brief, and spontaneous muscle contractions or twitches that involve small groups of muscle fibers. These twitches are visible under the skin and can happen in any muscle of the body, but they often occur in the arms, legs. Unlike myokymia, which typically involves finer muscle fibers like those around the eyes. Local Twitch Response (LTR) refers to an involuntary, reflexive contraction of a muscle or a portion of a muscle that occurs when a trigger point is s

1. A muscle's contractile units lie within individual muscle cells, which are also called muscle fibers. Each muscle fiber contains numerous bundles of protein filaments. These bundles, called myofibrils, are organized into repeating contractile units called sarcomeres. 2. A sarcomere contains two types of protein filaments: actin filaments and myosin filaments. Each actin filament is attached to a Z line, which is found at either end of a sarcomere. The thick myosin filaments lie between the Z