Contractions of motor units evoked by stimulation with pulses and variable interpulse intervals
The rate of motoneuronal fi ring is a major factor regulating the force of motor units (MUs). During voluntary activity of a muscle, its MUs generate tetanic contractions which are characterized by variable force and fusion degree. This study aimed at analysis of force changes during tetani evoked at random stimulation patterns and their mathematical decomposition into responses to individual pulses. It was demonstrated that longer interpulse intervals and lower initial levels of force result in higher force increase during next components of tetanic contractions, and that random stimulation pattern produces higher output of MUs (and higher economy of contraction) than the constant frequency during relatively weak contractions. The decomposition revealed considerable variability in twitch responses to successive pulses. Their basic parameters can be predicted with high accuracy on the basis of the force level at which the next contraction begins. The physiological signifi cance of successive action potentials generated by active motoneurones appears to show considerable variation, so the application of constant frequency patterns during experimental analysis of functionally isolated MUs can lead to several conclusions that do not correspond to activity of MUs during natural voluntary movements. This concerns parameters of successive components of tetanic contractions: the contraction time, the instantaneous force and its variability, the effectiveness and economy of the MU’s contraction.