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. 2020 Oct 5;4(10):941–953. doi: 10.1038/s41551-020-00619-3

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© The Author(s), under exclusive licence to Springer Nature Limited 2020

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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Fig. 1 — a, Rendering of the OSL. b, Schematic of the OSL, highlighting the transmission, electronics and load cell. c, Output view of the electric motor used in the OSL. d, Output view of the motor integrated with the open-source motor controller and embedded system. e, Side view of a single spring disk. f, Finite element analysis of a spring disk being deflected by the gear-shaped internal shaft. The colours represent the von Mises stress. g, Exploded view of six springs stacked inside the knee output pulley. h, Torque-angle relationship of the knee with 1–6 springs stacked inside. Each spring has a stiffness of approximately 100 N m rad⁻¹. We tested each condition five times.