# From Knowledge to Leverage: How to Use Musculoskeletal Simulation to Design Exoskeleton Concepts
## 1 Introduction  
Passive exoskeletons are wearable devices characterized by the absence of energy input from motors and usually also absence of control systems. They support the human body by structures through which forces can flow around and offload anatomical joints, allowing energy to be stored temporarily in elastic elements for subsequent release to level peak muscle loads. For a comprehensive classification of industrial exoskeleton types, please refer to [1]. Technical simplicity and potentially inexpensive production have enabled passive exoskeletons to penetrate the market for assistance of manual labor tasks. Typical applications are in the construction and production industries, where some work tasks involve repetitive motion of limbs and tools against gravity, for instance, in painting or drywall mounting and in warehouse or delivery work involving handling of boxes and other goods, as shown in Figure 1, which serves throughout this paper to exemplify the presented and generally applicable methodology. Due to passive exoskeletons' lack of net energy input, they can only have supportive functions and/or level peak power exertion. The combination of added mass and zero energy input means that they cannot decrease the total mechanical energy consumption of a work task. However, levelling the maximal effort by releasing elastic energy stored in springs can potentially decrease the metabolic energy consumption because even negative mechanical muscle work requires positive metabolic energy input. Ideally, elastic energy is stored in the springs during sequences of negative mechanical work. To avoid an increasein muscle effort in these sequences, the power transfer to the springs should not exceed the negative mechanical power. These physics-based limitations of passive exoskeletons narrow the window of opportunity for their use and require additional care in the combination of exoskeleton design and intended application field. The exoskeleton and human body form a combined mechanical system, where the human body with its control system and safety requirements is by far the more complicated part. The design task must consider the entire system and the mutual influence between the anatomical body and the exoskeleton, and so must simulation methods employed in the process. In this paper, we illustrate how musculoskeletal simulation methods can be exploited in the conceptual design of an exoskeleton for a typical industrial work task illustrated by the example of Figure 1.  
The process presented in the present study can be summarized in these steps:  
1. Perform a motion capture experiment to record the motions of the work task;
2. Import the data into a musculoskeletal simulation system to perform a baseline analysis;
3. Identify highly loaded tissues, i.e., particular muscles and joints;
4. Add idealized reaction forces or moments to the model to offload the identified tissues;
5. Repeat the analysis, record the idealized forces and moments, and plot them against joint angles and/or point connections over which a prospective exoskeleton could work;
6. Perform a linear regression to identify the linear elastic element that best approximates the idealized forces and moments;
7. Replace the idealized forces and moments with the identified linear springs in the model and evaluate their offloading effect;
8. Proceed with detailed design of the exoskeleton when suitable springs have been identified. Figure 1: A musculoskeletal model of box lifting from the floor to a shelf. Blue lines under the feet represent the simulated pressure distributions, which add up to the ground reaction forces represented by the red vectors. The blue and red dots represent the postural matching of marker coordinates between the experimental motion data and the model. Living human bodies working in concert with environments, such as exoskeletons, can be simulated by software systems for musculoskeletal modeling.