1. A relation between stimulation frequency and muscle force is usually determined with stimulus trains of constant frequency and described as a single-valued sigmoid curve. This relationship fails to explain a number of features of rate coding. 2. Single motor units were isolated in medial gastrocnemius or soleus muscles of cats deeply anesthetized with pentobarbital sodium. Motor units were classified as fast or slow. Each unit was stimulated with a train whose frequency varied linearly from less than 3 pulses per second (pps) to 20% above the unit's fusion frequency and back to about 3 pps with a period of 5 s. 3. All motor units showed a marked hysteresis during frequency-varying stimulation. A greater force was produced when frequency was decreasing than when it was increasing. The force output of each unit remained nearly maximal as stimulus frequency declined from its maximum to about one-half of the unit's fusion frequency; force rapidly declined with further decreases in frequency. The force-frequency relation could change with each trial as frequency increased but was highly reproducible when frequency decreased. This suggested a strategy by which central nervous system (CNS) control could maximize the force at any discharge rate and produce a predictable force-frequency relation. 4. Posttetanic potentiation, motor unit slowing, and a preload which causes a motor unit to operate on the negatively sloping portion of the length-tension curve may each contribute to the observed hysteresis under certain circumstances. None can explain why hysteresis was consistently seen in all motor units. A time-dependent rate of tension development and decay together with a catchlike property can account for all of the properties of hysteresis and appeared to be the primary cause of hysteresis in fully potentiated motor units.
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