Edman K A P, Josephson R K
Department of Experimental Medical Science, Biomedical Centre, F11, University of Lund, S-221 84 Lund, Sweden.
J Physiol. 2007 May 1;580(Pt.3):1007-19. doi: 10.1113/jphysiol.2006.119982. Epub 2007 Feb 15.
Force-velocity (F-V) relationships were determined for single frog muscle fibres during the rise of tetanic contraction. F-V curves obtained using isotonic shortening early in a tetanic contraction were different from those obtained at equivalent times with isovelocity shortening, apparently because changing activation early in the contraction leads, in isovelocity experiments, to changing force and changing series elastic extension. F-V curves obtained with isotonic and with isovelocity shortening are similar if the shortening velocity in the isovelocity trials is corrected for series elastic extension. There is a progressive shift in the scaling of force-velocity curves along the force axis during the course of the tetanic rise, reflecting increasing fibre activation. The time taken for F-V curves to reach the steady-state position was quite variable, ranging from about 50 ms after the onset of contraction (1-3 degrees C) to well over 100 ms in different fibres. The muscle force at a fixed, moderately high shortening velocity relative to the force at this velocity during the tetanic plateau was taken as a measure of muscle activation. The reference velocity used was 60% of the maximum shortening velocity (V(max)) at the tetanic plateau. The estimated value of the fractional activation at 40 ms after the onset of contraction was used as a measure of the rate of activation. The rate of rise of isometric tension in different fibres was correlated with the rate of fibre activation and with V(max) during the plateau of the tetanus. Together differences in rate of activation and in V(max) accounted for 60-80% of the fibre-to-fibre variability in the rate of rise of isometric tension, depending on the measure of the force rise time used. There was not a significant correlation between the rate of fibre activation and V(max). The steady-state F-V characteristics and the rate at which these characteristics are achieved early in contraction are seemingly independent. A simulation study based on F-V properties and series compliance in frog muscle fibres indicates that if muscle activation were instantaneous, the time taken for force to rise to 50% of the plateau value would be about 60% shorter than that actually measured from living fibres. Thus about 60% of the force rise time is a consequence of the time course of activation processes and about 40% represents time taken to stretch series compliance by activated contractile material.
在强直收缩上升期测定了单个蛙肌纤维的力-速度（F-V）关系。在强直收缩早期使用等张缩短获得的F-V曲线与在等速缩短的相同时间获得的曲线不同，这显然是因为在收缩早期改变激活状态，在等速实验中会导致力的变化和串联弹性伸长的改变。如果对等速实验中的缩短速度进行串联弹性伸长校正，那么用等张和等速缩短获得的F-V曲线是相似的。在强直收缩上升过程中，力-速度曲线沿力轴的标度存在渐进性偏移，这反映了纤维激活程度的增加。F-V曲线达到稳态位置所需的时间变化很大，范围从收缩开始后约50毫秒（1-3摄氏度）到不同纤维中超过100毫秒。将相对于强直平台期该速度下的力而言，固定的、适度高的缩短速度下的肌肉力作为肌肉激活程度的指标。使用的参考速度是强直平台期最大缩短速度（V(max)）的60%。收缩开始后40毫秒时的分数激活估计值用作激活速率的指标。不同纤维中等长张力的上升速率与纤维激活速率以及强直收缩平台期的V(max)相关。激活速率和V(max)的差异共同解释了等长张力上升速率中纤维间变异性的60-80%，这取决于所使用的力上升时间的测量方法。纤维激活速率与V(max)之间没有显著相关性。稳态F-V特性以及在收缩早期达到这些特性的速率似乎是相互独立的。基于蛙肌纤维F-V特性和串联顺应性的模拟研究表明，如果肌肉激活是瞬间的，力上升到平台期值的50%所需的时间将比从活纤维实际测量的时间短约60%。因此，约60%的力上升时间是激活过程时间进程的结果，约40%代表被激活的收缩物质拉伸串联顺应性所需的时间。
