Effects of insole design on ankle gear ratio, mechanics, and Achilles tendon loading during over-ground running

A lower ankle gear ratio, defined as the ratio of the resultant ground reaction force (GRF) lever arm to the Achilles tendon (AT) moment arm can lower muscle metabolic cost and AT strain during running. Gear ratio can be altered by changing footwear heel-to-toe drop, offering a practical intervention to systematically investigate how altered ankle joint gear ratios affect ankle joint mechanics and energetics during running. Eleven recreationally trained male runners completed overground runs at a self-selected pace using three custom insole conditions: UP (+14 mm drop), FLAT (0 mm), and DOWN (−5 mm). Gear ratios, AT strain and muscle fascicle dynamics were computed during overground running using synchronised motion capture and force plate data. Running economy (RE, ml·kg−1·min−1) was quantified from indirect calorimetry in a separate treadmill session. UP significantly reduced ankle gear ratios during the loading, push-off, and toe-off phases compared to DOWN (p < 0.001), without altering peak ankle moment (p = 0.200). Ratings of perceived exertion (RPE), heart rate and RE were similar across conditions (p = 0.223). AT force during loading was significantly reduced in the UP condition compared to DOWN (p = 0.019). AT strain energy was not significantly different (p = 0.394). Estimated muscle fibre length change was significantly greater in UP versus DOWN (p < 0.05). Modifying heel-to-toe drop via insole design alters ankle joint gearing, influencing muscle-tendon dynamics without compromising performance or increasing energetic cost. These findings highlight the potential of insole geometry to alter muscle-tendon mechanics during running.