Mechanisms underlying the regulation of motor unit contraction in the skeletal muscle.

The control of movements is made posible thanks to the activity of motor units in skeletal muscles. In the present paper the influence of frequency and pattern of motoneuronal firing on the tension of contraction and the tension-time area is presented and discussed. The most resistant to fatigue slow-twitch motor units are low susceptible to changes in a pattern of impulses and therefore they are well prepared to participate in long-lasting movements at low but rather stable levels of tension. Moreover, their contraction is very effective and it is performed at a low metabolic charge. Fast-twitch units have lower resistance to fatigue and they have higher tension but they have high susceptibility to a pattern of pulses and their tension can be effectively regulated by an increase or a decrease in the interpulse interval. Therefore, fast motor units are specialized to participate in the regulation of the movement force. The existence of different functional groups of motor units in skeletal muscles enables the performance of different motor tasks very effectively and at possibly low metabolic costs.