Nonlinear force addition of newly recruited motor units in the cat hindlimb

The present experiments were designed to examine the interaction of simultaneously active motor units. Pairs of medial gastrocnemius (MG) or soleus (Sol) units were stimulated individually and then together with constant frequency trains of 5-40 pulses per second. Stimulating two units asynchronously produced a smoother contraction than synchronous stimulation, but rarely a force increase. This contrasts with similar experiments on whole muscle bundles. A force increase may require that adjacent muscle fibers be active. The combined force of two motor units exceeded the algebraic sum of their separate forces by 12% in MG and 5% in Sol on average. The force a unit could sustain after a second unit fell silent was greater than the force the unit produced alone (21% in MG and 8% in Sol). We conclude that motor units produce more force when interacting than alone. During derecruitment the units remaining active produce more force than when recruited.