Human legs operate like springs with adjustable stiffness during locomotion, improving movement economy and versatility. The potential for the foot to contribute to this spring-like mechanism has been established. However, due to previous modelling approaches assuming a rigid-foot segment, it is unknown if ankle and foot quasi-stiffness can be actively regulated, quasi-stiffness being a measure of joint deformation under an applied load. In this study, we sought to determine if midfoot quasi-stiffness was increased in a similar manner as the ankle with increasing load demands, and if these changes were mediated by increased activation of the intrinsic foot muscles. We also quantified differences in measures of ankle joint quasi-stiffness attributable to the modelling assumption of a rigid-foot segment versus a multi-segment foot. Twenty-two healthy individuals performed a single-leg hopping task at three frequencies (2.0 Hz, 2.3 Hz, and 2.6 Hz) while measuring lower limb kinematics, kinetics and muscle activation (EMG). Kinetics and kinematics were calculated at the midfoot and using two different ankle models: rigid-foot model (shank-foot) and an anatomical ankle (shank-calcaneus). Midfoot quasi-stiffness increased with hopping-frequency (p < 0.05), while contact phase intrinsic foot muscle activation decreased (p < 0.05). The assumption of a rigid-foot model overestimated ankle ROM by ∼10° and underestimated ankle quasi-stiffness by 45-60%. This study demonstrates that midfoot quasi-stiffness increases with hopping-frequency; however, the mechanism for these changes remains unclear. Furthermore, this study demonstrates the need to assess the ankle and foot independently, using appropriate models, to avoid significant inaccuracies in basic ankle kinematic and kinetic outputs, such as range of motion and joint quasi-stiffness.

在运动过程中，人的腿就像弹簧一样具有可调节的刚度，从而提高了运动的经济性和多功能性。已经确定了脚有助于这种弹簧状机制的潜力。但是，由于先前的建模方法假定是脚的刚性段，因此尚无法有效地调节脚踝和脚的准刚度，准刚度是施加载荷时关节变形的量度。在这项研究中，我们试图确定脚中部准刚度是否以与脚踝相似的方式随着负荷需求的增加而增加，以及这些变化是否由固有的脚部肌肉的增加的激活介导。我们还量化了脚踝关节准刚度量度的差异，这归因于僵硬脚段与多段脚的建模假设。22位健康个体在测量下肢运动学，动力学和肌肉激活（EMG）的同时，以三个频率（2.0 Hz，2.3 Hz和2.6 Hz）执行了单腿跳跃任务。在脚中部并使用两种不同的脚踝模型来计算动力学和运动学：刚性脚模型（小腿脚）和解剖型脚踝（小腿跟骨）。中足准刚性随着跳跃频率的增加而增加（p <0.05），而接触相固有的脚部肌肉激活减少（p <0.05）。刚足模型的假设将脚踝ROM高估了约10°，而将脚踝的半刚度低估了45-60％。这项研究表明，脚中准刚度随跳频而增加。但是，这些更改的机制仍不清楚。此外，