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. 2019 Feb 25;9:2732. doi: 10.1038/s41598-019-39522-6

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Mechanistic model of eSMT force generation and transmission, and changes in the mechanical property induced by ECM remodelling. (a) Mechanistic model of eSMT consisting of an active element (AE), parallel passive element (Parallel PE), and serial passive element (Serial PE). (b) Images of collagen IV (red) and actin in the myotubes (green) of the eSMTs to measure the fibre orientation distribution of the ECM network (left). Images of well-aligned collagen fibres (middle) and less-aligned fibres (right). Scale bars represent 5 μm. (c) Schematics showing the regions of the parallel and serial ECMs relative to the myotubes, and ECM fibre orientation θ measured from the longitudinal direction of myotube. (d,e) Orientation distribution of collagen IV in the parallel (d) and serial (e) ECM regions. Most ECM fibres were aligned parallel to the longitudinal direction of the myotubes (0°) by pretension. SEM, n = 8, 12, 11, and 10 (parallel), n = 3, 7, 3, and 7 (serial). (f) Comparison of fibre orientation factors (η_o) to predict the elastic modulus of the ECM network (E_L). The out-of-phase co-stimulation induced the highest elastic modulus (largest η_o) for the Serial PE and a low elastic modulus for the Parallel PE. SEM, n is the same as (d,e). *P < 0.05, **P < 0.01, and ns, not significant. (g) Schematic depicting the mechanism underlying the changes in performance induced by ECM remodelling following the application of the out-of-phase co-stimulation. The out-of-phase co-stimulation decreases the stiffness of the parallel ECM for less impedance on muscle contraction and increases the stiffness of the serial ECM to increase force transmission to the load.