## 4 Discussion
Therefore, the adoption of this method may lead to a criterion contamination problem, whereby the actual criterion (i.e., the parameter that is used to measure a construct) includes variables it should not, ultimately leading to measurement errors. Moreover, QUEST overlooks some sub-dimensions of usability (e.g., learnability and efficiency) that may be informative for both research and practice purposes. Therefore, it can be concluded that QUEST may provide researchers with valid and reliable assessments of rehabilitative exoskeletons' usability, although they ought to be aware of its limitations and cautious against potential measurement errors. In this regard, the deployment of only its \"device\" subscale may be advisable, because of its resemblance with the construct of usability [27; 36; 37; 38]. Similarly, the conceptual model underlying AttrakDiff partially encompasses the construct of usability within its \"pragmatic quality\" subscale, and the overall method aims to assess the interactive qualities of a rehabilitative exoskeleton beyond its mere usability, such as an individual's emotional response to the use of the wearable robotic medical device. Again, this may be expected to lead to a criterion contamination problem, as the instrument includes two other sub-dimensions (i.e., \"hedonic quality\" and \"attractiveness\") that are not strictly representative of the construct of usability. So, deploying only the \"pragmatic quality\" subscale may be a potential solution to this issue, if support for its validity and reliability is provided. Furthermore, an additional notation is worthwhile regarding two other quantitative and subjective assessment methods of the usability of rehabilitative exoskeletons, such as VAS and PRE. These instruments aim to measure the subjective response to pain and the perceived physical effort in users during robotic exoskeleton-based rehabilitation exercises, respectively. However, as rehabilitation itself tends to imply difficult and painful exercises according to the severity of the condition suffered by patients, the assessments performed via VAS and PRE may not necessarily reflect the usability of the wearable medical device. As for objective methods, they constitute a minority among the retrieved assessment methods of usability in the use of rehabilitative exoskeletons. One major issue with this type of method is that it does not provide a clear indication of the validity for the usability construct. Rather, usability was conceptualized as enjoyment, difficulty, and comfort [33]; ease of use [39]; or torque and speed [43]. As the term \"usability\" is often intended as a synonym of \"feasibility\" within the engineering field, the authors of these studies might have deployed instruments that were consistent with this mental model. To answer RQ2, we retrieved the methods that were deployed to assess the cognitive workload associated with the use of rehabilitative exoskeletons. Specifically, we categorized cognitive workload assessment methods in terms of the deployed type of measure (RQ2a) and investigated their psychometric properties (RQ2b). Compared with the usability assessment methods, we were able to retrieve a smaller number of methods. Of them, we retrieved mostly quantitative and subjective assessment methods. None of the retrieved assessment methods deployed qualitative types of measurements. Although the limited number of studies included prevented us from drawing definite conclusions, this result may be thought to reflect a lack of interest in these methods' adoption on the part of researchers. Among the retrieved methods that were deployed to assess the cognitive workload associated with the use of rehabilitative exoskeletons, SWAT and NASA-TLX showed the best psychometric properties and were therefore proven to be appropriate to assess cognitive workload while performing robotic exoskeleton-based motor rehabilitation.