Cinderella hypothesis

The Cinderella hypothesis is based on earlier finding by Henneman et al. (1965), showing an orderly recruitment of motor units - the smaller functional units of the muscle - in response to static muscle load. Small, low-threshold motor units (type Ⅰ fibers) are always recruited first, before large ones, and these low-threshold units are assumed to remain constantly active until complete relaxation of the muscle. Motor units with higher thresholds are activated at higher force levels and are shut off as soon as the force level decreases. This means that the low-threshold or 'Cinderella' motor units are constantly active under sustained physical work, whereas motor units with higher thresholds are activated only during heavy physical work.