This study was designed to determine whether human thenar motor units can be classified into types by the same physiological criteria used for other mammalian limb motor units and to consider whether such classification is functionally relevant. Contractile responses of 25 human thenar single motor units were examined when their motor axons were stimulated intraneurally at rates from 1 to 100 Hz and intermittently at 40 Hz in a conventional 2-min fatigue test. Twitch and tetanic forces were measured together with various indexes of contractile rate. Twitch contraction times and subtetanic to maximum tetanic force ratios were both distributed continuously. 'Sag' in tension was not evident in unfused force profiles. Thus these units could not be divided into fast and slow types by the use of traditional contractile rate criteria. Most units were fatigue resistant, with force fatigue indexes (FI) ranging from 0.33 to 1.14. None could be classified as fatiguable (FI < 0.025). Seven units (28%) fell into the fatigue-intermediate (FI = 0.25-0.75) category, whereas 18 units (72%) had FI > 0.75, i.e., they were fatigue-resistant units. However, these units could not be classified by conventional FI and contractile rate criteria, because fatigue-resistant and fatigue-intermediate units had similar contractile rates. Additional FI were calculated to describe changes in contractile rate. During the fatigue test, units behaved in one of three ways, showing 1) little change in either force or rate; 2) contractile slowing during the contraction and relaxation phases, with little or no force loss; or 3) both force and rate reduction. If the presence or absence of force loss and slowing of relaxation during the fatigue test were used to separate the units, the resultant groups shared some properties of 'slow, fatigue-resistant'; 'fast, fatigue-resistant'; and 'fast, fatigue-intermediate' types of motor units, respectively, as described in cat. For example, the tetanic and twitch forces were weakest for units showing little force loss with no slowing and greatest for units showing force and rate reduction. The initial contraction time of units showing little force loss with no slowing tended to be longer than those of the other units. However, after posttetanic twitch potentiation, some of these trends disappeared. Furthermore, there were no obvious breaks in the distribution of force and one-half relaxation time FI, suggesting that human thenar unit contractile properties operate along a continuum.
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