{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,25]],"date-time":"2026-08-25T10:54:05Z","timestamp":1787655245403,"version":"build-2736575974"},"reference-count":65,"publisher":"The Company of Biologists","issue":"14","license":[{"start":{"date-parts":[[2023,7,31]],"date-time":"2023-07-31T00:00:00Z","timestamp":1690761600000},"content-version":"vor","delay-in-days":16,"URL":"http:\/\/creativecommons.org\/licenses\/by\/4.0"}],"funder":[{"DOI":"10.13039\/100000049","name":"National Institute On Aging","doi-asserted-by":"crossref","award":["F31AG069412"],"award-info":[{"award-number":["F31AG069412"]}],"id":[{"id":"10.13039\/100000049","id-type":"DOI","asserted-by":"crossref"}]},{"DOI":"10.13039\/100018041","name":"American Society of Biomechanics","doi-asserted-by":"crossref","id":[{"id":"10.13039\/100018041","id-type":"DOI","asserted-by":"crossref"}]},{"DOI":"10.13039\/100007059","name":"Northwestern University","doi-asserted-by":"crossref","id":[{"id":"10.13039\/100007059","id-type":"DOI","asserted-by":"crossref"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":[],"published-print":{"date-parts":[[2023,7,15]]},"abstract":"<jats:title>ABSTRACT<\/jats:title>\n               <jats:p>Regulating ankle mechanics is essential for controlled interactions with the environment and rejecting unexpected disturbances. Ankle mechanics can be quantified by impedance, the dynamic relationship between an imposed displacement and the torque generated in response. Ankle impedance in the sagittal plane depends strongly on the triceps surae and Achilles tendon, but their relative contributions remain unknown. It is commonly assumed that ankle impedance is controlled by changing muscle activation and, thereby, muscle impedance, but this ignores that tendon impedance also changes with activation-induced loading. Thus, we sought to determine the relative contributions from the triceps surae and Achilles tendon during conditions relevant to postural control. We used a novel technique that combines B-mode ultrasound imaging with joint-level perturbations to quantify ankle, muscle and tendon impedance simultaneously across activation levels from 0% to 30% of maximum voluntary contraction. We found that muscle and tendon stiffness, the static component of impedance, increased with voluntary plantarflexion contractions, but that muscle stiffness exceeded tendon stiffness at very low loads (21\u00b17\u00a0N). Above these loads, corresponding to 1.3% of maximal strength for an average participant in our study, ankle stiffness was determined predominately by Achilles tendon stiffness. At approximately 20% MVC for an average participant, ankle stiffness was 4 times more sensitive to changes in tendon stiffness than to changes in muscle stiffness. We provide the first empirical evidence demonstrating that the nervous system, through changes in muscle activations, leverages the non-linear properties of the Achilles tendon to increase ankle stiffness during postural conditions.<\/jats:p>","DOI":"10.1242\/jeb.244863","type":"journal-article","created":{"date-parts":[[2023,6,23]],"date-time":"2023-06-23T09:14:18Z","timestamp":1687511658000},"source":"Crossref","is-referenced-by-count":15,"title":["Non-linear properties of the Achilles tendon determine ankle impedance over a broad range of activations in humans"],"prefix":"10.1242","volume":"226","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-3435-8353","authenticated-orcid":false,"given":"Kristen L.","family":"Jakubowski","sequence":"first","affiliation":[{"name":"Northwestern University 1 Department of Biomedical Engineering , , Evanston, IL 60208 , USA"},{"name":"Northwestern University 2 Department of Physical Therapy and Human Movement Sciences , , Chicago, IL 60611 , USA"},{"name":"Shirley Ryan AbilityLab 3 , Chicago, IL 60611 , USA"},{"name":"Emory University and Georgia Tech 4 Wallace H. Coulter Department of Biomedical Engineering , , Atlanta, GA 30322 , USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0268-6686","authenticated-orcid":false,"given":"Daniel","family":"Ludvig","sequence":"additional","affiliation":[{"name":"Northwestern University 1 Department of Biomedical Engineering , , Evanston, IL 60208 , USA"},{"name":"Shirley Ryan AbilityLab 3 , Chicago, IL 60611 , USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9860-6427","authenticated-orcid":false,"given":"Eric J.","family":"Perreault","sequence":"additional","affiliation":[{"name":"Northwestern University 1 Department of Biomedical Engineering , , Evanston, IL 60208 , USA"},{"name":"Shirley Ryan AbilityLab 3 , Chicago, IL 60611 , USA"},{"name":"Northwestern University 5 Department of Physical Medicine and Rehabilitation , , Chicago, IL 60611 , USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9238-0674","authenticated-orcid":false,"given":"Sabrina S. M.","family":"Lee","sequence":"additional","affiliation":[{"name":"Northwestern University 2 Department of Physical Therapy and Human Movement Sciences , , Chicago, IL 60611 , USA"},{"name":"Simon Fraser University 6 Department of Biomedical Physiology and Kinesiology , , Burnaby, BC , Canada , V5A 1S6"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"237","published-online":{"date-parts":[[2023,7,27]]},"reference":[{"key":"2023073108532269200_JEB244863C1","doi-asserted-by":"publisher","first-page":"433","DOI":"10.1007\/BF02479137","article-title":"Mechanical behaviour of tendon in vitro","volume":"5","author":"Abrahams","year":"1967","journal-title":"Med. Biol. Eng."},{"key":"2023073108532269200_JEB244863C2","doi-asserted-by":"publisher","first-page":"1001","DOI":"10.1016\/S1095-6433(02)00143-5","article-title":"Tendon elasticity and muscle function","volume":"133","author":"Alexander","year":"2002","journal-title":"Comp. Biochem. Physiol. A Mol. Integr. 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