# Development and Preliminary Evaluation of a Lower Body Exosuit to Support Ankle Dorsiflexion
## 2 Exosuit Functionality and Design
### Exosuit Design
#### 2.2.1 Supporting and Force-Transmitting Structure  
Creating a supporting structure is one of the key challenges in developing an exosuit, as it constitutes a human-suit interface to apply force into the body or body regions in an effective, comfortable and safe manner [39]. The current structure is composed as a biomimetic design and is therefore based on the ligament and tendon structure of the ankle. Whereas tendons are soft tissues which connect muscle and bone to transmit force, ligaments connect bones and stabilize. The tendon of the m. tibialis anterior, as the primary muscle that facilitates dorsiflexion of ankle joint, inserts medial of the first metatarsal bone of the foot. The tendon of the synergism m. extensor digitorum longus inserts at the II-V phalanges. To maintain a stable lever arm, the tendons are held by the extensor retinaculum in place. The antagonists to dorsiflexion are the muscles of triceps surae which transmit force on the Achilles tendon and consequently induce plantarflexion at the ankle. In analogy to the biological structure, the supporting and force-transmitting structure of the exosuit follows a similar mechanism. The supporting structure comprises three main parts:  
1. Artificial tendon--Two Bowden cables are used to transmit the actuation force from the actor--which is located on the hip belt--to the foot and therefore mimic the tendons of the dorsiflexion and the plantarflexor muscles, respectively. The steel cables are stored in a pressure-stable Bowden tube to transmit the force over any curved path. The length of the Bowden cable can be adjusted to fit different leg length;  
Figure 2: (**a**) Frontal view and (**b**) rear view of underbody exosuit. 2. Insole--The force-transmission into the foot is realized by a seatbelt webbing loop, which is sewn under an insole in the area of the 1 st and 5 th capt ossis metatarsalis, where a good transmission of force is given, due to the small soft tissue thickness [40] and a large possible lever arm to the ankle joint. Due to the high stiffness of seatbelt webbing for tensile forces, it is suitable to transmit forces without much elongation. As the webbing loop distributes the force over the entire foot width, it is possible to use only one artificial tendon for dorsiflexion. Regarding different footwear, the use of an insole allows an easy adaptability by providing multiple sizes of insoles, which can be placed in the wearer's shoe and attached to the Bowden cable using a screw lock. To consider users with different gender and anthropometry, we provided three sizes of insoles (small, middle, large);
3. Webbing--To transmit the force into the lower leg and to maintain a lever arm of about 5 cm to the ankle joint, a structure of two webbing loops (5 cm width) with diagonal connections was designed. The first loop above the ankle joint acts as tension band wiring and therefore mimics the extensor retinaculum. The Bowden tube is attached to an anchor point on the frontal side of this webbing loop using a 3D-printed adapter which is sewn onto the textile. A second Bowden tube for plantar flexion is attached to the back side. This webbing loop is linked to an upper loop above the calf to transmit the counterforce from the attached Bowden tube to the muscle body of the calf. The requirement is a firmly holding interface to the body. The localization of the human-system interface above the thicker muscle body in combination with anti-slip pads made of cellular rubber on the inner side of the webbing allows a slip-resistant possibility to transmit the force into the body. Moreover, the pads distribute the force to a larger area of the skin and provide a buffer for better wearing comfort.