Summary of the subject-muscle-specific MSK properties of the six flexor muscles when the ankle joint is set to a 30° plantarflexion angle.

The moment arm L made by the tendons with the ankle joint and the muscle-tendon lengths lMT were identified by MRI segmentation and measurements in NMSBuilder [52] and OpenSim [48,49]. The optimal length and the tendon slack length were scaled from the generic values reported in [54] with the ratio of generic to subject-specific muscle-tendon lengths lMT at default 0° plantarflexion angle. The muscle maximum isometric force was estimated from the muscle volume VM, which was obtained from MRI segmentation for the TA, and from anatomical relationships [47] for the other muscles. The normalized muscle lengths , at 30° angle in ankle plantarflexion and assuming a rigid tendon, were calculated with Eq S1 in S1 Text (Section 1).