1. Single motor units of the flexor carpi radialis (FCR) muscle were activated with a series of constant-rate stimulus trains to study the relation between the frequency of activation and isometric tension development (F-T relation). The tension produced by each stimulus train was expressed as a percentage of the maximum tension-time area (A(max)) found for a given unit. 2. Between 25 and 75% A(max) a clear separation was seen in the rates needed to produce the same relative tension for the F-T curves of slow-twitch (type S) and fast-twitch (type F) units. Over the steepest portion of the F-T curve (25-50% A(max)), where tension output was most sensitive to changes in activation rate, type F units required substantially higher stimulation rates (30 pps) to achieve the same relative tension output as type S units. Furthermore, the frequency range that corresponded to the steep portion of the curve was 2.3 times greater for type F units. 3. For both type S and F units, twitch duration was deemed to be an important determinant of the F-T curve, as has been shown previously. A direct continuous relation was seen between the integrated twitch time (ITT) and the stimulus interval needed to produce 50% A(max) (r = 0.94, P < 0.001). Thus, units that had relatively brief twitches required higher activation rates to achieve the same relative percentage of A(max). 4. Comparison of F-T curves from FCR with those derived by other investigators for cat hindlimb units (medial gastrocnemius and peroneus longus; Ref. 34) revealed that significant differences in activation rates were needed to produce the same percentage of A(max) throughout the midrange of the F-T curve. At 50% A(max), type F units in FCR required activation rates ~20 pps higher than type F units in the hindlimb. Type S units in FCR required only slightly higher rates (~5 pps). 5. Based on a number of well-founded assumptions, F-T curves derived from FCR units were used to estimate the potential contribution of rate coding to total muscle tension by type S and F units. This analysis leads to the conclusion that rate modulation is a potentially important factor in the gradation of tension for the FCR muscle.
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