Smartphone User Interaction and its
Effect on the Median Nerve
The following experiment aims to observe how smartphone swiping speed and range would affect the median nerve and subsequently grip strength. It was found that slow, longer swipes produced less change in the grip strength and participants reporting lower visual analog scale values
B A C K G R O U N D
Modern handheld technology relies heavily on the movement of the thumb to navigate touchscreen interfaces; the amount of time spent using mobile devices has increased consistently since 2014 with an average daily screen time of 3 hours 43 minutes among U.S. users in 2019, with usage rates climbing even higher in 2020 due to the COVID-19 pandemic [2]. However, the cartilage of the thumb can be overexerted with repetitive scrolling motions which increases the risk of injury in the carpal tunnel [3,4]. Carpal tunnel syndrome (CTS) is a physiological disorder characterized by increased pressure in the carpal tunnel, particularly on the median nerve, typically due to repetitive motion [4,5]. Previous studies have demonstrated the fatiguing effects as a result of handheld device usage characterized by a large range of thumb motion [6,7] and increased thumb speed [8]. Specifically, reduction of grip strength has been linked to increased carpal tunnel pressure and compression on the median nerve [9,10].
Fig. 1: Anatomical Model of the Median Nerve [1]
O B J E C T I V E
The objective of this project is to measure the effects of mobile device usage on patient digit dexterity, wrist discomfort, and potential chronic conditions using grip strength as a quantitative indicator with swipe motion and speed of swipe as control variables. A qualitative survey will also be used as a secondary indicator relating user discomfort to median nerve pressure. In order to minimize the pressure on the median nerve and reduce the risk of CTS, it is hypothesized that optimal usage consists of swiping with a low range of motion (minimizing thumb movement), at a slow to moderate swiping speed of (30 mm/s) thereby reducing both cyclic loading [11,12] and the onset of thumb fatigue.
M E T H O D O L O G Y
Fig. 2: Procedural Flow Chart for Testing
A grip strength test is conducted for three trials, varied by device usage time to promote onset of thumb fatigue: once before scrolling, a second time after scrolling for 200 seconds, and a third time after scrolling continuously for 300 seconds [13]. To accommodate for the COVID-19 regional lockdown, 500ml plastic water bottles will be used as grip strength indicators since it is a readily available commodity in Toronto. The participant is to squeeze the water bottle and film the height of the water projection. Using the video recording frame rate, it is possible (through 1D fluid mechanics modelling) to measure the speed and calculate the pressure of the water jet. The pressure of the water jet can then be translated to the pressure (i.e. grip strength) induced by the hand. A decrease in pressure between experimental trials would indicate a decrease in grip strength, suggesting an abnormal strain on the median nerve [8].
Swiping Area (red)
Submit data to store time and finger location to Google Sheets through API
Choose 1 of two trials (200s/300s) and start swiping
Fig. 3: Website built for Finger Position Tracking and Speed
Fig. 4: Grip Strength Control Trial, 200s Trial, 300s Trial (Left to Right)
To validate the findings, finger tracking using a web page that logs user touchscreen scrolling events will be used to determine the user range of motion and swiping speed while swiping continuously. The finger position and swiping speed, when compared to change in grip strength, verifies if there exists a correlation between carpal tunnel pressure with swiping speed and range of motion.
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<-- See More
(Link to GitHub Repository: https://github.com/hsiehk/swipeplease)
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Finally, a qualitative survey will be used to assess if any pain or discomfort (artefacts of fatigue) was felt before or after the experiment using the Visual Analog Scale (VAS) [13].
R E S U L T S
From the grip strength data, it was found that on average there is a slight decrease after using smartphone devices for 200s and another slight decrease in grip strength after using devices for 300s on the next trial (Fig. 5). This trend suggests that there may be a correlation between grip strength and the amount of time spent swiping; these findings help explain the visual decrease in the height of the water projectile in the videos (Fig. 4).
Fig. 5: Grip Strength Data for 12 Participants (20-60 years old)
Using the time and location data taken from the finger tracking website (Fig. 3), a heat map of the swiping can be created for each trial.
Fig. 6: Participant Swiping Heat Map for 200s (left) and 300s (right)
In the 200s trial, a clear swiping pattern can be observed but in the 300s trial no distinct swiping pattern can be found. This may be because we didn’t specify a swiping method to observe a person’s normal swipe so as not the bias the data and participants may have started swiping differently in the 300s trial due to fatigue and to alleviate some of the stress they were feeling in their median nerve. Although we did control the dominant hand scrolling and scrolling only with the thumb, we did not have a set direction or line for the participant to follow and thus resulted in the free-form finger positioning shown in the heat maps.
Looking at the normalized swipe range data and comparing it to the respective grip strength data
(Fig. 7), it was found that a longer range of motion produced less of a difference in grip strength after swiping for extended periods of time than if you used smaller swiping strokes. This may be because with a longer stroke, you would be swiping a lot less than if you had shorter strokes and shorter strokes would increase the frequency of loading in your muscles in the thumb in order to swipe the same distance as a longer thumb. The data was normalized since it was assumed that longer thumbs would produce longer swiping strokes relative to that of a shorter thumb.
Fig. 7: Swipe Motion Range Normalized to Thumb Length Compared to Change in Grip after Swiping
The speed data shows that for 200s an increase in swiping speed also produced a larger change in grip strength -- grip strength decreased more for individuals who were swiping faster (Fig. 8). The 300s data, though, shows relatively little change in grip strength regardless of speed. This is unexpected since both the 300s and 200s grip strength data are subtracted from the control grip strength data to get the change in grip strength, so the 300s data should not produce higher grip strength than the 200s or control trials. One explanation is that like an athlete does before exercising, the 200s trial acted as a warm-up for the muscles and thus enabled the grip strength to remain unchanged or even increase.
However, we should also consider the speed they are swiping since the longer strokes may be swiping slower than the shorter strokes and thereby having fewer cycles when loading the abductor muscle and producing a smaller change in grip strength.
Fig. 8: Swipe Speed Compared to Change in Grip after Swiping
The more likely scenario is that, as shown in Fig. 6, participants started swiping differently to alleviate some of the stress felt in the thumb and thereby decreasing the effects of swiping on grip strength.
From the qualitative survey we found that participants who picked higher VAS scores also had lower grip strength after their subsequent trials. After the 200s trial, a clear shift in the VAS score can be seen from 0 and 1 to 0 and 5 and 0.5 to 6 for 300s (Fig. 9); however, looking at the data we also found that pain is very subjective person to person. Thus, we are only using this data to see if there is a trend to perceived pain and grip strength.
C O N C L U S I O N
Due to the COVID-19 pandemic, a reliable means of testing grip strength was not possible amongst all participants. As such, although the data suggests slow, long swipes to produce the least change in grip strength and a smaller VAS score, thereby inferring less stress on the median nerve, a universal conclusion cannot be made due to the small sample size and rudimentary test design. For future consideration, it may be best to use a standard grip strength tester (CAMRY), increase sample size, and limit participant swipes in a specific path.
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Comprehensive data for recent trends in the prevalence of CTS is unavailable, but historical data shows a steady increase in both incidence and prevalence from 1993 to 2013 [9]. An understanding of the biomechanics underlying typical smartphone use and the related impact on the median nerve motivates change in the design considerations for both physical smartphones, associated software, and UI/UX.
R E F E R E N C E S
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