Department of Physical Therapy, College of Medical Science, Jeonju University, Jeonju, South Korea.
Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ, USA.
Sci Rep. 2023 Jan 18;13(1):948. doi: 10.1038/s41598-023-28236-5.
Although the phenomenon of residual force depression has been known for decades, the mechanisms remain elusive. In the present study, we investigated mechanisms of residual force depression by measuring the stiffness to force ratio during force redevelopment after shortening at different velocities. The results showed that the slope of the relationship between muscle stiffness and force decreased with decreasing shortening velocity, and the y-intercept increased with decreasing shortening velocity. The differing slopes and y-intercepts indicate that the stiffness to force ratio during isometric force redevelopment depends on the active shortening velocity at a given muscle length and activation level. The greater stiffness to force ratio after active shortening can potentially be explained by weakly-bound cross bridges in the new overlap zone. However, weakly-bound cross bridges are insufficient to explain the reduced slope at the slowest shortening velocity because the reduced velocity should increase the proportion of weakly- to strongly-bound cross bridges, thereby increasing the slope. In addition, if actin distortion caused by active shortening recovers during the force redevelopment period, then the resulting slope should be similar to the non-linear slope of force redevelopment over time. Alternatively, we suggest that a tunable elastic element, such as titin, could potentially explain the results.
尽管残余力抑制现象已经存在了几十年，但其中的机制仍然难以捉摸。在本研究中，我们通过测量在不同速度下缩短后力再发展期间的力与刚度比，研究了残余力抑制的机制。结果表明，肌肉刚度与力之间关系的斜率随缩短速度的降低而减小，而截距随缩短速度的降低而增大。斜率和截距的差异表明，在给定肌肉长度和激活水平下，等长力再发展期间的力与刚度比取决于主动缩短速度。主动缩短后较大的力与刚度比可能可以用新重叠区中弱结合的横桥来解释。然而，弱结合的横桥不足以解释在最慢缩短速度下斜率的降低，因为降低的速度应该增加弱结合到强结合的横桥的比例，从而增加斜率。此外，如果由于主动缩短而引起的肌动蛋白扭曲在力再发展期间恢复，则所得斜率应与随时间变化的力再发展的非线性斜率相似。或者，我们建议像titin 这样的可调弹性元件可以用来解释这些结果。
