An investigation of an ergonomics intervention to affect neck biomechanics and pain associated with smartphone use

Abstract

BACKGROUND:

Neck discomfort and pronounced neck flexion have been associated with smartphone use.

OBJECTIVE:

Eye glasses with a 90 deg prism in each lens were investigated as a potential intervention to reduce awkward head and neck postures during activities involving viewing the device.

METHODS:

Sixteen smartphone users with neck pain and 9 asymptomatic users performed a texting task on a smartphone with and without the prism glasses, in sitting and standing postures in a laboratory setting.

RESULTS:

Cervical erector spinae and upper trapezius muscle activity, head posture and motion, performance, discomfort and other subjective perceptions were assessed. Prism glasses reduced neck extensor muscle activity, neck flexion, and head tilt compared to the direct view. In the symptomatic group, the intervention produced less neck and shoulder discomfort compared to the direct view.

CONCLUSIONS:

This intervention could offer an alternative way of interacting with a smartphone while texting in stationary postures, by reducing exposure to pronounced flexed neck and head posture commonly seen in users, and thereby could reduce neck discomfort associated with smartphone use.

Keywords

Smartphone intervention musculoskeletal discomfort neck discomfort prism glasses

1 Introduction

Prevalence of neck pain in general populations across the globe varies across studies. In their review of epidemiological studies on neck pain, Hoy et al. [1] reported point prevalence from 3.3 to 22.7 percent in five studies that met their inclusion criteria, while 12 month prevalence ranged from 17.1 to 73 percent in nine studies that met their inclusion criteria. Neck discomfort is prevalent in many sedentary occupations, including dentistry [2, 3], tattooing [4], and occupations involving notebook or tablet computer work [5, 6]. Neck discomfort has been associated with adopting awkward neck postures for prolonged periods of work [7 –10] due to the sustained recruitment of neck extensor muscles. This can result in muscle fatigue in the short term and potentially more extensive problems longer term [11].

Neck and shoulder discomfort have been associated with smartphone use [12, 13]. Similar to other tasks associated with neck pain, smartphone tasks (emailing, texting, web browsing, etc.) are typically performed with users looking downwards [14]. In a laboratory study, an average head flexion angle of 33–45 deg was measured during smartphone use [14].

Smartphone ownership is prevalent throughout the world, with a global median of 43 percent and a peak of 88 percent in South Korea [15]. In each country surveyed, smartphone ownership prevalence was higher in millennials than in people over age 34; ownership by millennials exceeds 90%in Korea, Australia, the United States, Canada, and Germany.

Berolo et al. [12] found that cumulative smartphone use exceeding 2.375 hrs/day was associated with increased risk for neck and shoulder discomfort. In the years since that study mobile device use (exposure) has increased from an average of 0.8 hrs/day to 2.8 hrs/day in the US, while there has been no decrease in desktop/laptop use (avg. about 2.5 hrs/day) (slide 14 in the report by Meeker [16]). A recent study by Szeto et al. [17] reported that neck discomfort experience was correlated with both neck flexion and amount of time using a smartphone (the latter two were measured during a three hour field collection effort). In a cross-sectional examination of discomfort in young Swedish adults, Gustafsson et al. [13] found odds ratios for discomfort in neck and shoulder regions increased with the number of self-reported text messages sent and received; data collection was performed in 2007 and the highest response option category was > 20 messages/day. A study conducted just 4 years later in the US showed the median number of text messages sent by people in a comparable age group was 50 per day (with a mean of 109.5) [18].

Although ergonomics recommendations have suggested avoiding sitting with head bent forward to prevent neck pain [19], such behavioral recommendations have not provided a specific means by which to avoid neck flexion during smartphone use. In studies of dental professionals, prism glasses (PG), which angle the user’s vision by an amount based on the design of the prism and angle of the lenses relative to the temples of the glasses, have been shown to reduce neck flexion [20, 21]. PG were also shown to significantly reduce muscle activity and discomfort in novices performing a simulated dental task [21], and were shown to reduce discomfort in a longitudinal study of dental professionals [22]. Given the positive effects of PG in that application area, a short term laboratory study was conducted to examine effects of PG as a potential intervention to reduce awkward head and neck postures during smartphone use. It was hypothesized that use of PG would reduce neck flexion, neck extensor muscle activity, and related discomfort during smartphone use in stationary viewing situations, in people with and without chronic neck pain.

2 Methods

2.1 Experimental design

2.1.1 Independent variables.

Effects of smartphone viewing method (direct view and PG), posture (sitting and standing), and symptom group (with and without neck pain) were studied while participants performed a 10 minute texting task in each test condition (where test condition refers to the combination of viewing method and posture). The study employed a nested (within symptom group) repeated measures study design that blocked on participants. Statistical analysis tested for main effects and interactions of the independent variables. The sequence of test conditions was counterbalanced to control for carryover effects.

2.1.2 Dependent measures.

Objective measures included neck flexion in the sagittal plane, head tilt (refers to fore-aft rotation of the head) [21], phone angle (Fig. 1), and muscle activity (cervical erector spinae (CES) and upper trapezius (UT)). Head movement was assessed, to determine if postural fixity was affected by the intervention. Performance was quantified by typing speed and accuracy. Subjective perception of discomfort was assessed at the start and end of each test condition; subjective perceptions about the intervention were collected at the end of the study.

Fig. 1

Head, neck, and phone angle measurements.

2.2 Participants

Twenty-five individuals (11 females and 14 males) aged 26.2±5.3 years participated in the study, 16 with ongoing neck pain symptoms during the last 12 weeks, and 9 without neck pain during the last 12 weeks. Qualifications for participation included being at least 18 years of age, having at least 1 year of experience using a touchscreen phone, and not requiring eye glasses to read the phone (eye glasses would interfere with the PG; contact lenses were acceptable). Exclusion criteria included a history of neck surgery, neck trauma, or debilitating neck pain, medical diagnosis of fibromyalgia, cervical radiculopathy, headache aggravated by reading, or allergy to hypoallergenic adhesive tape.

