[Biceps brachii motor unit frequency and recruitment during static work (author's transl)].

The recruitment and the changes in firing frequency of biceps brachii motor units (MU) were studied during static work in normal man. The activity of single MUs have been followed from minimal to maximal levels of force, using three selective wire-electrodes inserted in the muscle. 1. The increase of MUs firing rate contributes to the force increase during the whole of the range of forces. However, the contribution of increased firing rate is more important at low force levels. 2. The firing rate variability is more important for low levels of force. This may be one of the mechanisms by which the summation of the twitches leads to a stable contraction. 3. At each level of force, the diverse MUs fire at the same mean frequency, except during the "catching up phase" which follows the start of the 'MUS activity. 4. The force generated by the muscle varies linearly with the number of active MUs, i.e. the MU recruitment has a constant contribution to the increase of force. 5. The comparison between firing rate and MU recruitment suggests that the first mechanism is more important f or low levels of force, while MU recruitment becomes more important at higher force levels.