Understanding User Acceptance of Electrical Muscle Stimulation in Human-Computer Interaction

Multiple theories and models aim to provide a better understanding of the diverse factors influencing user acceptance (e.g., Brown et al. [8], Davis et al. [12]). According to Dillon, “user acceptance” refers to the “demonstrable willingness within a user group to employ information technology for the tasks it is designed to support”[13]. User acceptance models typically do not only reflect on the actual status of technology but also project the challenges and opportunities that the technology will have to face in the long run. User experience and user acceptance are two approaches used to define how a user would adopt a new technology [33]. One can think of them as related constructs. However, each of them provides insights obtained from two different points in time. On one side, the user acceptance targets the user’s expectation from the system, on the other side the user experience targets the users’ opinion based on actual interaction with the system. Surveys have been used in the past to assess user acceptance, for example, for data glasses [40] or smart kitchens [53]. Shahu et al. [74] offered an initial glimpse into the acceptance of EMS technology using an online survey, highlighting that various scenarios have an impact on user acceptance. Surveys provide the advantage of high experimental control, while capturing misconceptions about a novel technology, and allow for the inclusion of a broad range of scenarios and diverse respondents. This breadth also comes at the cost of the participants not gaining actual experience with the technology during the study. This is because EMS, being haptic and unfamiliar to many participants, cannot be fully experienced in a video. However, this limited exposure is also a benefit: it allows to showcase the to-be-evaluated scenarios in a manner approximating market entry when prospective users get first in contact with technology via media reports, advertisements, and accounts of peers.

EMS does not only affect the perception channels of humans like traditional output methods (e.g., displays or audio) [73] but also actuates the human to execute certain actions (e.g., move the arms) [22]. While applications of EMS already exist in fields such as fitness training (e.g., strengthening the muscles [25]) or health (e.g., overcoming certain health conditions [27]), it was not until recently that the HCI field started to explore this technology [80]. Over the last decade, within HCI, research has started to look into different use cases where EMS could be integrated as part of an user interface [72]. Faltaous et al. [21] grouped EMS application in action manipulation and perception manipulation:

In a first step, we selected scenarios from existing EMS applications across HCI, health, and sports. In particular, we selected ten out of the most-cited⁴ research articles using EMS, which implies interest for future work as indicated by the number of citations. Additionally, we selected two baseline scenarios, showcasing products available on the market for application in the medical or sports domain. Thus, both scenarios do not show research prototypes but actual applications of EMS which might already be known to users. The first scenario is a commercial fitness application, where EMS is used to strengthen muscles. The second scenario is a rehabilitation scenario, where EMS is used to aid the treatment of a stroke patient who has difficulty walking. Both baseline scenarios describe applications in which EMS is currently successfully applied on the market in contrast to the research prototypes.

We analyzed these potential scenarios by conducting a pre-test to assess their suitability for the online survey format. For this, we prepared a 30-second video and a brief textual description for each scenario. The video and textual description both present the goal of each project and show the benefits of the EMS technology in each scenario. We based our videos on videos uploaded to YouTube and other video platforms and changed the length to 30 seconds to have a comparable level of detail. We showed the potential videos in combination with the textual description to 3 pilot participants. These participants had no prior experience with EMS. After each participant, we iteratively improved the descriptions and videos to make sure that the overall concepts and benefits could be understood by survey respondents. This resulted in six scenarios focusing on EMS research in HCI (cf. Table 1): three targeting action manipulation and three targeting perception manipulation.

We based our design on four main technology acceptance models: Technology Acceptance Model (TAM) [11], Unified Theory of Acceptance and Use of Technology (UTAUT) [84], User Acceptance of Wearable Devices (UAWD) [87] and Almere model [31]. We started by taking TAM as the base for the model that we would apply to our research, extending it with the above-mentioned three additional models. After combining overlapping constructs, we filtered out all the non-relevant constructs that could not be applied at this early stage of the EMS technology (e.g., brand name) and that could not be generalized to every use case (e.g., visual attractiveness). This approach leads to nine constructs affecting the user acceptability of EMS: social value, perceived usefulness, perceived enjoyment, anxiety, trust, intention of use, functionality, compatibility, and attitude. Table 2 lists the constructs and questions. Each question is formulated as a statement and uses a 7-point Likert item stating to which degree the respondent agrees or disagrees.