Participants were assigned into the no pain group or the symptomatic neck pain group based on their Neck Disability Index (NDI) Score [23]. The NDI contains 10 questions used to determine the severity of neck pain affecting a person’s daily life; it has a high degree of reliability and internal consistency [23]. Subjects with an Index score of≤8%out of a possible score of 100%were assigned to the no pain group (no neck disability) and the rest were assigned to the symptomatic neck pain group. An NDI score > 8%has been used to identify subjects with neck pain in prior research [24 –26]. To minimize risk of injury, if any of the 10 items in the NDI exceeded a score over 3, potential participants would have been excluded from the current study.

2.3 Testing protocol

The protocol was approved by the university’s Institutional Review Board (identification number 2016H0005). Informed consent was obtained for each participant prior to testing. Demographic (age, gender) and anthropometric data (height, shoulder height, popliteal height, seat pan height, sitting shoulder height) were gathered, and then the participant demonstrated how he/she typically interacted with his/her smartphone by typing a brief text message. The participant was then introduced to the 90 deg prism glasses (i90 Heads-up Tablet and Smartphone Glasses, Lane Franklin LLC, Portland OR, USA; Fig. 2) and given time to practice using them by typing a practice text message provided by the researcher.

Fig. 2

Use of 90 degree prism glasses (enlarged image of the glasses provided at right) while seated and typing text on a smartphone. The lower portion of the participant’s face has been covered to preserve anonymity and an ActivPAL accelerometer is taped to the forehead.

Next, the participant’s skin was prepared for application of surface electromyographic (emg) electrodes, by shaving the skin and cleaning with alcohol at the four electrode sites. Electrodes (Delsys DE-2.1 single differential) were positioned for the left and right CES and left and right UT muscles. CES electrodes were placed 2 cm from the midpoint at the level of C3 [27] and UT electrodes were placed 2 cm lateral to the midpoint of a line from C7 to the acromion [28]. To assess head movement, a tri-axial accelerometer (ActivPAL^™, PALTechnologies) was taped on the subject’s forehead; the device collected data continuously throughout the session.

A height-adjustable chair with arm supports was adjusted to the subject’s popliteal height and elbow height. While seated, subjects used the chair’s armrests and backrest. A procedure for recording each subject’s neutral baseline (for posture and emg) was performed while sitting and standing. The participant was instructed to gently move the head (repeated mild neck flexion and extension and head rotation) with eyes closed, and then settle into what felt to him/her to be a comfortable, neutral, balanced head and neck posture, which was held for 10 seconds. Posture was recorded by two video cameras positioned to the right of the subject at the level of the acrominon, one to assess posture while standing and one while sitting. Maximum muscle exertions were not part of this protocol, in the interest of minimizing risk of injury to participants in the neck pain group.

Each test condition was 10 min long. Test conditions were separated by a 5 min. break. Longer breaks were provided as needed to return a subject to a lower level of discomfort if discomfort had increased during the test condition just completed.

During each condition subjects typed a text message of pre-assigned content. The assigned paragraph was typed repeatedly during the 10 min. period. The text message was emailed to the researcher at the end of the condition, for assessment of speed and accuracy. Five different paragraphs were used in this study (one for practice and the others for the four test conditions). Paragraphs were derived from novels; they were each about 100 words long and had Flesch-Kincaid Grade Level readability scores of about 9. The order of the four paragraphs was counterbalanced and the sequences were randomly assigned to the subjects. Before typing, the subject was instructed to focus on accuracy rather than speed. Each paragraph was printed on paper about 3.8 cm x 7.6 cm in size; it was taped on the subject’s smartphone, just above the display.

Immediately before and after each test condition, the subject was asked about his/her level of neck and shoulder discomfort, using the Borg CR-10 [29]. At the end of the study, when all four conditions were finished, subjects completed a survey concerning their preferences between the direct view and the use of PG in terms of neck comfort, visual comfort, productivity, accuracy, and suggestions for the prism glasses. Session length was about 1.5 hr.

2.4 Data collection and processing

EMG data were collected via a Delsys Bagnoli-8 Surface Electromyographic System (Natick, MA, USA) and a MotionMonitor data acquisition system (Innsport, Chicago, IL). EMG data were collected at 1000 Hz, band pass filtered (20 and 500 Hz), and notch filtered at intervals of 60 Hz. Data were collected in 10 s long samples during Minute 3 through Minute 9 of each 10 min. test condition, for a total of 7 samples for each condition. EMG data were processed using a custom MATLAB program (Matlab 2014R), that included several steps (rectification, 75 ms moving average smoothing, and Hanning filter). Test condition data were normalized by dividing by the baseline activity. Normalized 10th and 50th percentile muscle activity values were of interest.

Video footage analysis was performed in Adobe Photoshop CS5.1. Images at baseline, the third minute, and the ninth minute were captured from the footage. From this footage, neck flexion, head tilt, and phone tilt angles were measured with tools in Photoshop. The baseline (neutral) measurement was subtracted from neck flexion and head tilt angles, to create the dependent variables from those measurements. ActivPAL^™ software was used to export a csv file containing the ActivPAL data. The standard deviation of the ActivPAL data was calculated during each 10 min test condition and was used to quantify the variation of the head motion activity (dependent variable: head motion) throughout each test condition. Finally, performance assessment was conducted using word count and compare functions in Microsoft Word, to quantify typing speed and accuracy, respectively.

2.5 Statistical analysis

The data were checked for normality (Shapiro-Wilk test) and equality of variance. Subject demo-graphics were compared via the two sample t-test (for comparing independent samples of size less than 30) with pooled or Satterthwaite methods as appropriate. Proc Mixed ANOVA procedures were used to examine effects of viewing method, posture, and group on these dependent variables: head tilt angle, neck flexion angle, head motion, phone tilt angle, and typing accuracy.

