Hinks Avery, Jacob Kaitlyn B E, Patterson Makenna A, Dalton Benjamin E, Power Geoffrey A
Department of Human Health and Nutritional Sciences, College of Biological Sciences, University of Guelph, Guelph, ON N1G 2W1, Canada.
Department of Human Health and Nutritional Sciences, College of Biological Sciences, University of Guelph, Guelph, ON N1G 2W1, Canada.
J Sport Health Sci. 2024 Oct 23;14:101000. doi: 10.1016/j.jshs.2024.101000.
Residual force enhancement (rFE), defined as increased isometric force following active lengthening compared to a fixed-end isometric contraction at the same muscle length and level of activation, is present across all scales of muscle. While rFE is always present at the cellular level, often rFE "non-responders" are observed during joint-level voluntary contractions.
We compared rFE between the joint level and single fiber level (vastus lateralis biopsies) in 16 young males. In vivo voluntary knee-extensor rFE was measured by comparing steady-state isometric torque between a stretch-hold (maximal activation at 150°, stretch to 70°, hold) and a fixed-end isometric contraction, with ultrasonographic recording of vastus lateralis fascicle length (FL). Fixed-end contractions were performed at 67.5°, 70.0°, 72.5°, and 75.0°; the joint angle that most closely matched FL of the stretch-hold contraction's isometric steady-state was used to calculate rFE. The starting and ending FLs of the stretch-hold contraction were expressed as % optimal FL, determined via torque-angle relationship.
In single fiber experiments, the starting and ending fiber lengths were matched relative to optimal length determined from in vivo testing, yielding an average sarcomere excursion of ∼2.2-3.4 µm. There was a greater magnitude of rFE at the single fiber (∼20%) than joint level (∼5%) (p = 0.004), with "non-responders" only observed at the joint level.
By comparing rFE across scales within the same participants, we show the development of the rFE non-responder phenomenon is upstream of rFE's cellular mechanisms, with rFE only lost rather than gained when scaling from single fibers to the joint level.
残余力增强（rFE）定义为在相同肌肉长度和激活水平下，主动拉长后等长力相较于固定端等长收缩时增加，在所有肌肉尺度中均存在。虽然在细胞水平rFE始终存在，但在关节水平的自主收缩过程中，常观察到rFE“无反应者”。
我们比较了16名年轻男性关节水平和单纤维水平（股外侧肌活检）的rFE。通过比较拉伸-保持（在150°最大激活，拉伸至70°，保持）和固定端等长收缩之间的稳态等长扭矩来测量体内自主伸膝rFE，并超声记录股外侧肌肌束长度（FL）。固定端收缩在67.5°、70.0°、72.5°和75.0°进行；使用与拉伸-保持收缩的等长稳态FL最匹配的关节角度来计算rFE。拉伸-保持收缩的起始和结束FL表示为%最佳FL，通过扭矩-角度关系确定。
在单纤维实验中，起始和结束纤维长度相对于体内测试确定的最佳长度相匹配，平均肌节偏移约为2.2 - 3.4 µm。单纤维水平的rFE幅度（约20%）大于关节水平（约5%）（p = 0.004），“无反应者”仅在关节水平观察到。
通过比较同一参与者不同尺度的rFE，我们表明rFE无反应者现象的发展位于rFE细胞机制的上游，从单纤维扩展到关节水平时，rFE只是丧失而非获得。
