1. Multiunit integrated electromyographic (EMG) signals and single-unit EMG potentials were recorded during isometric and lengthening (stretch reflex) contractions of soleus and medial gastrocnemius (MG) muscles in 20 decerebrate cats. Patterns of motor-unit recruitment and rate modulation were examined in isometric muscles and during constant-velocity stretches. 2. Analysis of multiunit EMG activity and its relationship to active force revealed a marked difference between isometric and lengthening contractions. While the force-EMG relationship for isometric contractions was characteristically linear, the relation recorded during stretch-reflex responses showed a disproportionate early EMG increase, which was most obvious at low force levels, suggesting that the efficacy of force production is reduced in lengthening muscle. 3. Single-unit recruitment patterns were found to be qualitatively similar in isometric and lengthening contractions. In each case, motor units were recruited in order of increasing spike voltage. The numbers of newly recruited units declined steeply with each successive increment in active force. For a given unit, the force at which recruitment occurred was found to be greater in lengthening contractions than in isometric contractions, and in lengthening contractions it was also found to depend on the level of initial force. 4. Two patterns of motor-unit rate modulation were observed during muscle stretch, depending on whether a given unit was firing before the beginning of stretch or wether it was recruited during the course of stretch. Motor units that were active prior to stretch were found to increase firing rate at stretch onset and to vary their rate very little thereafter. Motor units recruited in the course of stretch began firing at an initial rate proportional to their force threshold, gradually increased their firing rate with increasing force, and sometimes reached an apparent maximum rate. 5. These results are discussed in terms of the mechanical properties of lengthening muscle and reflex regulation of these properties. Each identified pattern of motor-unit recruitment and rate modulation is evaluated for its potential contribution to the regulation of muscle properties, especially the prevention of muscle yield. We conclude that at low to moderate levels of initial force, recruitment of new motor units is likely to be the most effective compensatory mechanism.
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