Fast and slow motor units: their recruitment for tonic and phasic contraction in normal man

Most skeletal muscles include at least two types of MU's. Then, it has been stated that the properties of each MU may imply its specific recruitment in static or dynamic work. To test this hypothesis, the activities of biceps brachii MU's have been recorded during voluntary static contraction followed without any break by dynamic contraction. The MU's activity was recorded by means of wire-electrodes. From 45 tests carried out on 15 normal subjects in which 78 MU's were studied it has been found that: (1) the same MU may discharge both in static and in dynamic work; (2) in both types of work the MU's firing rate and recruitment only depend on the external force. For example, a single MU which does not discharge for a given level of static force will discharge during the following dynamic work, but can also be recruited for a higher level of static force. Thus, it can be said that the qualification "tonic" and "phasic", characterize the MU's twitches and do not imply any static-dynamic differentiation. A comparison has been made between the patterns of MU activity which occur in movement against load and those which have been found in movement against inertia. It is suggested that while the latter are preprogrammed, the former fall under a feed-back control.