α-motor units can be subdivided according to their contractile properties, into fast-twitch (F) and slow-twitch (S) fatigue-resistant types (Table 3) (Burke et al., 1973). In addition, fast-twitch α-motor units can be further categorized into fast-twitch fatigable [FF] and fast-twitch fatigue-resistant [FR] types, based on the length of time they sustain contraction. The basis of this classification is the duration of the twitch contraction time (Burke et al., 1973). F- and S-MNs also exhibit different afterhyperpolarization duration (AHP) properties. AHP is the phenomenon by which the membrane potential undershoots the resting potential following an action potential. S-MNs have a longer AHP than F-MNs, indicating that S-MNs have a longer “waiting period” before they can be stimulated by an action potential. Thus, they cannot fire at the same frequency as F-MNs (Eccles et al., 1957), so the larger FF-MNs take longer to reach an activation threshold. Similarly, other electrical properties differ between S- and F-MNs (Table 3), including their input resistance (a measure of resistance over the plasma membrane) and rheobase (a measure of the current needed to generate an action potential). S-MNs have a higher input resistance than F-MNs, underlying Hennenman’s size principle which postulates that S-motor units are the first to be recruited during movement, followed by FR and then FF units (Henneman, 1957; Mendell, 2005). Hence, a slow movement generating a small force will only recruit S-MNs, whereas a quick and strong movement will also recruit F-MNs, as well as S-MNs.