The Sign Test was used to analyze effects of viewing method and posture on muscle activity, discomfort data, and typing speed, due to the data being non-normal and having unequal variance which were not resolvable by transforming the data. These analyses were performed separately for each participant group. In preparation for using the Sign Test to assess effects of viewing method on muscle activity, the mean value of the normalized emg data for the prism view was divided by the mean value of the normalized emg data for the direct view (form of the dependent variable: emg_prism/emg_direct). This calculation was performed for each muscle, for each subject. Separate assessments were performed for standing and sitting. Similarly, to assess effects of posture, the mean value of the normalized emg data for sitting was divided by the mean value of the normalized emg data for standing view (form of the dependent variable: emg_sit/emg_stand). Separate assessments were performed for each viewing method. A significant Sign Test meant that the median value of the muscle activity ratio was not equal to 1, indicating muscle activity levels in the two conditions being compared were not the same.

In order to assess effects of test condition on discomfort, the difference between post- and pre- test discomfort ratings was calculated for each test condition. Then these discomfort differences were compared between the test conditions. The effect of viewing method was assessed by subtracting the post-pre discomfort difference for the prism view from the post-pre discomfort difference for the direct view (form of the dependent variable: discomfortdifference_direct - discomfortdifference_prism). This was done for both postures, for each participant. The effect of posture was assessed by subtracting the post-pre discomfort difference for the sitting posture from the post-pre discomfort difference for the standing posture; this was done for both viewing methods, for each participant (form of the dependent variable: discomfortdifference_stand - discomfortdifference_sit). A significant Sign Test meant that the median value of the difference between test conditions was not equal to 0, indicating the post-pre-test difference in discomfort in the two conditions being compared were not the same.

To assess effects of viewing method on typing speed, character count for the prism view was divided by the character count for the direct view (form of the dependent variable: speed_prism/speed_direct). Separate assessments were performed for standing and sitting. Similarly, to assess effects of posture the character count during sitting was divided by the character count during standing (form of the dependent variable: speed_sit/speed_stand). Separate assessments were performed for each viewing method. A significant Sign Test meant that the median value of the ratio of the typing speeds was not equal to 1, indicating the typing speeds in the two conditions being compared were not the same.

3 Results

Viewing method had a significant effect on many of the dependent variables, including head posture and movement, muscle activity, discomfort, and typing speed.

3.1 Participant characteristics

Table 1 provides demographic characteristics and NDI scores for the symptomatic and no pain groups; mean NDI scores were 21.6%and 3.6%, respectively (t-test with unequal variance, p < 0.001). Table 2 provides information about smartphone use preferences and patterns for the two participant groups. There were no significant differences between groups on age, weight, height, phone weight, and total self-reported time spent using a smartphone per day (independent t-test, all p > 0.05).

Table 1
Demographic characteristics by gender for study participants in the symptomatic group and the no pain group. NDI refers to Neck Disability Index

Symptomatic Group Mean (SD) No Pain Group Mean (SD) p-value

Female n 9 2

NDI Score, % 22.9 (8.2) 2.5 (4.2) 0.0172

Age (years) 26.1 (5.6) 25.5 (0.7) ns

Weight (kg) 61.7 (13.8) 60.6 (6.2) ns

Height (cm) 164.9 (6.5) 169.2 (5.4) ns

Male n 7 7

NDI Score, % 20.0 (5.5) 3.1 (3.0) < 0.001

Age (years) 26.6 (4.9) 26.3 (6.6) ns

Weight (kg) 77.0 (13.8) 83.6 (7.1) ns

Height (cm) 174.4 (7.3) 181.1 (8.0) ns

		Symptomatic Group Mean (SD)	No Pain Group Mean (SD)	p-value
Female	n	9	2
	NDI Score, %	22.9 (8.2)	2.5 (4.2)	0.0172
	Age (years)	26.1 (5.6)	25.5 (0.7)	ns
	Weight (kg)	61.7 (13.8)	60.6 (6.2)	ns
	Height (cm)	164.9 (6.5)	169.2 (5.4)	ns
Male	n	7	7
	NDI Score, %	20.0 (5.5)	3.1 (3.0)	< 0.001
	Age (years)	26.6 (4.9)	26.3 (6.6)	ns
	Weight (kg)	77.0 (13.8)	83.6 (7.1)	ns
	Height (cm)	174.4 (7.3)	181.1 (8.0)	ns

Table 2

Subjects’ smartphone type, preferences, and patterns of use. Subjects used their own phones during the study, in the orientation they preferred

	Symptomatic (n = 16)	No pain (n = 9)	p-value
Phone Brands [proportion]	iPhone = 50%	iPhone = 55%
	Android = 50%	Android = 45%
Phone Orientation [proportion]	100%Portrait	89%Portrait
		11%Horizontal
Typing Style [proportion]	87.5%both hands type	100%both hands type
	6%left hand holds, right hand types
	6%both hands hold, right hand types
Phone Weight (gram) [mean (SD)]	140.7 (19.5)	140.9 (20.6)	ns
Total self-reported time on Smartphone (hours/day) [mean (SD)]	3.8 (3.5)	2.9 (1.7)	ns

3.2 Head posture and motion and phone tilt

Head tilt and neck flexion were significantly reduced with use of the prism glasses. Both angles were close to the baseline neutral head posture, in comparison with the head tilt and neck flexion when viewing the phone directly (Table 3, Fig. 3). Compared to the direct view, head motion was reduced when using PG. Head tilt was also affected by posture, with head tilt while standing (–15.9 deg, sd = 22.0) being slightly more declined and further from neutral than when sitting (–11.9 deg, sd = 20.6). Head posture and motion in both participant groups were similarly affected by viewing method and posture (no effect of group).

