Faculty of Health and Life Sciences, University of Exeter, Exeter, UK.
Institute of Health and Sport Sciences, University of Tsukuba, Ibaraki, JAPAN.
Med Sci Sports Exerc. 2023 Nov 1;55(11):2035-2044. doi: 10.1249/MSS.0000000000003229. Epub 2023 Jun 30.
The amount of muscle belly elongation induced by passive lengthening is often assumed to be equal to that of fascicles. But these are different if fascicles shorter than the muscle belly rotate around their attachment sites. Such discrepancy between fascicles and muscle belly length changes can be considered as gearing. As the muscle fascicle arrangement is 3D, the fascicle rotation by passive lengthening may occur in the coronal as well as the sagittal planes. Here we examined the fascicle 3D dynamics and resultant gearing during passive elongation of human medial gastrocnemius in vivo .
For 16 healthy adults, we reconstructed fascicles three-dimensionally using diffusion tensor imaging and evaluated the change in fascicle length and angles in the sagittal and coronal planes during passive ankle dorsiflexion (from 20° plantar flexion to 20° dorsiflexion).
Whole muscle belly elongation during passive ankle dorsiflexion was 38% greater than the fascicle elongation. Upon passive lengthening, the fascicle angle in the sagittal plane in all regions (-5.9°) and that in the coronal plane in the middle-medial (-2.7°) and distal-medial (-4.3°) regions decreased significantly. Combining the fascicle coronal and sagittal rotation significantly increased the gearing effects in the middle-medial (+10%) and distal-medial (+23%) regions. The gearing effect by fascicle sagittal and coronal rotations corresponded to 26% of fascicle elongation, accounting for 19% of whole muscle belly elongation.
Fascicle rotation in the coronal and sagittal planes is responsible for passive gearing, contributing to the whole muscle belly elongation. Passive gearing can be favorable for reducing fascicle elongation for a given muscle belly elongation.
被动拉长引起的肌腹伸长量通常被认为与肌束相等。但是，如果肌束比肌腹短，则围绕其附着点旋转。这种肌束和肌腹长度变化之间的差异可以被认为是齿轮传动。由于肌束排列是三维的，被动拉长时肌束的旋转可能发生在冠状面和矢状面。在这里，我们检查了人体内侧比目鱼肌在体内被动伸长过程中的肌束 3D 动力学和由此产生的齿轮传动。
对于 16 名健康成年人，我们使用弥散张量成像三维重建肌束，并评估被动踝关节背屈（从 20°跖屈到 20°背屈）过程中矢状面和冠状面肌束长度和角度的变化。
被动踝关节背屈时整个肌腹伸长比肌束伸长多 38%。在被动伸长时，所有区域的矢状面肌束角度（-5.9°）和中部-内侧（-2.7°）和远部-内侧（-4.3°）区域的冠状面肌束角度均显著减小。冠状面和矢状面肌束旋转的结合显著增加了中部-内侧（+10%）和远部-内侧（+23%）区域的齿轮传动效应。肌束矢状面和冠状面旋转的齿轮传动效应相当于肌束伸长的 26%，占整个肌腹伸长的 19%。
冠状面和矢状面的肌束旋转是被动齿轮传动的原因，有助于整个肌腹伸长。被动齿轮传动有利于减少给定肌腹伸长的肌束伸长。
