MOTOR UNIT RECRUITMENT PATTERNS DURING REFLEX COMPENSATION OF MUSCLE YIELD INVESTIGATED BY COMPUTER-SIMULATIONS

An important function of the stretch reflex in the soleus muscle in th e decerebrate cat preparation is to compensate for the tendency of mus cle suddenly to yield during ramp increases in length. As the level of background (i.e. pre-stretch) force increases, there is a systematic change in the curvature of the force trajectory during this reflex com pensation, from concave to convex with respect to increasing force. Th e hypothesis that this change in curvature was due to background force -dependent changes in the recruitment pattern of motor units was inves tigated with a combined computer simulation/experimental technique. Th e simulation consisted of 20 model motor units for the soleus muscle, each based on a distributed moment muscle model. The timing of recruit ment of the motor units was optimized to allow the simulation outputs to fit a set of experimental data records on the reflex response to st retch initiated at five different levels of pre-stretch force. The res ulting recruitment patterns showed that a tendency for recruitment to be concentrated progressively in the early portion of the stretch as p re-stretch force increased could account for the changes in reflex for ce curvature. These results are consistent with the skewed distributio n of intrinsic electrical thresholds of motoneurons, in which low-thre shold units are much more frequent than high-threshold ones. Therefore the changes in recruitment pattern and reflex force curvature may be due primarily to the intrinsic properties of motoneurons.