We recruited respondents through mailing lists and social media. Participation was incentivized with ten Amazon vouchers of € 25, which were raffled to respondents after the survey closed. Overall, 101 respondents filled in the questionnaire completely (mean time 29.58 mins). Respondents self-identified gender, our results include 57 male, 38 female, 2 other gender, and 4 preferred to not specify. Respondents specified their age, averaging 30.3 years (min: 18; max: 75; sd: 11.10). Overall 91% of the respondents came from Germany, Egypt, and the United States.

We measured response time to check for outliers completing the survey too quickly or slowly. We used the Tukey method of the 1.5 quartile distance for survey completion time. All respondents met the inclusion criteria based on completion time, we did not exclude any respondents. To create a uniform scale from positive to negative, we inverted the polarity of the negative Likert items (i.e., stronger agreement equals a positive attitude towards the scenario). Where multiple questions applied to a single construct, we grouped these responses as shown in Table 2.

The survey data was analyzed using quantitative techniques for ordinal Likert data responses. We applied the Aligned Rank Transform (ART) procedure [86] to our data before performing repeated measures analyses of variance (ANOVA) for each of the questions with the within-subject factor scenario (8 levels) and the between-subject factor previous experience with EMS (EMS Exp. – 2 levels). For previous experience with EMS, we grouped respondents into two groups: respondents who did not use EMS before (58) and respondents who used EMS once or more often (43) based on their self-reported prior experience with EMS. In addition to the rating, participants described their experience. About twelve participants already participated in user studies involving EMS, seven explored EMS themselves (e.g., as a medical student), four used it as medical treatment (e.g. after knee surgery), four used it in the sports domain as a training tool, three conducted research with EMS themselves, and 13 did not specify their experience in detail.

We explored significant effects for all comparisons in more detail using Holm-Bonferroni-corrected post-hoc pair-wise t-tests. We conducted the evaluation based on the individual scenarios as well as with categorization of action and perception (cf., Table 1).

We also did a control analysis in which we checked the control factors of tech-savviness (low vs. high – split in half based on median), gender, country (Western vs. Middle East), and age. We found one statistical difference for country. Respondents from Western countries (Med = 4.33, Mad = 1.48) are less anxious than respondents from middle eastern countries (Med = 3.33, Mad = 1.48), F(1, 97) = 14.082, p < .001. We found no further statistically significant main and interaction effects.

Figure 2 gives an overview of the nine survey constructs for each of the eight EMS scenarios. Table 3 gives an overview of statistical comparisons for each construct based on the scenario and previous experience with EMS as reported by respondents including the effect size of each factor. In the following, we provide an overview of the analysis of the survey results highlighting the main findings.

The need for EMS that combines functional electrode placement and socially acceptable wearable technologies is clear from our results. Social aspects play an important role in EMS acceptance as indicated by our participants in the ratings of the Social Value construct (cf., Section 3.7.3) and their comments regarding External Image in the interviews (cf., Section 4.2.3 – External Image). While experience influenced the participants’ willingness to use the technology alone at home, this influence was not observed in public. Multiple participants mentioned that cables “coming out of the body" (i.e., from electrodes on the body) might not be appropriate in a public space. This is in line with Dunne et al. [14] work on the social acceptability of wearables, where they found that users are afraid that their wearables attract (negative) attention. Throughout the interviews, we received multiple suggestions to integrate the electrodes into clothing or accessories. Our results support what has been hypothesized by Knibbe et al. [39], that future EMS devices should fulfill wearability criteria, including aesthetics and social acceptability. While the technology is not there yet, there are some approaches to include EMS in smart textiles [64]. However, not all parts of the body are always covered with clothing. Moreover, the position of the muscle is defined by human physiology and constrains electrode placement [64]. Thus, the design space with regard to electrode placement is limited, which requires more adjustable systems (e.g., Chen et al. [10]). Our results highlight the social constraints that must be considered when developing EMS in terms of electrode placement and user appearance.

