## 4 Discussion  
In this study, we analysed the contribution that the TA and EDL have on foot dorsiflexion and pronation and supination through a biomechanical simulation of the foot-ankle district. We simulated both normal conditions and a set of conditions typical in stroke patients. Our aim was to quantify to what extent dorsiflexion was affected by these two muscles and to promote awareness among clinicians of the key role played by the EDL in stroke patients with EVFD. Figure 6: The effect of limiting the length excursion of the SO during the simultaneous contraction of the TA and EDL. Different colours represent the effects of different length excursions from 100% (black curve), indicating no limitation, to 25% (yellow curve), indicating that the SO can only lengthen by 25% of its full range. Foot joint angles are reported in panels (**A–E**); TA force and EDL force are reported in panels (**F**) and (**H**) respectively; SO muscle length in (**G**). In the model, the TA was considered because it is the main dorsiflexor muscle that inserts on the medial surface of the foot. EDL was included in the model because it is the main dorsiflexor muscle that inserts on the lateral surface of the foot. The results of our modelling study demonstrate that dorsiflexion is only achievable through the synergistic and balanced action of the TA and EDL. Specifically, the contraction of the TA or EDL alone cannot lift the foot beyond the neutral sagittal position (0 degrees). This condition does not allow foot clearance during swing [39], which is one of the prerequisites of normal gait [40]. The lack of dorsiflexion typically results in the adoption of compensatory mechanisms to move the trailing limb forward, such as pelvic elevation and greater hip and knee flexion in the sagittal plane or the lateral bending of the trunk and limb circumduction in the frontal plane. In the clinical field, functional electrical stimulation in patients with drop foot shows similar results. Proper stimulation of both the TA and EDL is required to achieve a balanced foot dorsiflexion [41; 42]. Taken individually, neither muscle can produce neutral dorsiflexion, i.e., without deviations in the frontal plane and with a magnitude suitable for most daily activities. In stroke patients with EVFD (with normal TA activity and missing EDL measured by surface electromyography), a surgical procedure named SPLATT (split tibialis anterior tendon transfer) and calf muscle lengthening are often recommended [43]. In SPLATT, the TA tendon is cut longitudinally into two parts; one branch remains in place, and the other is transferred to the lateral side of the foot and fixed to the cuboid. This provides a mechanical contribution similar to that which the paretic EDL can no longer provide. Our results highlight the contribution of the TA and EDL to ankle joint rotation in both the sagittal and frontal planes. Starting with the foot hanging freely, the TA contraction alone determined up to 18 degrees of dorsiflexion (from \(-18^{\circ}\) to \(0^{\circ}\)). This brought the ankle back to a neutral position without further dorsiflexion. Despite being commonly known as the main dorsiflexor muscle, the main action of the TA (alone) is to produce foot supination. In our study, when only the EDL was contracted, it rotated the ankle by 12 degrees into dorsiflexion (from \(-18^{\circ}\) to \(-6^{\circ}\)) and the foot shifted into pronation (see Figure 4). Only the combined contraction of the TA and EDL resulted in the physiological dorsiflexion peak (e.g., 15 degrees) needed for walking and with the foot neutral in the frontal plane. Textbooks used as reference by students and clinicians alike also indicate that the EDL is a muscle that assists foot dorsiflexion and counteracts supination [44; 45]. Nevertheless, when addressing a lack of foot dorsiflexion, studies only take into account the TA and seldom consider the EDL and its role during walking, e.g., in stroke patients.