Table 3
Results from statistical analysis of the effects of posture (sit v. stand), viewing method (direct view v. prism glasses), and symptom group (symptomatic v. no pain) on head posture and movement and phone tilt

Head tilt angle Neck flexion angle Head motion Phone tilt angle

Posture F = 6.09 ns ns F = 8.08

p = 0.0215 p = 0.0092

effect size = 0.2 effect size = 0.5

View method F = 314.5 F = 96.6 F = 24.5 F = 6.95

P < 0.0001 P < 0.0001 P < 0.0001 P = 0.0148

effect size = 3.3 effect size = 1.9 effect size = 0.9 effect size = 0.5

Symptom group ns ns ns ns

P×V ns ns ns ns

P×S ns ns ns ns

V×S ns ns ns ns

P×V×S ns ns ns ns

	Head tilt angle	Neck flexion angle	Head motion	Phone tilt angle
Posture	F = 6.09	ns	ns	F = 8.08
	p = 0.0215			p = 0.0092
	effect size = 0.2			effect size = 0.5
View method	F = 314.5	F = 96.6	F = 24.5	F = 6.95
	P < 0.0001	P < 0.0001	P < 0.0001	P = 0.0148
	effect size = 3.3	effect size = 1.9	effect size = 0.9	effect size = 0.5
Symptom group	ns	ns	ns	ns
P×V	ns	ns	ns	ns
P×S	ns	ns	ns	ns
V×S	ns	ns	ns	ns
P×V×S	ns	ns	ns	ns

Fig. 3

Main effects of viewing method on the mean values of head tilt and neck flexion relative to baseline neutral; also depicted are mean values of head motion for both viewing methods.

Phone tilt was significantly affected by posture and viewing method, but differences were small. Mean phone tilt angle was 52.8 deg (sd = 11.2) during direct viewing and 57.0 deg (sd = 5.6) when using the PG. Mean phone angle was 52.9 deg (sd = 9.2) during sitting and 57.0 deg (sd = 8.6) during standing. Phone tilt did not differ between participant groups.

3.3 Muscle activity

3.3.1 Cervical erector spinae m.

The effect of viewing method was statistically significant and consistent across the two groups of subjects. Normalized muscle activity while using the PG was significantly different from normalized muscle activity recorded during direct viewing, for sitting and standing postures. Across the left and right CES, when comparing the 10th and 50th percentile muscle activity statistics, normalized activity when using the prism glasses was about 60–80 percent of the normalized activity during the direct view, for both groups of subjects (Table 4). There was no effect of posture (sit v. stand) on CES activity, in either group of subjects.

Table 4
Statistical analysis of the normalized 10th and 50th percentile muscle activity. Values in the table are ratios of the normalized muscle activity recorded during each pair of test conditions listed in the first column. Descriptive statistics include average, standard deviation, and median values of the ratios. Results of the Sign tests for median = 1 are provided in columns labeled ‘p-value’

Cervical Erecter Spinae Trapezius

LEFT RIGHT LEFT RIGHT

Test condition comparison %-ile Avg (SD) Median p-value Avg (SD) Median p-value Avg (SD) Median p-value Avg (SD) Median p-value

Symptomatic Group (n = 16)

Standing Prism 10th 0.75 (0.18) 0.0005 0.79 (0.17) 0.0042 0.72 (0.24) 0.004 0.83 (0.39) 0.004

0.76 0.77 0.77 0.81

Standing Direct 50th 0.58 (0.24) < 0.0001 0.69 (0.25) 0.004 0.59 (0.32) 0.0005 0.67 (0.50) 0.004

0.53 0.59 0.66 0.69

Sitting Prism 10th 0.72 (0.13) < 0.0001 0.88 (0.43) 0.0042 1.39 (1.77) ns 1.05 (0.70) 0.021

0.76 0.79 0.96 0.94

Sitting Direct 50th 0.59 (0.18) < 0.0001 0.84 (0.56) 0.004 1.64 (2.14) ns 1.10 (0.60) ns

0.61 0.69 0.99 0.94

Sitting Direct 10th 1.08 (0.23) ns 0.96 (0.33) ns 0.59 (0.27) 0.0005 0.75 (0.47) ns

1.03 1.06 0.51 0.75

Standing Direct 50th 1.09 (0.30) ns 0.92 (0.37) ns 0.49 (0.29) 0.0005 0.67 (0.53) ns

0.99 1.02 0.43 0.57

Sitting Prism 10th 1.03 (0.20) ns 0.96 (0.23) ns 0.96 (0.70) ns 0.84 (0.49) ns

1.03 0.98 0.73 0.67

Standing Prism 50th 1.04 (0.21) ns 0.97 (0.31) ns 1.38 (1.77) ns 1.18 (1.31) ns

1.00 0.92 0.90 0.94

No Pain Group (n = 9)

Standing Prism 10th 0.71 (0.18) 0.039 0.70 (0.18) 0.0039 1.06 (0.39) ns 1.05 (0.47) ns

0.69 0.64 0.98 0.91

Standing Direct 50th 0.58 (0.21) 0.0039 0.55 (0.23) 0.0039 0.99 (0.48) ns 0.98 (0.62) ns

0.55 0.47 1.02 0.77

Sitting Prism 10th 0.78 (0.20) 0.039 0.79 (0.17) 0.039 0.86 (0.25) ns 0.98 (0.32) ns

0.76 0.80 0.89 1.16

Sitting Direct 50th 0.64 (0.24) 0.039 0.67 (0.24) 0.039 1.07 (0.30) ns 1.12 (0.21) ns

0.57 0.69 1.06 1.16

Sitting Direct 10th 0.89 (0.29) ns 0.99 (0.35) ns 0.44 (0.41) 0.0391 0.58 (0.47) ns