Besides having a device that is designed to look natural on the body, EMS also uses the human body as an output device. This induced movement should still look like existing human movements and dynamics. How the body moves when stimulated by EMS is important for factors like the perceived social image of the user, as elaborated by our participants in the interviews (cf., Section 4.2.2 – Design Requirements). EMS could be integrated into acceptable wearables, but this might still lead to unnatural looking movements that attract (negative) attention. Social appearance anxiety, described as “fear of negative evaluation of one’s appearance" [44], is a particular threat to acceptance when the technology can produce un-natural looking movements. While EMS in general mimics the signal of the brain and therefore provides similar input to the muscle than the users themselves, the fine-grained control of muscles is still an open challenge. Particularly muscles that are covered by other muscles cannot always be actuated precisely (if at all) with electrodes placed on the skin. The actuation should be designed in a way that the movements look as close to user initiated movements as possible. For example, the work by Takahashi et al. [79], where they explored new EMS electrode placement for increasing dexterity. Another issue is the Trust shown to the user’s actions and movements are perceived as robotic, sudden, or random. In some situations, where the application is stationary and the interface is clearly visible (e.g., in VR) the spectator’s experience [70] will differ from scenarios where the user is using it in public in an unobtrusive way (e.g., cruise control [62] hidden by long pants). As a result, the spectator’s Trust and caution towards the EMS user might also differ. For this reason, we recommend evaluating the perceived visual appearance of behavior elicited by actuation and perceived trustworthiness when evaluating acceptance of EMS. Although exploratory work in EMS may results in actuated movements that do not match human dynamics, refinement towards more natural movements is important to improving acceptance of EMS in everyday life.

Beyond visual appearance and perceived awkwardness, users were afraid of specific issues like control, pain, and agency. Prior work noted that their participants’ felt that EMS feedback was uncomfortable [2], unnatural [54], weird [61], induced numbness as well as tingling [10, 42, 48] and unfamiliar sensationas  [23, 37]. Furthermore, research showed that participants have fear of losing control [38, 61]. This observation is further supported by our findings. Fearing that a system failure might hurt them directly or even hurt others, causes a high level of Anxiety . Participants were particularly afraid of failure in safety measures, for example not being able to override the EMS signal and lose their sense of agency. This is particularly clear in our survey results for the non-experienced respondents, whi were more anxious to hurt themselves compared to experienced respondents (cf., Section 3.7.4 – Anxiety ). Furthermore, it is in line with prior research exploring the users’ desired sense of Agency when interacting with technology [45] and their fear of control loss [62, 74]. Even if the safety of an EMS system can be ensured through technical security measures in software and hardware, the EMS system would still need to provide options for manual interrupt or override to support the user’s sense of Agency and – in consequence – feeling of comfort. How to design interventions that can address Anxiety and Agency in wearable EMS is an open research challenge.

Safety measures for EMS should be introduced first, before exploring the user experience. We have found that potential users would refrain from using such a system just by thinking of consequences based on speculation. Unlike traditional user interaces (e.g., clicking a wrong button), a failure in the case of EMS not only produces the wrong action or result but also potentially unexpected sensations directly actuated on the body. EMS raises interesting challenges in the context of interface guidelines of Shneiderman et al. [77], for example that systems should provide an easy reversal of actions. While this is easily implemented in conventional computing systems, EMS actuates the human body in the real world and reversal of action may not be possible.

The interviews demonstrated that Urge to Use, such as a necessary use case, is the main motivation to use EMS (Section 4.2.1 – Urge to Use). Novelty alone seems not to be enough to drive an Urge to Use. This is further supported by the survey results, where scenario tends to influence the responses more than previous experience with EMS. We also found that the Perceived Usefulness is influenced by the users’ prior experience with EMS. EMS provides unusual feedback and sensations directly to the body, and unfamiliarity makes judging utility more complicated for users.

