## 4 Discussion and Conclusions
A final consideration must be made regarding the lack of a general gravity compensation of the exoskeleton weight. This has not been directly addressed for two reasons. The first one is the presence of the polyarticulated arm, that thanks to a parallel spring in joint 4 (P4) of the passive arm sustains most of the weight of the exoskeleton. The second factor is more related to control aspects. In fact, when the robot operates in the Kinematic mode, the need for force compensations is not required, since the robot is controlled in position. When instead the robot operates in the assistive mode, additional contributions--in terms of injected current to joint 4--are added when moving against gravity (Assistance and Support) as depicted in Figure 5.  
Most of the state-of-the-art upper-limb exoskeletons, presented so far, focus on the shoulder complex and limit the normal mobility of the rest of the body, forcing the patient into a fixed standing or sitting position [5; 6; 9]. In terms of structure, the active part of the Float exoskeleton weighs 10.0 kg. However, it is difficult to compare with other devices due to the different number of active DOFs: e.g., ANYexo [5] has six DOF and weighs 12.98 kg, Harmony [9] has seven DOFs per arm and weighs 15.65 kg. Despite these differences, it is possible to compare the exoskeletons density per degree of freedom--namely the ratio between exoskeleton weight and number of DoFs. We observe in this way that the Float exoskeleton presents a density of 2 kg/DoF while ANYexo has 2.16 kg/DoF and Harmony 2.24 kg/DoF. This demonstrates how Float, even if marginally and qualitatively, represents a lighter solution for the rehabilitation of the upper limb. For what concerns the power capabilities of the device, Float has a total nominal power of 300 W (computed as the sum of the power of all the actuated joints) which leads to a power-to-weight ratio of 30 W/kg. In literature this information is not always available, the authors found data regarding the ARMin V and ANYexo devices, which show, respectively, a power-to-weight ratio of 24.7 W/kg and 18.5 W/kg. This highlights a slightly better performance of the Floatexoskeleton also in terms of power density, further supporting the lightweight and power efficiency of the developed solution  
Moreover, Float provides a physiological ROM for the entire shoulder complex with a safe design, compliant with the main requirements for medical devices. We invested time and effort to build a system that can be safely used in clinical practice. This is a consistent paradigm shift when compared to classical research exoskeletons. Compared to other approaches using compliant technologies (e.g., series elastic actuators) [5], we employed a classical approach (without torque sensing). Nevertheless, our approach guarantees robustness and the control strategies implemented in Float cover most of the needs for orthopedic patients. A clinical pilot trial is currently underway to perform a user-centered evaluation of Float, on orthopedic patients at the INAIL Motor Rehabilitation Center (Crm) of Volterra, Italy. Additionally, the user-centered design and the development of a new version of Float is also adopting the principles of the International Classification of Functioning, Disability and Health (ICF) to further approach the user needs [17; 26; 27] within the exploratory and the generative phases. ICF is enabling a categorization of the needs of patients with shoulders pathologies that can be extended to the ecologically valid design of rehabilitative tasks that must be allowed by the new machine. [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] wrote the manuscript. [PERSON], [PERSON], [PERSON] and [PERSON] prepared the original figures. [PERSON] designed the original version and kinematics of the exoskeleton. [PERSON] performed the analyses. [PERSON], [PERSON] and [PERSON] designed and assembled the mechanical part of the systems. [PERSON], [PERSON] and [PERSON] wrote the software.