## 1 Introduction  
Humans have relied heavily on the use of their hands throughout history, since they provide a wide range of problem-solving possibilities. Therefore, replacing an upper limb represents an enormous technological challenge. Despite the large number of available prosthetic alternatives, such as a myoelectric hand, body-powered hand, passive hand, and body-powered hook, among others, prosthesis rejection rate (41-74%) remains high [1]. It is worth noting that 80% of the global population of more than 600 million people with disabilities live in developing countries, demonstrating a significantly higher prevalence of disability than in their high-income counterparts [2; 3]. However, current research on the development of medical devices prioritizes the needs and infrastructure of developed countries [2]. Furthermore, high-tech myoelectric prostheses have recently been presented as the most attractive alternative due to their stylized appearance and broad variety of functions [4]. Nonetheless, the majority of consumers cannot afford such technology. Consequently, an imbalance has emerged between the wide diversity of high-tech devices for developed countries and the relative lack of appropriate prosthetics for developing countries [2]. Given these circumstances, it is imperative for new prosthetic research to focus on the needs of the vast majority of users. Body-powered technology is an appropriate alternative for those in developing countries, since it meets the crucial \"4 A's\" (availability, accessibility, appropriateness, and affordability) [5]. Interestingly, body-powered prostheses are popular in developed countries as well, where they are frequently preferred due to their high functionality, robustness, performance, and light weight [1; 6; 7; 8]. A higher functionality was demonstrated in Cybathlon [9], a competition that evaluates the functionality of medical devices. For the upper-limb prostheses category, recent experimental designs competed against the most advanced prostheses available on the market. Although prostheses such as Michelangelo [10], BeBionic [10], and I-Limb [11] were competing, the prosthesis with the highest valuation in Cybathlon 2016 and Cybathlon 2020 was a body-powered prosthesis [12], which shows that a simpler technology does not necessarily mean less functionality. Thus, the evolution of body-powered technology will have an impact not only on developing countries but also on developed ones. Hands or hooks are used as terminal devices on body-powered prostheses. Although body-powered hands have a better appearance, their use for daily-living activities is limited; body-powered hooks, in contrast, are ideal for day-to-day functions, since they have higher functionality, lower weight [13], direct feedback, robustness, and low acquisition and maintenance costs [14], which make them preferred by users [6; 15]. The technology of a body-powered hook is simple: there are no sensors, batteries, or electrical/electronic components. Therefore, the user controls the opening and closing of the prosthesis through body movement (typically by shoulder abduction). There are two categories of body-powered devices: voluntarily closed (VC) and voluntarily open (VO). VC devices are normally open, and user activation closes them. Once released, an elastic band restores the system to its open state [16]. Their main advantage is that users can control the grip force; however, they must maintain the grip force to hold an object, causing fatigue and discomfort [13]. In contrast, VO devices are normally closed, and user activation opens them. An elastic band closes the mechanism to its inactive position, which leads to more comfort, since the user only needs to apply force when they desire to open the prosthesis [17]. Grasping force is determined by the strength of the passive band; therefore, the user cannot hold objects that require more grip force than the band provides.