## 2 Requirements Definition  
The requirements for the human-robot interaction torque estimation method were defined by applying a survey to a group of physiotherapists and conducting a literature review. The idea was to ask people in the health area about the requirements and to define the movements for the LLRR, i.e., the isotonic exercises to be used within the simulation study. Appendix B describes the study in more detail. In addition, it specifies the questions and answers used in the questionnaire. A total of 26 physiotherapists answered the anonymous questionnaire, and we asked about the Range of Motion (ROM), speed, and force to be used within the simulations and maximum tolerable error for the human-robot torque estimation methods. The survey results show that the physiotherapists defined the squat and press exercises to evaluate the performance of the human-robot interaction torque estimation methods. Moreover, they suggested four additional requirements for these methods. The methods used during the information search strategy are described below. Additionally, the methodology used to select studies based on inclusion and exclusion criteria is presented. Finally, the steps that were used to extract information are reported. A literature review was carried out to define the requirements of a human-robot torque estimation method for a robotic system usable for isotonic exercises. The review was conducted using the primary databases available, such as Scopus, ScienceDirect, IEEE Xplore, and Google Scholar, among others. This review was conducted, covering publications mainly between 2006 and 2022. The search was initially oriented to find the main metrics and characteristics, which are currently employed to evaluate the behavior of these methods. A list of keywords or phrases was defined to be used within the search. The search strategy included combining several Boolean operators to filter the results. Only literature in English and Spanish was reviewed. The keywords were: (a) Human-robot, (b) Interaction, (c) Torque, (d) Estimation, (e) Requirements, (f) Force, (g) Measurements (h) Sensors, (i) BSIPs, (j) Uncertainty, (k) Rehabilitation. Inclusion and exclusion criteria were also defined for the study selection. Inclusion criteria were (a) studies published between January 2006 and March 2022, and (b) full access to the research text. Exclusion criteria included: (a) publications that do not fit the subject of study of this review, (b) publications in languages other than Spanish and English, (c) letters to the editor, and (d) opinion articles. We extracted the essential data of each of the publications in a spreadsheet. Finally, let us enumerate the 18 chosen requirements of a human-robot interaction torque estimation method for an LLRR, which is usable for isotonic exercises. The literature review inspired the first 13, and the physiotherapists' answers inspired the last five:  
1. Non-dependence of additional sensors [11; 13]. 2. Low phase lag in the estimation [15]. 3. Low sensor noise sensitivity [15]. 4. Low sensitivity to BSIPs uncertainties [12]. 5. The average percentage error must be lower than 20 to 22% when using accurate model parameters [12]. 6. Small error band [10]. 7. Approximately 0.5 s of settling time or lower [10]. 8. Overshoot of approximately 25% or lower within the estimation [10]. 9. Coefficient of Correlation (R\({}^{2}\)) greater or approximately 0.935 for the hip joint and 0.924 for the knee joint [11]. 10. Root Mean Squared Percent Error (RMSPE) lower than 8.74% for the hip joint and 10.26% for the knee joint [11]. 11. A maximum of 5% error when moving just one joint, i.e., the distal one [28]. 12. Finite-time convergence [29]. 13. It should not require calibration each time that the user wears the LLRR, i.e., it requires a maximum of one calibration per user. [13]. 14. It works in all ROM, and the limiting angles of the joints must be configurable. 15. It works with slow exercises, executing between 1-25 repetitions per set. 16.