## 2 Design Requirements  
When working at sitting posture the leg muscle forces are exerted consistently against the gravity forces, and an assistive body support device that can relieve the knee muscle loadings during sitting would be of great help to the physical workers. Furthermore, for industrial application the body support device needs to be controlled adaptively to workers of various body sizes, and to working conditions of different sitting postures, and even to walking motion. There are two specific examples of body weight support devices developed so far. One type is a locking mechanism that sustains the body weight at an arbitrary seated position [3]. A worker could get the benefits of saving muscle effort in static situations using the locking mechanism by making use of passive mechanisms. However, this has some drawbacks in dynamic situations. For example, when a worker wants to change a body posture, the supporting mechanism can either provide some assistive torque to the human body, or make itself back-drivable so that a worker can change its configuration very easily as wished. The other type is a support device that assists a wearer's body continuously using actuators, for example the body weight support device developed by Honda [5]. The objective of this device was to reduce the knee muscle loadings of the worker in an arbitrary posture or motion by a preselected assistance ratio. However, due to its high energy consumption, it had to provide a limited amount of assistant torques to the knee to keep a long operation time. This study proposed a body support device that featured the advantages of both types, i.e., to support a human body with high energy efficiency during sitting, and to make its joints back-drivable so that a worker could change its configuration very easily as wished. As more wearable assistive devices such as the similar examples [3; 4; 5] illustrated in the introduction are developed for production line or nonuniform assembly line work, unburdening carrying and keeping heavy parts from physical workers, the need for this self-supporting device would increase, thereby relieving muscle pain and fatigue due to keeping awkward postures or routing repeatedly. In order to secure the applicability of a proposed device in industrial workers, a target scenario was set as in Figure 1 and Table 1. The performance goals of the robotic knee-assistive device are set as follows:  
1. Minimize the number of active actuators. 2. Assist up to a maximum body weight of 85 kg. 3. Energy consumption less than 1 Wh. 4. Specific operating conditions:  
- Moving distance < 15 m  
- Walking speed < 4 km/h  
- Working time in sitting pose > in walking motion  
- Phase transition between sitting and walking > 1 cycle/min  
The specific conditions of the work scenario referred to the data in the literature, on physical workers at an assembly line and workshop. Their weights were between 72 kg to 101 kg, and heights were between 173.6 cm and 183.3 cm (the 50 th to 95 th percentile male) [21]. In the paper the working conditions of workplaces were categorized into three scenarios (A, B, and C in Figure 1) according to the tasks. Among these three scenarios, A is considered the most arduous, since it features the longest sitting time. In this study the target condition for the body support device was set to be scenario A. More specifically, the range of knee motion during sitting was set to be between 0 to 90 degrees, and the walking speed was set to be 4 km/h. Prior studies [22; 23] reported that the required knee torque was between 0.44 and 0.77 Nm/kg during the transition motion between sitting and standing. Dynamic loadings of the knee joint during walking were analyzed by the open-source program OPENSIM [24; 25; 26] using an experimental data set of GIL02. The data set contains the walking kinematics and dynamics of a 14.6-year-old female of weight 66 kg, with legs 0.9 m long.