# Lessons Learned from Investigating Robotics-Based, Human-like Testing of an Upper-Body Exoskeleton
## 2 Materials
### Problem Analysis and Requirements
#### 2.2.2 Exoskeleton Interface  
The interfaces that connect the human body and the exoskeleton are crucial for defining the kinematic chain and facilitating their interaction, significantly influencing operational efficiency. Simulating and analyzing these interfaces is vital for evaluating control effectiveness and mechanical motion transmission. While various approaches for digital simulation and numerical analysis have been explored [41; 42; 43], physical simulations are essential to gain insights into the actual interaction, despite inherent simplifications in these methods.  
Figure 3 illustrates the highlighted interfaces of the exoskeleton \"Lucy\" (left) and the coupling regions on a streamlined human body (right). The highlighted interfaces, such as arm shells on the upper arm and belts encircling the shoulders and hips are essential for understanding the configured kinematic chain, the transmitted support load via the coupling, and the maintenance of stability, safety, and coordinated movements. The primary focus is on the interface where supporting forces are applied, particularly at the arm shells marked with dotted lines.  
One critical aspect to consider is that the human arm is not rigidly fixed to the exoskeleton. In typical upper-body exoskeletons, the human arm is positioned in an arm shell fixed with a strap (Figure 2). During application, the human arm can potentially move slightly inside the arm shell, necessitating consideration of sliding motions and rotations in interface modeling or motion generation. The softness of human tissue adds another layer of complexity. Soft tissue deformation under applied forces results in parts of the interaction force not contributing to the lifting motion. Furthermore, shear forces appear due to relative motion at the coupling. Realistically modeling a human arm with soft tissue is challenging due to variations in dimensions and softness among different subjects [44; 45].  
Additionally, the spatially and temporally changing interaction pressure distribution of the force poses challenges. The exoskeleton applies force as pressure on the contact surface between the human arm and the arm shell. With movement and soft tissues, the distribution may change significantly within different movements. Friction forces between the human arm and the interface, along with effects like softness and movement, must be considered in the design of the test station. Clothing worn by the individual inside the arm shell also introduces effects as soft objects that deform, move, and alter friction, clothes contribute to the complexities of the interaction, alongside the pure human arm.  
The relevance of these considerations extends beyond the main arm shell interface to all interfaces connecting the human to the exoskeleton. Counterforces resulting from assistive forces on the upper limbs transmit through the exoskeleton, impacting all interfaces. Consequently, examining interactions at the lower back and shoulder becomes crucial for comprehensive investigations and understanding the flow of forces.  
Figure 3: Highlighted interfaces of the exoskeleton “Lucy” (**left**) and the coupling regions on a streamlined human body (**right**).