0.84 0.96 0.22 0.38

Standing Direct 50th 0.84 (0.29) ns 0.98 (0.39) ns 0.35 (0.36) 0.0039 0.41 (0.35) 0.039

0.82 0.99 0.18 0.31

Sitting Prism 10th 0.95 (0.22) ns 1.1 (0.39) ns 0.42 (0.39) 0.039 0.57 (0.45) 0.039

0.99 0.99 0.17 0.30

Standing Prism 50th 0.93 (0.21) ns 1.16 (0.39) ns 0.49 (0.45) ns 0.66 (0.56) ns

0.91 1.03 0.15 0.28

		Cervical Erecter Spinae	Trapezius
Symptomatic Group (n = 16)
Standing	Prism	10th	0.75 (0.18)	0.0005	0.79 (0.17)	0.0042	0.72 (0.24)	0.004	0.83 (0.39)	0.004
			0.76		0.77		0.77		0.81
Standing	Direct	50th	0.58 (0.24)	< 0.0001	0.69 (0.25)	0.004	0.59 (0.32)	0.0005	0.67 (0.50)	0.004
			0.53		0.59		0.66		0.69
Sitting	Prism	10th	0.72 (0.13)	< 0.0001	0.88 (0.43)	0.0042	1.39 (1.77)	ns	1.05 (0.70)	0.021
			0.76		0.79		0.96		0.94
Sitting	Direct	50th	0.59 (0.18)	< 0.0001	0.84 (0.56)	0.004	1.64 (2.14)	ns	1.10 (0.60)	ns
			0.61		0.69		0.99		0.94
Sitting	Direct	10th	1.08 (0.23)	ns	0.96 (0.33)	ns	0.59 (0.27)	0.0005	0.75 (0.47)	ns
			1.03		1.06		0.51		0.75
Standing	Direct	50th	1.09 (0.30)	ns	0.92 (0.37)	ns	0.49 (0.29)	0.0005	0.67 (0.53)	ns
			0.99		1.02		0.43		0.57
Sitting	Prism	10th	1.03 (0.20)	ns	0.96 (0.23)	ns	0.96 (0.70)	ns	0.84 (0.49)	ns
			1.03		0.98		0.73		0.67
Standing	Prism	50th	1.04 (0.21)	ns	0.97 (0.31)	ns	1.38 (1.77)	ns	1.18 (1.31)	ns
			1.00		0.92		0.90		0.94
No Pain Group (n = 9)
Standing	Prism	10th	0.71 (0.18)	0.039	0.70 (0.18)	0.0039	1.06 (0.39)	ns	1.05 (0.47)	ns
			0.69		0.64		0.98		0.91
Standing	Direct	50th	0.58 (0.21)	0.0039	0.55 (0.23)	0.0039	0.99 (0.48)	ns	0.98 (0.62)	ns
			0.55		0.47		1.02		0.77
Sitting	Prism	10th	0.78 (0.20)	0.039	0.79 (0.17)	0.039	0.86 (0.25)	ns	0.98 (0.32)	ns
			0.76		0.80		0.89		1.16
Sitting	Direct	50th	0.64 (0.24)	0.039	0.67 (0.24)	0.039	1.07 (0.30)	ns	1.12 (0.21)	ns
			0.57		0.69		1.06		1.16
Sitting	Direct	10th	0.89 (0.29)	ns	0.99 (0.35)	ns	0.44 (0.41)	0.0391	0.58 (0.47)	ns
			0.84		0.96		0.22		0.38
Standing	Direct	50th	0.84 (0.29)	ns	0.98 (0.39)	ns	0.35 (0.36)	0.0039	0.41 (0.35)	0.039
			0.82		0.99		0.18		0.31
Sitting	Prism	10th	0.95 (0.22)	ns	1.1 (0.39)	ns	0.42 (0.39)	0.039	0.57 (0.45)	0.039
			0.99		0.99		0.17		0.30
Standing	Prism	50th	0.93 (0.21)	ns	1.16 (0.39)	ns	0.49 (0.45)	ns	0.66 (0.56)	ns
			0.91		1.03		0.15		0.28

Guide to table interpretation: Example: first row of results for the symptomatic group shows that when comparing the 10th percentile muscle activity in the left CES while standing and using prism glasses to standing and viewing directly, the ratio of the muscle activity is only 0.75, meaning CES activity when using the prism glasses is only 75%as high as when viewing directly during standing; the Sign test determined that the ratio was significantly different (p = 0.0005) from 1.0, meaning the 10th percentile muscle activity levels in the two conditions were not the same.

3.3.2 Upper trapezius m.

There was a difference in normalized muscle activity between the prism and direct views when standing, for the symptomatic group. Normalized activity during prism viewing was about 60–80 percent of the normalized activity during the direct view, bilaterally. This effect was not seen during sitting in the symptomatic group, and was not seen for either sitting or standing posture in the no pain group. Both groups saw a marked effect of posture on the normalized muscle activity in the left UT. Normalized activity during sitting was about 40–60 percent of the normalized activity during standing, when directly viewing the phone for the 10th and 50th percentile left UT, for both participant groups.

3.4 Discomfort

The only significant effects on discomfort occurred in the symptomatic group. The increase in neck discomfort (from pre-task to post-task) was greater in the direct view than in the prism view, for both standing (p = 0.0005) and sitting (p = 0.0225) postures; the median increase was 1 point greater on the discomfort scale when comparing the extent to which neck discomfort increased during the direct view v. during the prism view, for both postures. A similar effect occurred in this group for shoulder discomfort, but only in the sitting posture (p = 0.0005); the median increase was 0.75 points greater on the discomfort scale when comparing the extent to which shoulder discomfort increased during the direct view v. during the prism view, for both postures. Figure 4 provides a view of the pre- and post- task discomfort reported by each participant for each test condition.

