Botterman B R, Iwamoto G A, Gonyea W J
J Neurophysiol. 1986 Aug;56(2):494-506. doi: 10.1152/jn.1986.56.2.494.
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 (Amax) found for a given unit. Between 25 and 75% Amax 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% Amax), 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. 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% Amax (r = 0.94, P less than 0.001). Thus, units that had relatively brief twitches required higher activation rates to achieve the same relative percentage of Amax. Comparison of F-T curves from FCR with those derived by other investigators for cat hindlimb units (medial gastrocnemius and peroneus longus) revealed that significant differences in activation rates were needed to produce the same percentage of Amax throughout the midrange of the F-T curve. At 50% Amax, type F units in FCR required activation rates approximately 20 pps higher than type F units in the hindlimb. Type S units in FCR required only slightly higher rates (approximately equal to 5 pps). 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.
通过一系列恒速刺激串激活桡侧腕屈肌（FCR）的单个运动单位，以研究激活频率与等长张力发展之间的关系（F-T关系）。每个刺激串产生的张力表示为给定单位的最大张力-时间面积（Amax）的百分比。在25%至75%Amax之间，可以清楚地看到慢肌纤维（S型）和快肌纤维（F型）单位的F-T曲线在产生相同相对张力所需的频率上存在明显差异。在F-T曲线最陡的部分（25%至50%Amax），即张力输出对激活频率变化最敏感的部分，F型单位需要显著更高的刺激频率（30次/秒）才能达到与S型单位相同的相对张力输出。此外，F型单位对应于曲线陡峭部分的频率范围是S型单位的2.3倍。如先前所示，对于S型和F型单位，抽搐持续时间被认为是F-T曲线的一个重要决定因素。在积分抽搐时间（ITT）与产生50%Amax所需的刺激间隔之间存在直接的连续关系（r = 0.94，P小于0.001）。因此，抽搐相对短暂的单位需要更高的激活频率才能达到相同的Amax相对百分比。将FCR的F-T曲线与其他研究者为猫后肢单位（腓肠肌内侧头和腓骨长肌）得出的曲线进行比较，发现在F-T曲线的整个中间范围内，产生相同百分比的Amax需要不同的激活频率。在50%Amax时，FCR中的F型单位所需的激活频率比后肢中的F型单位高约20次/秒。FCR中的S型单位只需要略高一点的频率（约等于5次/秒）。基于一些有充分依据的假设，利用从FCR单位得出的F-T曲线来估计S型和F型单位的频率编码对总肌肉张力的潜在贡献。该分析得出结论，频率调制是FCR肌肉张力分级中一个潜在的重要因素。
