Non-Invasive Evaluation of Plantar Fascia Strain During Locomotion Considering its Insertion Site by Motion Capture-Ultrasound Combination

The human foot functions are primarily achieved by the elastic plantar fascia (PF), whereas plantar fasciitis is often caused by daily walking and running. PF strain is its potential risk factor. The PF is strained when the foot arch lowers and/or the toe dorsiflexes, as it spans the foot arch and the metatarsophalangeal joint. However, PF strain has often been alternatively assessed by the relative length change between the first metatarsal and the calcaneus $(\mathcal{E}_{\text{MH}1-\text{CAL}})$ in $3\mathrm{D}$ motion capture studies, without considering the effect of metatarsophalangeal dorsiflexion. Although some studies assessed PF strain, including the effect of metatarsophalangeal movement, they did not consider the sesamoid potentially increasing PF moment arm. Here we propose a motion capture-based method for estimating $\text{PF}$ strain $(\varepsilon{\text{PF}})$ during locomotion considering the location of PF insertion and sesamoid bone under the first metatarsal as PF path by combining ultrasound measurements. We compare $\varepsilon_{\text{PF}}$ and $\varepsilon_{\text{MH}1-\text{CAL}}$ in walking at preferred speed in eight females. The profiles of $\varepsilon_{\text{PF}}$ and $\varepsilon_{\text{MH1}-\text{CAL}}$ overlap during loading response, but then $\varepsilon_{\text{PF}}$ increases while $\varepsilon_{\text{MH}1-\text{CAL}}$ decreases. The peak $\varepsilon\text{PF}$ is significantly greater than peak $\varepsilon_{\text{MH1}-\text{CAL}}(4.7 \pm 1.2 \% \text{vs} 2.9 \pm 0.9 \%)$. The moment arm calculated from the metatarsophalangeal angle and the metatarsophalangeal movement-induced PF lengthening is significantly greater than that calculated by pre-existing methods without considering the sesamoid (relative difference: 32-49^%). We emphasize the importance of considering the metatarsophalangeal movement and the moment arm to metatarsophalangeal joint for PF strain evaluation. Our method offers PF strain assessment satisfying these requirements.