Brown I E, Loeb G E
Department of Physiology, Queen's University, Kingston, ON, Canada.
J Muscle Res Cell Motil. 2000 Jan;21(1):33-47. doi: 10.1023/a:1005687416896.
The interactive effects of length and stimulus frequency on rise and fall times and on sag were investigated in fast-twitch feline caudofemoralis at normal body temperature. The length and stimulus frequency ranges studied were 0.8 1.2 L0 and 15 60 pps. Isometric rise times were shortest under two sets of conditions: short lengths + low stimulus frequencies and long lengths + high stimulus frequencies. In contrast the isometric fall time relationship showed a single minimum at short lengths + low stimulus frequencies. Velocity was shown to have an additional effect on fall time, but only at higher stimulus frequencies (40 60 pps): fall times were shorter during movement in either direction as compared to isometric. The effects of sag were greatest at shorter lengths and lower stimulus frequencies during isometric stimulus trains. Potential mechanisms underlying this last effect were investigated by comparing isometric twitches elicited prior to and immediately following a sag-inducing stimulus train. Post-sag twitches produced less force, reached peak force earlier and initially decayed more quickly compared to pre-sag twitches. However, the final rate of force decay and the initial rate of force rise (during the first 15 ms) were unaffected by sag. We construct a logical argument based on these findings to hypothesize that the predominant mechanism underlying sag is an increase in the rate of sarcoplasmic calcium ion removal. All of the above findings were used to construct a model of activation dynamics for fast-twitch muscle, which was then extrapolated to slow-twitch muscle. When coupled with a previous model of kinematic dynamics, the complete model produced accurate predictions of the forces actually recorded during experiments in which we applied concurrent dynamic changes in length. velocity and stimulus frequency.
在正常体温下，研究了快速收缩的猫科动物股后肌中长度和刺激频率对上升时间、下降时间以及松弛的交互作用。所研究的长度和刺激频率范围分别为0.8 L0至1.2 L0以及15至60次/秒。在两组条件下，等长收缩的上升时间最短：短长度 + 低刺激频率和长长度 + 高刺激频率。相比之下，等长收缩下降时间的关系在短长度 + 低刺激频率时呈现出单一最小值。结果表明，速度对下降时间有额外影响，但仅在较高刺激频率（40至60次/秒）时：与等长收缩相比，在任何一个方向运动时下降时间都更短。在等长刺激序列中，松弛在较短长度和较低刺激频率时影响最大。通过比较在引起松弛的刺激序列之前和之后立即引发的等长单收缩，研究了这一最终效应的潜在机制。与松弛前的单收缩相比，松弛后的单收缩产生的力量更小，达到峰值力量的时间更早，并且最初衰减更快。然而，最终的力量衰减速率和力量上升的初始速率（在最初15毫秒内）不受松弛影响。基于这些发现，我们构建了一个逻辑论证，以推测松弛的主要机制是肌浆钙离子去除速率的增加。上述所有发现都被用于构建快速收缩肌肉的激活动力学模型，然后将其外推至慢速收缩肌肉。当与先前的运动动力学模型相结合时，完整的模型对在实验中我们同时施加长度、速度和刺激频率动态变化时实际记录的力量做出了准确预测。