Fig. 4

Neck discomfort for each participant, pre- and post- task performance (a –d); shoulder discomfort for each participant, pre- and post- task performance (e –h). No pain group participants are designated by C##; symptomatic group participants are designated by P## and a line drawn under the subject designator. The intent of the figure is to provide an overall sense of the increase of discomfort between pre-task (lighter foreground “mountain range”) and post-task (darker background “mountain range”) rather than look specifically at an individual subject’s data.

3.5 Performance

Accuracy was consistently high across participants and testing conditions (grand mean = 99.1%, sd = 0.6). A wide range of typing speeds was exhibited by the participants, with the fastest subject typing 5–7 times more characters than the slowest subject, across the four test conditions. Typing speed was affected by viewing method. Reductions in typing speed associated with using the prism glasses ranged from 9 to 20%, across the two postures and the two participant groups (Table 5). There also appeared to be a modest reduction in speed while sitting in comparison with standing when directly viewing the phone, for both participant groups. The no pain group also displayed a similar decrement in speed while sitting in comparison with standing when using the prism glasses.

Table 5
Statistical analysis of text typing speed on the smartphone, for both groups of study participants. Values in the table are ratios of the character counts from each pair of test conditions listed in the first column. Descriptive statistics include average, standard deviation, and median values of the ratios. Results of the Sign tests for median = 1 are provided in columns labeled p-value

Test condition comparison Symptomatic Group No Pain Group

Avg (SD) Median p-value Avg (SD) Median p-value

Standing Prism 0.84 (0.13) 0.021 0.91 (0.08) 0.039

Standing Direct 0.80 0.90

Sitting Prism 0.85 (0.10) 0.0005 0.84 (0.10) 0.004

Sitting Direct 0.83 0.86

Sitting Direct 0.93 (0.09) 0.021 0.91 (0.14) 0.039

Standing Direct 0.93 0.87

Sitting Prism 0.95 (0.14) ns 0.84 (0.12) 0.004

Standing Prism 0.94 0.87

Test condition comparison	Symptomatic Group	No Pain Group
Standing	Prism	0.84 (0.13)	0.021	0.91 (0.08)	0.039
Standing	Direct	0.80		0.90
Sitting	Prism	0.85 (0.10)	0.0005	0.84 (0.10)	0.004
Sitting	Direct	0.83		0.86
Sitting	Direct	0.93 (0.09)	0.021	0.91 (0.14)	0.039
Standing	Direct	0.93		0.87
Sitting	Prism	0.95 (0.14)	ns	0.84 (0.12)	0.004
Standing	Prism	0.94		0.87

Guide to table interpretation: Example: first row, results for the symptomatic group show that when comparing text typing speed while standing and using prism glasses to the speed while standing and looking directly, the ratio of the speed is only 0.84, meaning the typing speed when using the prism glasses is only 84%as high as when viewing directly, while standing; the Sign test determined that the ratio was significantly different (p = 0.021) from 1.0, meaning the text typing speed levels in the two conditions were not the same.

3.6 Preferences and other subjective comments

In both groups, there was a strong preference for the prism glasses over the direct view in terms of neck comfort (Fig. 5; question on survey, ‘Which would you prefer in terms of NECK comfort’, with response options ‘Direct View’ and ‘Prism View’). For visual comfort, productivity, and accuracy, most participants preferred the direct view. After completing the preference survey, participants were asked if they had any comments or suggestions concerning the prism glasses. The primary positive comment made by over half of the participants concerned comfort in the neck region that they experienced when using the prism glasses (mentioned by 14 participants in the symptomatic group and 2 in the no pain group). The primary concern mentioned by almost half of the participants concerned the weight of the prism glasses (mentioned by 10 in the symptomatic group and 2 in the no pain group).

Fig. 5

Participants’ viewing mode preferences in terms of neck comfort, visual comfort, productivity, and accuracy.

4 Discussion

Pronounced neck flexion is commonly observed in people viewing smartphones. This study assessed the potential for the use of prism glasses to foster a more neutral head and neck posture during smartphone use in stationary seated or standing situations. Consistent with the initial hypothesis, the use of prism glasses was shown to reduce awkward head postures (pronounced neck flexion and downward head rotation) and neck extensor muscle activity during a 10 minute interval of continuous typing (texting) on a smartphone. These reductions were seen in study participants with and without neck pain. Neck discomfort was also significantly reduced in the neck pain group when using the PG in comparison to viewing the phone directly; discomfort was not different in the no pain group.

4.1 Head posture, muscle activity, and head motion

4.1.1 Head posture.

Using the PG while seated produced neck flexion and head tilt angles in the current study that were comparable to mean values reported for subjects wearing PG while performing simulated dental hygiene tasks [21]. Current study: neck flexion = 36 deg, head tilt = 24 deg. Smith et al.: neck flexion = 41 deg, head tilt = 20 deg.

4.1.2 Muscle activity.

In the sitting-direct view test condition, UT activity was significantly lower than in the standing-direct view condition. This result was anticipated because the chair’s arm rests were supporting the participant’s arms, thus requiring less shoulder muscle activity. This result aligns with recommendations for forearms to be supported in some way (e.g. against the thighs, or a table, or an arm rest) to reduce the risk of developing musculoskeletal pain when using a smartphone for texting [19]. It is unlikely that the reduced UT activity was due to the reduced typing speed performance, as other studies that have found differences in typing speeds between test conditions when typing on a phone held in the hand [30] or typing on a tablet on a table [31] but saw no effects in trapezius muscle activity. Cervical muscle activity was also significantly reduced when using the prism glasses, in comparison to the direct view. This corresponds with the reduction in neck flexion and head tilt while using the prism glasses. When using the prism glasses the center of mass of the head is closer to the spine and as a result the neck extensor muscles do not have to produce as much force to support that posture. Similar reduction in CES activity was seen concomitant with reduced head tilt and neck flexion [32], during a reading task on a computer monitor.