For all constructs, we found that scenario significantly influenced the ratings of the respondents (cf., Figure 3). This was especially pronounced in the Functionality construct, where the participants perceived the health-related scenarios as providing realistic purposes. In the interviews, all participants mentioned that the value a scenario adds should be directly relevant to their personal benefit. For example, if they are planning to learn a new musical instrument and EMS might support them by guiding their finger movements, this would be valuable enough that they would accept the technology. In contrast, some scenarios did not provide enough value (e.g., changing the food texture [Perception 1]) to encourage acceptance. These results are in line with previous work where participants shared about their opinion in four different EMS applications in HCI [74] (i.e, motor skill learning [15, 80], virtual reality applications feedback [50, 51], interacting with media player [46] and road safety [62]). They found it non-essential to integrate the EMS experience in these situations.

Future applications of EMS with high utility could change acceptance of EMS. For example, EMS that can give users “superhuman powers.” If EMS could improve human reaction times (e.g., preemptive action scenario), this would be a high utility scenario and complex social and ethical implications. Participants also discussed ethical challenges of EMS (i.e., Section 4.2.6 – Ethical Perspective) is similar terms to autonomous vehicles [32]. For example, if a system failure resulted in hurting someone other than the user, who would be legally responsible for the resulting action? Although it was clear in our results that the use-case plays the most significant role in acceptance, ethical and legal aspects need to be further investigated as use cases increase their utility.

Throughout the survey data analysis, we found that respondents with previous experience provided more favourable responses to EMS compared to respondents without previous experience. A core reason for this difference is users’ Anxiety. In the survey responses (cf., Table 3) and throughout the interviews (cf., Section 4.2.4 – Causes of Anxiety), non-experienced participants emphasized that they were afraid of hurting themselves. This impression is also solidified by non-experienced participants mentioning that they would prefer the help of professionals while using EMS and, consequently, not feeling comfortable using it home alone. In contrast, experienced participants prefer home usage due to social reasons. The insecurity of the non-experienced participants was tied to having never perceived EMS. They were, for example, afraid that the EMS device could force them to move beyond their normal limits. While the latter might be addressed by providing explanations and reassurances, the lack of knowledge about how EMS actually feels creates a significant entry hurdle.

This high entry hurdle has practical relevance: while user acceptance (as in TAM [12]) describes the willingness to start using a product or system, user experience [29] is decisive for whether a user will continue using it. As our results demonstrate that user acceptance is lower with non-experienced users, we identified targeted actions such as specifically designed demo cases to lower the entry barrier. An initial positive user experience can increase the willingness to use with experienced participants: some even mentioned that with a negative experience (e.g., tickling) in the beginning, they would still be willing to continue using EMS. This highlights how our work’s focus on user acceptance complements existing work on EMS’ user experience [38] by adding an orthogonal perspective.

EMS has the ability to take control of the users’ body if the users are willing to share them, asking users to hand over some control over their own bodies to the computer. The challenge of sharing the control was a recurring theme in our results. Nine of the interview participants reflected on the fear of losing control and not having enough autonomy (cf., Section 4.2.5 – Agency). One aspect that was mentioned is not knowing the extent to which the system would be actuating them. Particularly, non-experienced participants expressed concerns about how they would maintain their control and were unsure if they could easily overpower EMS actuation. Similar issues were discussed in the field of autonomous vehicles, for example handing over and regaining control from driving systems. The issue of sharing control and when would it be suitable for humans to take over control from a vehicle has been extensively investigated [6, 17, 68]. While there are differences between the two cases (e.g., nature of interaction, system failure consequences, etc.), control sharing still needs to be researched for EMS. For example, while driving it is clear how the user is required to intervene (e.g., driving wheel). Control sharing metaphors for EMS are still an open challenge. While classical interface guidelines such as Golden Rules by Shneiderman et al. [77], which suggest that the user should maintain control of the system (i.e., be the initiator of action), it raises the challenge of how that should be done in cases where users want to hand over control.

