## 4 Discussion  
In the present study, we systematically reviewed the assessment methods of usability and cognitive workload in the use of rehabilitative exoskeletons. To answer RQ1, we retrieved the methods that were deployed to assess the usability of rehabilitative exoskeletons. Specifically, we categorized usability assessment methods in terms of the deployed type of measure (RQ1a) and investigated their psychometric properties (RQ1b). We were able to retrieve mostly quantitative and subjective assessment methods, as well as qualitative and subjective ones. These methods most often consist of questionnaires and interviews, for both individuals and groups. This finding may be thought of as reflecting the need for researchers to take advantage of usually inexpensive  
Figure 2: Critical appraisal of the retrieved assessment methods of usability and cognitive workload in the use of rehabilitative exoskeletons based on the adaptation of the checklist from [PERSON] and colleagues [22]. methodologies in order to conduct usability assessments of rehabilitative exoskeletons whereby end-users are directly involved in providing informed feedback [11]. On the one hand, quantitative methods allow for standardized evaluations of a rehabilitative exoskeleton's overall usability, thus being more straightforward and less ambiguous when it comes to the interpretation, comparability, and generalizability of results across different experiments. On the other hand, qualitative methods may offer deeper insights into potential issues that might foster or hinder the usability of a rehabilitative exoskeleton, thus providing researchers with valuable information regarding specific usability issues encountered by participants while using the wearable robotic device during some rehabilitation exercise; in turn, this may enable an easier resolution of problems and may reveal especially important information during the early phases of a rehabilitative exoskeleton's development. It is reasonable to expect the validity and reliability of qualitative methods (i.e., interviews and focus groups) to be affected by the facilitators' ability to conduct the discussion with the interviewees or to manage a smooth interaction between participants. In addition, the lack of a clearly defined conceptual model and an obscure description of the methodological procedure may render qualitative methods less effective. Nevertheless, these issues can apply to quantitative methods too. Considering all of the above, it can be concluded that quantitative and qualitative methods should not be considered as competing nor as being mutually exclusive. Rather, these two types of measurements can be integrated and can complement each other to compensate for each other's limitations and maximize each other's pros. Among the retrieved methods that were deployed to assess the usability of rehabilitative exoskeletons, SUS, QUEST, and AttrakDiff showed the best psychometric properties and therefore proved appropriate to assess usability while performing robotic exoskeleton-based motor rehabilitation. The only notable caveat about SUS concerns the fourth item of the questionnaire, namely \"I think that I would need the support of a technical person to be able to use this system\". As exoskeleton-based rehabilitation requires a therapist's assistance throughout the exercises' performance, the final score for this item may not necessarily reflect the actual usability of the exoskeletal device. This conclusion is supported by findings by [PERSON] and colleagues [30], who reported high internal consistency values as an indicator of the reliability of SUS, except for item 4, thus suggesting the presence of a low correlation between this item's score and the total score of SUS. Regarding QUEST, it aims to also evaluate other dimensions of satisfaction with the use of a given rehabilitative exoskeleton that are not directly related to usability, such as safety, size, comfort, weight, and durability.