{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,23]],"date-time":"2026-10-23T20:46:02Z","timestamp":1792788362332,"version":"4.5.0"},"reference-count":0,"publisher":"The Company of Biologists","license":[{"start":{"date-parts":[[2026,9,29]],"date-time":"2026-09-29T00:00:00Z","timestamp":1790640000000},"content-version":"am","delay-in-days":0,"URL":"http:\/\/creativecommons.org\/licenses\/by\/4.0"}],"funder":[{"name":"Eunice Kennedy Shriver National Institute of Child Health and Human Development","award":["R01HD090642"],"award-info":[{"award-number":["R01HD090642"]}],"id":[{"id":"https:\/\/ror.org\/04byxyr05","id-type":"ROR","asserted-by":"crossref"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":[],"abstract":"<jats:p>Exoskeletons assist and augment movement, but their effects on proprioceptive feedback remain poorly understood due to challenges in making direct measures of sensory signals in humans. Here, we leveraged a benchtop animal model to explore how mechanical context akin to an elastic exoskeleton operating on a human lower limb joint influences primary muscle spindle firing. In an anesthetized rat preparation, we applied controlled stretches to the medial gastrocnemius with engineered springs (0\u20130.5 N\/mm) in parallel to the muscle-tendon unit (MTU) while modulating muscle activation to maintain overall system stiffness. Fascicle length was measured with sonomicrometry, force and MTU length with a servo motor, and spindle instantaneous firing rate (IFR) using dorsal root recordings. Trading off increases in exoskeleton stiffness with reductions in muscle activation decreased biological muscle force (3.1\u00b10.6 N to 1.6\u00b10.6 N, p&amp;lt;0.001) and stiffness (4.4\u00b11.5 N\/mm to 2.3\u00b11.3 N\/mm, p&amp;lt;0.01), and increased fascicle length (7.9\u00b11.3 mm to 8.3\u00b11.5 mm, p&amp;lt;0.005). We found significant correlations between spindle firing and each independent fascicle dynamics factor we investigated (p&amp;lt;0.005). Thus, parallel stiffness modified muscle fascicle dynamics but did not alter spindle firing, possibly due to internal trade-offs in the salient fascicle dynamics that drive spindle behavior. Leveraging in-situ experiments that enable monitoring of afferent feedback in complex mechanical contexts such as added parallel stiffness can provide a window into the effects of wearable devices on underlying sensory systems.<\/jats:p>","DOI":"10.1242\/jeb.252345","type":"journal-article","created":{"date-parts":[[2026,9,29]],"date-time":"2026-09-29T06:51:01Z","timestamp":1790664661000},"source":"Crossref","is-referenced-by-count":0,"title":["Emulating the influence of exoskeleton stiffness on primary afferent feedback in rat isolated muscle-tendon unit"],"prefix":"10.1242","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-8813-2531","authenticated-orcid":false,"given":"Amro A.","family":"Alshareef","sequence":"first","affiliation":[{"name":"1 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4528-5607","authenticated-orcid":false,"given":"Paul","family":"Nardelli","sequence":"additional","affiliation":[{"name":"2 School of Biological Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Surabhi N.","family":"Simha","sequence":"additional","affiliation":[{"name":"3 Coulter Department of Biomedical Engineering, Emory University and Georgia Institute of Technology, Atlanta, Georgia, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8835-8055","authenticated-orcid":false,"given":"Timothy C.","family":"Cope","sequence":"additional","affiliation":[{"name":"2 School of Biological Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA"},{"name":"3 Coulter Department of Biomedical Engineering, Emory University and Georgia Institute of Technology, Atlanta, Georgia, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6854-9444","authenticated-orcid":false,"given":"Lena H.","family":"Ting","sequence":"additional","affiliation":[{"name":"3 Coulter Department of Biomedical Engineering, Emory University and Georgia Institute of Technology, Atlanta, Georgia, USA"},{"name":"4 Department of Rehabilitation Medicine, Emory University, Atlanta, Georgia, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5588-6297","authenticated-orcid":false,"given":"Gregory S.","family":"Sawicki","sequence":"additional","affiliation":[{"name":"1 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA"},{"name":"2 School of Biological Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"237","published-online":{"date-parts":[[2026,9,29]]},"container-title":["Journal of Experimental Biology"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/journals.biologists.com\/jeb\/article-pdf\/doi\/10.1242\/jeb.252345\/3719590\/jeb252345.pdf","content-type":"application\/pdf","content-version":"am","intended-application":"syndication"},{"URL":"https:\/\/journals.biologists.com\/jeb\/article-pdf\/doi\/10.1242\/jeb.252345\/3719590\/jeb252345.pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,9,29]],"date-time":"2026-09-29T06:51:01Z","timestamp":1790664661000},"score":1,"resource":{"primary":{"URL":"https:\/\/journals.biologists.com\/jeb\/article\/doi\/10.1242\/jeb.252345\/373039\/Emulating-the-influence-of-exoskeleton-stiffness"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,9,29]]},"references-count":0,"URL":"https:\/\/doi.org\/10.1242\/jeb.252345","relation":{},"ISSN":["0022-0949","1477-9145"],"issn-type":[{"value":"0022-0949","type":"print"},{"value":"1477-9145","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,9,29]]},"article-number":"jeb.252345"}}