4.1.3 Head motion.

Head motion activity tended to be greater during direct viewing than when using PG. While wearing PG, due to the limited viewing area of the prism (mentioned by nearly 1/3 of participants), participants tended to maintain fixed postures, which resulted in measurement of less head motion activity. Thorn et al. [33] demonstrated that it is possible for certain motor units to continue to fire when a posture is maintained for a protracted period of time. There is the potential for less head movement with the use of PG, which could contribute to sustained recruitment of certain muscle fibers. This could pose a risk for selective fibre injury in low-threshold motor units that are exposed to prolonged activation. This suggests that a modification of the design of the PG to provide a larger viewing area could be beneficial. However, this would have to be accomplished without adding weight to the PG, because weight was already a concern of some of the study participants.

4.2 Discomfort

In the symptomatic group the use of the PG resulted in a significantly lower increase in neck and shoulder discomfort at the conclusion of the task when compared to the direct view, irrespective of whether participants were sitting or standing. This is consistent with the finding of less neck muscle activity, less neck flexion, and less head tilt found during the use of the PG. Similar results were found in another smartphone study [25], which compared discomfort changes after a 10 minute task from 40 participants (20 neck pain and 20 healthy) who used a smartphone or a computer to type in three sitting conditions (bilateral texting, unilateral texting, and computer typing). Their results indicated that changes in discomfort level after each task were significantly higher in the symptomatic group compared to the healthy group [25].

With regards to the current study, Fig. 4a-d shows that several subjects in the symptomatic group began each condition with some level of mild to moderate neck discomfort, while only a few subjects in the no pain group reported even mild discomfort at the outset of each test condition. Yet, a substantial majority of participants in each group experienced an increase in neck discomfort by the conclusion of the direct view conditions, in both postures. In contrast, after using the PG most subjects in both groups did not experience an increase in neck discomfort, with one marked exception being one of the no pain group participants; at the conclusion of the standing-prism view condition, only a small number of no pain group participants and one symptomatic participant were shown to have noticeable increases in neck discomfort. As seen in Fig. 4e-h, development of shoulder discomfort was less consistent in comparison to neck discomfort. Figure 4 shows that after only 10 minutes of direct viewing most subjects in each group were susceptible to developing some level of neck discomfort (Fig. 4a and c), with some subjects in both groups also developing some shoulder discomfort (Fig. 4e and g), and that most subjects in both groups benefited from the use of the PG (Fig. 4b, d, f, and h), as revealed in less discomfort or less of an increase in discomfort by task end. More subtle prisms (5 deg.) have also been shown to have a positive effect on health outcomes. In a 12 month randomized control prospective field trial, Lindegard et al. [22] found prescription glasses with 5 deg. prisms to be effective, in terms of improvements in clinical diagnoses and self-reported work ability, in dental professionals with neck/shoulder pain at baseline who chose to wear PG regularly while working.

4.3 Performance

Some performance decrement was seen with respect to typing speed while using the PG. Accuracy was not reduced. This reflects the ability of participants to perform the texting task while wearing the prism glasses, and reflects adherence to instructions to focus on accuracy over speed. Drury [34] discussed the importance of researchers making the choice to emphasize speed or accuracy in an experiment, rather than leaving the choice to research participants, as the latter introduces more variability into the data. In the current study, all participants had a 10 minute practice session with the PG prior to the start of data collection. Though the majority said they felt comfortable with the PG after a few minutes of practice at the beginning of the experiment, during the post-experiment interview about one-third reported difficulty getting accustomed to the PG. A task speed reduction of 10 percent, which is within the range of the speed reduction seen in the current study, was reported by Smith et al. [21] when comparing prism and direct views for simulated dental hygiene tasks. More time on task while wearing the prism glasses would likely improve text typing speed, similar to improvements that were seen when participants had more exposure to a vertical keyboard in the study by van Galen et al. [35].

4.4 Subjective perceptions

The majority of the symptomatic participants preferred the PG, because at the end of the typing task they felt less neck discomfort when using the PG, compared to the direct view. By contrast, only a couple of no pain group participants spontaneously mentioned the neck feeling comfortable with use of the PG. While the no pain group benefited biomechanically from the PG, the effect on discomfort did not reach significance in that group.

The sensitivity of the symptomatic group could also be seen from the impression of 10 of 16 participants from that group that the PG were too heavy versus only 2 of 9 from the no pain group reporting that perception. The prototype PG used in this study were lighter than models on the market, weighing 128 g, compared to other models that weigh 180 g. As a point of reference, plastic frame reading glasses (‘readers’) that can be purchased at a drug or general merchandise store might weigh 20 g. The i90 PG frame was made from lightweight aluminum. Most of the weight of the glasses is from the prisms, and most of the weight was supported by the bridge of the wearer’s nose. This concentration of weight could have caused some discomfort. It is unknown if the prisms could be made with a lighter weight material. Since those who recognized the benefits of the reduced discomfort were in the symptomatic group and that is the group most likely to purchase such glasses to reduce their neck discomfort, weight reduction deserves further attention. Another potential alternative that might reduce the weight of the prisms could be a prism with a smaller angle. The PG used by Lindegard et al. [20, 22] contain a 5 deg prism about the size of the bifocal portion of regular corrective lens. In their study, reductions in head and neck flexion were each about 5 deg. Some compromise angle between 5 and 90 deg might provide an optimum balance between significant reductions in muscle activity and awkward postures while providing improved comfort and productivity.

Additionally, about 30%of participants in the current study had an impression that the viewing area of the PG was too small. This could have restricted head motion activity as measured in this study. Enlarging the viewing area without sacrificing the requirement for less weight might improve the usability and desirability of the PG.

