# Development and Biomechanical Evaluation of a Back Exoskeleton to Support Lifting Tasks
## 2 Exoskeleton Functionality and Design
### Exoskeleton Design
#### 2.2.1 Supporting and Force-Transmitting Structure  
An important component of exoskeleton functionality is its structural design, which enables effective force transmission from the actuator to the body. To meet the two key requirements of efficient force transfer and high wearing comfort with user adaptability, carefully selected materials with specific properties are used. The structure consists of four main elements: (1) shoulder vest, (2) hip belt, and (3) thigh straps, which serve as human-system interfaces, ensuring comfort and adaptability. Additionally, (4) two steel cables connect the actuator to the body to facilitate force transmission. The supporting structure comprises four main parts:  
1. Force Transmission--Two steel cables (1.5 mm diameter, tensile strength 1570 Nm/mm\({}^{2}\); 7 \(\times\) 7) are used to transmit the actuation force from the actor--which is located at the back of the shoulder west--to the back of the thighs, using the thigh straps, to ensure a supporting force along the spine and hip. To maintain a consistent distance between the body and the cable, two pulleys (PA6 plastic, 35 mm \(\O\), low-friction ball bearing) are used in the lumbar spine area and at the lower part of the hip belt. This creates a 10 cm lever arm around the hip, enabling the lifting of the upper body and extension of the thighs. Therefore, two-way support is provided for both the upper and lower body around the hip, which acts as a fulcrum. 2. Shoulder Vest--A shoulder vest is created using a soft and unstretchable 1000 D Cordura\({}^{\text{\textregistered}}\) nylon fabric. It has two functions: (1) to guarantee high wearing comfort  
Figure 2: (**a**) rear view and (**b**) frontal view of the exoskeleton. and (2) to transmit the actuation strain to the torso. Regarding wearing comfort, the vast is soft and adaptable to different body shapes by adjusting the size using Velcro tapes at the shoulders and sides of the torso. Moreover, the closed-cell EVA foam material at the back helps prevent painful pressure points. To effectively transmit the actuation force to the upper body, the used fabric is stiff when tension is applied (tensile strength \(>1800\) N). The backbone between the shoulder vest and hip belt transmits the force from the actuator to the hip and is attached to the vest by a ball bearing, allowing free mobility of the upper torso. 3. Hip Belt with Backbone--The hip belt is also made of Cordura(r) fabrics with an EVA foam inlet and a size-adaptable strap. In the lumbar region, a spinal support structure (aluminum, 2 mm thickness) is attached to facilitate the transmission of compressive forces along the spinal axis to the pelvic region. As the flexibility of the hip belt contrasts with the force transmission of the backbone, a PA12 plastic inlay is integrated into the hip belt to prevent deformation and effectively apply force to the hip. In addition to the backbone stiffness along the longitudinal axis, it is flexible along the sagittal axis, allowing for bending forward and backward movements. For lateral bending movements along the transverse axis, the backbone is mounted to the hip belt using a needle bearing. To adjust the size of the exoskeleton according to the length of the wearers back, a mechanism enables the backbone to move in and out. 4. Thigh Straps--Using straps, the actuation force can be applied to the thighs to support hip extension during lifting. The straps must be tightened comfortably to match the shape of the thighs. In contrast, a high stiffness is required to transmit the force from the attached steel cable. To fulfill both requirements, the thigh straps are made of inner and outer components. The inner component is made of elastic Lycra(r) fabric (25% Spandex, 75% Nylon) fabric to be able to tighten it firmly to the body without slipping. The outer component comprises a plastic inlay to which the steel cable is attached. Both components are connected to each other, and the straps are tightened using a Velcro fastener.