PERCEIVED EXERTION AND
DETACHABLE SMARTWATCH DESIGN
Glanceability and low access time are arguably the key assets of a smartwatch. Smartwatches are designed for, and excel at micro-interactions– simple tasks that only take seconds to complete. However, if a user desires to transition to a task requiring sustained usage, we show that there are additional factors that prevent possible longer usage of the smartwatch. In this paper, we conduct a study with 18 participants to empirically demonstrate that interacting with the smartwatch on the wrist leads to fatigue after only a few minutes. In our study, users performed three tasks in two different poses while using a smartwatch. We demonstrate that only after three minutes of use, the change in perceived exertion of the user was anchored as “somewhat strong” on the Borg CR10 survey scale. These results place an upper bound for smartwatch usage that needs to be considered in application and interaction design.
Based on our determination of these limitations, we explored novel product designs for a smartwatch. We believe that smartwatches can retain micro-interactions and glanceability, but also get better at long and complex interactions. We propose a smartwatch that a user can detach, and use as more than a wearable depending on their context, requirements, and preference. Detaching the watch enables it to morph into different forms, and thereby become a better interaction device, better display, and a better sensor suite.
To do so, we conducted need finding product research for potential applications. First, we interview participants to elicit usage themes for a detachable watch. The results of these study are used to build prototype applications that showcase the range of use-cases where a detachable smartwatch offers additional functionality compared to an always-worn one, and highlights the affordances and benefits enabled due to detachability.
Figure 1. The two poses in which our study was conducted, (A) sitting with elbows resting on armrest, and (B) standing with arm raised.
We conduct a within-subject study, which simulates typical users’ target selection tasks with a smartwatch in different poses over time. We simulate it using three different forms of abstract smartwatch input primitives:
Touch: Participants tap a target once with their finger.
2. Dwell: Tap and hold a target for a specified time (500ms).
3. Swipe: Swipe a target in a particular direction (shown on screen) for a specified movement of at least 35 pixels.
The participants perform the inputs in two different poses (Figure 1), which mimicks commonly held smartwatch usage positions: A) sitting with elbows rested; and B) standing with arm raised. We denote each combination of input and pose as a condition in our study. The participants completed eight 30-second trials in each condition.
We measure participants’ self-reported exertion with the Borg CR10 scale , once before each condition to establish a baseline, and at the end of each 30 second trial. Participant provided their responses verbally and a researcher recorded their response on paper. We recruited 18 participants (8 female, 10 male) via word of mouth. The participants’ ages ranged from 22 to 35 (median = 25.5) and their smartwatch experience varied from non-users to power users.
Participants start the study in one of two poses (sitting or standing). The poses are counter balanced across participants and the conditions (input task) within each pose are randomized. Participants complete 8 trials of 30 seconds in a given condition with a 5-minute break after each condition to rest and to reduce any carryover effects. For each trial, participants select a target as many times as they can in the allotted time. Touch and dwell targets are solid circles with a 35 pixel radius, while swipe targets have arrows indicating a particular direction participants swipe in to select the target. If the participants accurately select the target, it disappears and the next target appears at a random position on the screen. We time-stamp and log all touch events, the count of accurate touches, and total touches during each trial.
We collect our data using the ordinal Borg CR10 Scale. The data was not normally distributed according to a Shapiro-Wilk test (p<0.05). We transform our data using ART analysis and the associated ARTool. This transformation aligns the perceived exertion scores for each main or interaction effect and assigns them ranks. This procedure allows us to conduct a parametric three-way repeated measures ANOVA (INPUT x POSE x TIME) for each effect on the transformed data. Table 1 shows the results of our ANOVA.
Table 1. Table summarizing results of ANOVA with pose, input and time as independent variables and perceived exertion as the dependent variable after performing the ART procedure.
Figure 2: Boxplot and linear regression model for PE of subjects in standing pose for all three conditions: Single (left), Dwell (middle), and Swipe (right)
Usage Time: There is a significant effect of usage time on exertion. The raw exertion scores of participants show a consistent upward trend over time. Figure 2 show the boxplots of the raw (non-transformed) exertion values in each input primitive in the standing pose. On the transformed data, we conduct a post-hoc Tukey's pairwise comparisons at each time point and observe a significant difference (p<0.05) between all pairs of time points except between 3.5 and 4 minutes. Moreover, the least-squared mean value of exertion increases with increase in time. Together, it confirms that participants consistently reported higher perceived exertion with increase in usage time.
We also want to ascertain after how much time does a smartwatch use becomes unsuitable for continuous interactions. A self-reported value of 4 on the CR10 scale is anchored as `somewhat strong'. We use this as a threshold for when continued usage would become a significant usability issue. In the standing pose, the median exertion score exceeds `somewhat strong' at t=2.5min for Single Touch and Swipe. It exceeds `somewhat strong' at t=3min for dwell. For any form of input primitive, a user is expected to be `somewhat strongly' tired after only 3 minutes of smartwatch use in the standing pose.
DETACHABLE SMARTWATCH DESIGN
The findings of our quantitative study were used to inform a novel/futuristic detachable smartwatch design.
By providing the user with the ability to quickly detach the smartwatch from the wrist, they are free to take on different poses while working with the watch. For example, the smartwatch can transition from the smartwatch's default on-wrist interactive state to one more analogous to a phone where the device is held in one or two hands.
The ability to detach the watchface offers several affordances that considerably increases the input space available on a smartwatch. To better understand how people may use it, we conducted an elicitation study to identify usage themes.
We conducted a study with six participants with a goal to elicit usage themes; a common technique used to evaluate prototypes for futuristic designs. We provided the participants with a functional prototype that used magnets to connect the watch face to the wristband. We first encouraged the participants to talk freely of ideas and use-cases that they might have, and then provided them with prompts for affordances such as the ability to rotate and dock a watchface. The study was audio recorded and we conducted a thematic analysis of the participant feedback.
Based on our thematic analysis, the following usage themes emerged:
Contextual Mode Switch: The transition between different dispositions can act as a trigger to change usage or context mode. For example, if a watch is detached and merely resting on the table, it would adopt a "Do Not Disturb" mode. Thus depending on the context, the watch can perform different functions and/or take different profiles without explicit user intervention.
Heterogeneity: When detached, the smartwatch is not a wearable device anymore, rather depending on the location, it's a suite of sensors to sense different activities on-body or in the environment.
Modularity: It is not necessary that the whole watch gets detached; parts of it can remain on the wrist. Different activities may require only a part of the watch e.g., accelerometer to track exercises) and thus only specific "modules" of the watch can be detached which may interact with each other. This theme encompasses the affordance of the interaction between the band and the watch. They are just two pieces of a larger device.
Based on these themes, several prototype applications were developed such as detaching the watchface to use as a phone, as a camera device, as a fitness tracker and a handheld gaming device. We conducted another study to evaluate the potential usefulness of a detachable watch via the affordances it enables. As a proxy for each feature of the detachable watch, we use functional prototypes of a subset of the proposed applications.
12 participants (5 female, mean age=26) took part in our study. 7 of the 12 had either owned or previously used a smartwatch. The participants were recruited using word of mouth and snowball sampling.
We demoed our applications to the participants and explained how the applications work. We advised the participants to focus on the concepts (and not on the finesse of the implementation). For each of the applications, the participants answered two questions: (1) "Is this useful?", and (2) "Is this enjoyable?" on a 7-point Likert scale. The participants were also encouraged to provide lower ratings if it was a feature they could not foresee using for any reason. We also captured free-form feedback at the end to better understand their preferences. The participants spent around 20~minutes exploring different applications before giving feedback.