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This study presents the first implementation of the muscle length feedback reflex and establishes a minimal sensorimotor unit based on mechanical intelligence. Specifically, the proposed reflex module integrates sensing, computation, and actuation within its physical architecture, enabling autonomous contraction when muscle length exceeds a threshold, entirely without electronic components. Additionally, a muscle spindle\u2010inspired mechanism allows for adaptive modulation of this threshold. When incorporated into musculoskeletal robots, the module reproduced the patellar tendon reflex. Furthermore, by integrating this mechanical reflex with high\u2010level phase\u2010dependent threshold modulation control, bipedal locomotion was generated as an emergent behavior. These findings demonstrate that essential local sensorimotor control can be physically realized within mechanical systems, providing a minimal, robust, and scalable design principle for musculoskeletal robotics and advancing embodied intelligence beyond traditional control architectures.<\/jats:p>","DOI":"10.1002\/aisy.70590","type":"journal-article","created":{"date-parts":[[2026,10,22]],"date-time":"2026-10-22T01:00:26Z","timestamp":1792630826000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Purely Mechanical Muscle Length Feedback Enables Embodied Reflexive Control in Musculoskeletal Robots"],"prefix":"10.1002","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-3806-4803","authenticated-orcid":false,"given":"Shunsuke","family":"Koseki","sequence":"first","affiliation":[{"name":"Department of Robotics, Graduate School of Engineering Tohoku University  Sendai Japan"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6179-5706","authenticated-orcid":false,"given":"Mitsuhiro","family":"Hayashibe","sequence":"additional","affiliation":[{"name":"Department of Robotics, Graduate School of Engineering Tohoku University  Sendai Japan"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1217-3892","authenticated-orcid":false,"given":"Dai","family":"Owaki","sequence":"additional","affiliation":[{"name":"Department of Robotics, Graduate School of Engineering Tohoku University  Sendai Japan"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2026,10,21]]},"reference":[{"key":"e_1_2_11_2_1","doi-asserted-by":"publisher","DOI":"10.1016\/S0301\u20100082(98)00081\u20101"},{"key":"e_1_2_11_3_1","volume-title":"Neuroscience for Rehabilitation","author":"Cohen H. 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