You're relaxing on the sofa when suddenly your eyelid starts twitching. Or perhaps it's a muscle in your arm, your leg, or your foot that begins to spasm – sometimes for a few seconds, sometimes for hours or even days. It's an unsettling sensation that affects about 70% of people at some point in their lives. Muscle twitches fall into two main types. There's myoclonus , where a whole muscle or group of muscles twitch or spasm. Then there's fasciculation , where single muscle fibres twitch –