_applied sciences_  
_Article_  
Investigating and Measuring Usability in Wearable Systems: A Structured Methodology and Related Protocol  
[PERSON]@  
* They should be unobtrusive, i.e., they have to operate in synergy with the body and not limit the user's functions and mobility;
* They can be used as a communications media. In a macro-generalization, they can be categorized into four families: 1. Systems dedicated to empowering human functions; exoskeletons used in different workplaces are a good example of this family;
2. Systems integrating monitoring functions (for humans and/or the environment); smart garments, smart bracelets, smart watches, and smart shoes belong to this category and are applied in sport, fitness, and medicine;
3. System performing actions on the human body; orthoses and other systems for protection or rehabilitation can be included in these examples;
4. Systems able to replace human functions such as prostheses for the upper or lower limbs, and even insulin pumps replacing the physiological insulin management can be included in this group. For all these typologies, usability becomes a mandatory requirement more than a simple feature. The ISO norms define \"usability\" as the \"extent to which a system, product or service can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use\" [2; 3]. In this regulatory framework, it is clear how usability is an outcome of use, but no specific processes or methods for taking account of usability in the design development or evaluation have been described. Firstly, this paper tried to address this topic in specific relation to usability assessment. Some general guidelines to define methods and usability assessment are provided in [4], in particular in clause 5. The methodologies proposed by the technical report are divided into methods either involving users or not. In the iterative design process, user participation is essential, so that the observation of users, performance-related measurements, critical incidents analysis, questionnaires, interviews, and thinking aloud are the recommended methods. Despite this enormous importance in terms of their health-related applications, only a few studies have faced the usability aspects in a complete setting, mainly by adopting simple questionnaires to investigate them. In the next subsection, a survey of the usability assessment methods in wearable systems is presented. Related Works  
[PERSON] et al. chose the system usability scale (SUS) questionnaire to investigate the usability and acceptability of wearable sensors in the elderly. The SUS [5; 6; 7; 8] measures the usability of a device/system/technology through a ten-item questionnaire with five response options for respondents from 1, strongly disagree, to 5, strongly agree, resulting in a potential minimum score of 0 and a maximum score of 100. [PERSON] et al. also applied the intrinsic motivation inventory (IMI) to assess participants' experiences related to the target activity involved with wearing the device. IMI [9] is a multidimensional questionnaire containing twenty-two items on a seven-point Likert scale, ranging from 1, not at all true, to 7, very true. The measure assesses six subscales, namely interest/enjoyment, perceived competence, effort/importance, pressure/tension, value/usefulness, and perceived choice. It is indirectly related to usability but is otherwise interesting. Similarly, [PERSON] et al. [10] explored the user experience of wearing an activity tracker in a similar cohort of older adults; usability and acceptance were evaluated through the technology acceptance model (TAM) [11], SUS, and the user satisfaction evaluation questionnaire [12]. In a wider perspective, [PERSON] et al.