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Shifting ankle-foot gearing regulates speed of plantarflexor (i.e., calf muscle) contraction, which influences economy of force production. Here, we tested the hypothesis that manipulating ankle-foot gearing via stiff-insoled shoes will change the force-velocity operation of plantarflexor muscles and influence whole-body energy cost differently across walking speeds. We used\n                    <jats:italic>in vivo<\/jats:italic>\n                    ultrasound imaging to analyze fascicle contraction mechanics and whole-body energy expenditure across three walking speeds (1.25, 1.75, and 2.0\u2009m\/s) and three levels of foot stiffness. Stiff insoles increased leverage of the foot upon the ground \u00a0(p\u2009&lt;\u20090.001), and increased dorsiflexion range-of-motion (p\u2009&lt;\u20090.001). Furthermore, stiff insoles resulted in a 15.9% increase in average force output (p\u2009&lt;\u20090.001) and 19.3% slower fascicle contraction speed (p\u2009=\u20090.002) of the major plantarflexor (Soleus) muscle, indicating a shift in its force-velocity operating region. Metabolically, the stiffest insoles increased energy cost by 9.6% at a typical walking speed (1.25\u2009m\/s, p\u2009=\u20090.026), but reduced energy cost by 7.1% at a fast speed (2.0\u2009m\/s, p\u2009=\u20090.040). Stiff insoles appear to add an extra gear unavailable to the human foot, which can enhance muscular performance in a specific locomotion task.\n                  <\/jats:p>","DOI":"10.1038\/s41598-020-65626-5","type":"journal-article","created":{"date-parts":[[2020,5,29]],"date-time":"2020-05-29T06:02:27Z","timestamp":1590732147000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":50,"title":["Gearing Up the Human Ankle-Foot System to Reduce Energy Cost of Fast Walking"],"prefix":"10.1038","volume":"10","author":[{"given":"Samuel F.","family":"Ray","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kota Z.","family":"Takahashi","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2020,5,29]]},"reference":[{"key":"65626_CR1","doi-asserted-by":"publisher","first-page":"88","DOI":"10.1097\/00003677-200504000-00006","volume":"33","author":"AD Kuo","year":"2005","unstructured":"Kuo, A. 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