4.5 Study strengths and limitations

The current study examined the biomechanical efficacy and effects on discomfort, usability, desirability, and performance outcomes of wearing PG while texting on a smartphone. To eliminate the possibility of unequal familiarity with smartphone use, all participants had at least 1 year of experience using a smartphone. Second, in order to minimize any biased preference that symptomatic individuals might have of the intervention, a group of individuals who did not experience chronic neck pain was included. Finally, the study collected multiple types of data (methodological triangulation), including objective and subjective data to ensure a holistic assessment of the intervention [36].

The duration of each test condition was only 10 min., so it cannot be predicted if the benefit of the PG for this task would persist beyond 10 min. Further, participants were all younger adults (age range 19–36 yr); future studies should expand the age range to determine if similar results are seen in other age groups. Notably, for adults over 50 yrs of age a correction for presbyopia would likely need to be incorporated. Validity of the results would be enhanced if the study was replicated and similar results were achieved. Finally, the study was conducted in a controlled setting, and as such further usability assessment is recommended to determine feasibility and usefulness in real world settings and conditions, including optimal patterns of wear for a positive effect.

5 Conclusion

Prior research has shown promising benefits of prism glasses as an intervention that could possibly reduce neck pain and discomfort during simulated [21] and actual dental work [22]. The current study examined the biomechanical efficacy and effects on discomfort, as well as usability, desirability, and performance outcomes of wearing prism glasses during smartphone use in stationary conditions. Results showed that the prism view generated lower neck muscle activity, more neutral postures, and less neck discomfort, compared to the direct view, in symptomatic participants; biomechanics were similarly improved in the no pain group. Although typing speed when using the prism glasses was significantly lower than in the direct view, accuracy was not adversely affected. It is believed the speed decrement would decline with additional practice.

The prism glasses tested in this study need to be further developed to address two important concerns raised by study participants: 1) prism glasses being too heavy and 2) viewing area being too small. As the majority of the weight is from the 90 deg prisms, it would be important to address these potential barriers to adoption through design modifications, such as a smaller prism angle or glasses made from lighter weight material, followed by further testing, particularly in individuals with neck pain that is exacerbated by texting and other smartphone uses that require sustained viewing of the display.

Conflict of interest

None to report.

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		Cervical Erecter Spinae				Trapezius
		LEFT		RIGHT		LEFT		RIGHT
Test condition comparison		%-ile	Avg (SD) Median	p-value	Avg (SD) Median	p-value	Avg (SD) Median	p-value	Avg (SD) Median	p-value
Symptomatic Group (n = 16)
Standing	Prism	10th	0.75 (0.18)	0.0005	0.79 (0.17)	0.0042	0.72 (0.24)	0.004	0.83 (0.39)	0.004
			0.76		0.77		0.77		0.81
Standing	Direct	50th	0.58 (0.24)	< 0.0001	0.69 (0.25)	0.004	0.59 (0.32)	0.0005	0.67 (0.50)	0.004
			0.53		0.59		0.66		0.69
Sitting	Prism	10th	0.72 (0.13)	< 0.0001	0.88 (0.43)	0.0042	1.39 (1.77)	ns	1.05 (0.70)	0.021
			0.76		0.79		0.96		0.94
Sitting	Direct	50th	0.59 (0.18)	< 0.0001	0.84 (0.56)	0.004	1.64 (2.14)	ns	1.10 (0.60)	ns
			0.61		0.69		0.99		0.94
Sitting	Direct	10th	1.08 (0.23)	ns	0.96 (0.33)	ns	0.59 (0.27)	0.0005	0.75 (0.47)	ns
			1.03		1.06		0.51		0.75
Standing	Direct	50th	1.09 (0.30)	ns	0.92 (0.37)	ns	0.49 (0.29)	0.0005	0.67 (0.53)	ns
			0.99		1.02		0.43		0.57
Sitting	Prism	10th	1.03 (0.20)	ns	0.96 (0.23)	ns	0.96 (0.70)	ns	0.84 (0.49)	ns
			1.03		0.98		0.73		0.67
Standing	Prism	50th	1.04 (0.21)	ns	0.97 (0.31)	ns	1.38 (1.77)	ns	1.18 (1.31)	ns
			1.00		0.92		0.90		0.94
No Pain Group (n = 9)
Standing	Prism	10th	0.71 (0.18)	0.039	0.70 (0.18)	0.0039	1.06 (0.39)	ns	1.05 (0.47)	ns
			0.69		0.64		0.98		0.91
Standing	Direct	50th	0.58 (0.21)	0.0039	0.55 (0.23)	0.0039	0.99 (0.48)	ns	0.98 (0.62)	ns
			0.55		0.47		1.02		0.77
Sitting	Prism	10th	0.78 (0.20)	0.039	0.79 (0.17)	0.039	0.86 (0.25)	ns	0.98 (0.32)	ns
			0.76		0.80		0.89		1.16
Sitting	Direct	50th	0.64 (0.24)	0.039	0.67 (0.24)	0.039	1.07 (0.30)	ns	1.12 (0.21)	ns
			0.57		0.69		1.06		1.16
Sitting	Direct	10th	0.89 (0.29)	ns	0.99 (0.35)	ns	0.44 (0.41)	0.0391	0.58 (0.47)	ns
			0.84		0.96		0.22		0.38
Standing	Direct	50th	0.84 (0.29)	ns	0.98 (0.39)	ns	0.35 (0.36)	0.0039	0.41 (0.35)	0.039
			0.82		0.99		0.18		0.31
Sitting	Prism	10th	0.95 (0.22)	ns	1.1 (0.39)	ns	0.42 (0.39)	0.039	0.57 (0.45)	0.039
			0.99		0.99		0.17		0.30
Standing	Prism	50th	0.93 (0.21)	ns	1.16 (0.39)	ns	0.49 (0.45)	ns	0.66 (0.56)	ns
			0.91		1.03		0.15		0.28