We acknowledge the following limitations to our work. Firstly, we recruited participants for the online survey through social media and mailing lists. Although previous research has shown this method to be feasible [4, 5, 43], we acknowledge that it can introduce biases in our sample composition. We in particular acknowledge that a high number of participants in our sample have prior experience with EMS. Although the number of participants who had prior experience with EMS through participating in HCI-related studies was relatively high (12 of 43), participants had various other experiences with EMS (physiotherapy, sports, studying medicine – see Subsection 3.6). We believe that this reflects a state in the future where people have tried EMS-based applications with friends or medical personnel and, thus, have experienced it a few times in real life. Further, we gave all participants an introduction to how EMS would be used to ensure equal grounds. The goal here was to make sure that, even if participants used EMS differently (i.e., in medical treatment without actually inducing a signal that makes limbs move), they understood the potential of EMS as it is currently used in HCI. Although we iteratively in a human-centered approach improved the videos and descriptions of the used applications, it remains possible for users to have a misunderstanding of one of the applications or EMS in general. This is, however, a similar situation as in the future when users get to know EMS systems through advertisements or explanations of family and friends. We addressed this limitation by explicitly distinguishing between novice and experienced users. The selection of interview participants might also influence the qualitative results. We chose both novice and experienced users (i.e., participants that used EMS multiple times during sports training, medical treatment, or participating or conducting research in HCI using EMS) from the survey sample to ensure that both groups are represented since we found that the experience has a strong influence (cf. Table 3). However, this limited our pool of potential volunteers. Nevertheless, particularly among the experienced users, a diverse set of participants volunteered for the interview, with previous experience in using EMS for medical treatment or as output technology in HCI research.

Interviewer : [reading items from the original survey as in Table 2, then proceeds]

Q1. When would you use such a system? Does the location influence your willingness to use? Why?

Q2. How would you think of people wearing such electronic devices (think about playing games, interacting with a system, health, and sports)? Why? When/where would that be acceptable in your point of view?

Q3. What if such a device is embedded in clothing or smartwatch – would that change your opinion and make it more acceptable?

Q4. How important is appearance for you in general? How about such a device?

Interviewer : [reading items from the original survey as in Table 2, then proceeds]

Q1. What application scenario would you consider useful?

Q2. When would you agree to use EMS?

Interviewer : [reading items from the original survey as in Table 2, then proceeds]

Q1. Can you indicate what you expect as perceived enjoyment?

Q2. What benefit would you need so that you would enjoy using it?

Interviewer : [reading items from the original survey as in Table 2, then proceeds]

Q1. Why are you afraid to hurt yourself/others? Does the fact that you can override the technology help that you feel safe?

Q2. What about receiving feedback vs. feeling being actuated why?

Q3. What needs the system to do in order to provide a higher security level or more assurance for you? Would a safety switch or communication of intent help to trust the computer?

Interviewer : [reading items from the original survey as in Table 2, then proceeds]

Q1. Why or why not do you trust EMS? If wearing EMS would convey a better experience, performance, or treatment, for which scenario/experience would you be certain of using it? (Which not?) Why?

Q2. Would you trust being actuated with EMS for each of sports, action, perception, and health? Why this specifically?

Q3. EMS provides the control over parts of your body to a computer. This rarely happens in other occasions. What would you need to accept that a computer partly controls your body?

Q4. Is it different in health? Why? What about Doctor prescribing for example the navigation system to let you walk a longer way home to have a healthy and active lifestyle?

Interviewer : [reading items from the original survey as in Table 2, then proceeds]

Q1. What negative implication do you expect? How to overcome them?

Q2. Do you think in general there is a scenario where you would like/can use EMS in your daily life? How would it look like? How should one of the scenarios change so that you would use it?

Q3. Does the location of the electrodes influence your rating? Would putting them on your head differ from limbs or from rest of the body?

Interviewer : [reading items from the original survey as in Table 2, then proceeds]

Q1. Why do you (not) think that EMS could provide realistic functionality? More details?

Interviewer : [reading items from the original survey as in Table 2, then proceeds]

Q1. What everyday task you do that might benefit from EMS?

Interviewer : [reading items from the original survey as in Table 2, then proceeds]

Q1. What would you make using the EMS technology? How should technology be/not be? Would you rather use perception or action scenarios?

Q2. Do you have experience with EMS? What? Did your opinion towards EMS change after trying it out?