Participants
This cross-sectional study was conducted between February and August 2021 at the Department of Ergonomics, University of Social Welfare and Rehabilitation Sciences. A total of 60 volunteer students (age 24.57 ± 4.38, range 18 to 40 years, 53.3% male) were recruited from a sample of convenience within four University Medical centers in Tehran, Iran. Inclusion criteria were a history of smartphone use for more than one year and 2 h per day. Exclusion criteria were a history of neck pain, radicular pain, neurological symptoms, and surgery to the neck and/or upper limbs. All subjects provided written informed consent prior to the study. The study was approved by the ethical committee of the University of Social Welfare and Rehabilitation Sciences, number IR.USWR.REC.1400.035.
Procedure
Demographic information was collected through a checklist that included sex, age, education, daily duration of smartphone use, areas of discomfort or symptoms when working with a smartphone, and the purpose of use (e,g. social media, web surfing, reading news, gaming, etc.). To identify ‘Addicted’ and ‘Non-Addicted’ participants the Smartphone Addiction Scale-Persian short Version SAS-SV-Pr was used. We assessed cervical flexor muscles endurance using the Neck Flexor Muscle Endurance Test (NFMET) [22], Neck extensor endurance using the Neck Extensor Muscle Endurance Test (NEMET), cervical position sense using a laser pointer custom-made device, and neck ROM using a manual goniometer.
Neck Flexor Muscle Endurance Test (NFMET)
To perform this test, the subject is placed in a supine hook-lying position and performs a maximum isometric chin retraction, followed by a head and neck lift of approximately 2.5 cm above the table. While the subject maintains the chin tuck posture, the examiner puts their hand just below the occipital bone of the subject’s head (Fig. 1) and through verbal commands (‘Hold your head up’) encourages the subject to separate the head from the examiner’s hand and try as hard as possible to avoid touching the hand. When the subject is no longer able to maintain his/her head up and touches the hand, the test was completed (Fig. 1). The total length of time in seconds that the subject held the head in the ‘up position’ without touching the examiner’s hand is recorded by a stopwatch as the time for muscle endurance [22, 23].
Neck Extensor Muscle Endurance Test (NEMET)
To perform this test, the procedure described by Ljungquist et al., was adopted [24]. The subject is lying in a prone position on the table while their arms are at their side and the head is over the end of the table and supported by the examiner (Fig. 2). To avoid the displacement of the thorax, it was supported by a strap. To monitor the head position, an inclinometer was attached to the posterior aspect of the head and fixed by a Velcro band. A 1-kg weight was suspended from the headband so that the weight was located just short of the floor. The subject's head was positioned in the neutral sagittal plane position and the cervical spine in a horizontal line with his/her chin. As the support of the examiner was removed the stopwatch recorded the time for the sustained position. The test was terminated when the weight returned to the floor or the neck horizontal position change more than 5° as shown by the inclinometer [23].
Joint position sense (JPS) assessment
In this test JPS was measured according to the experimental procedure presented by Revel et al. [25]. The ability to relocate the natural head posture (anatomic position) after performing either active cervical extension or left and right rotations, while blindfolded, was measured. The subject wears a light headpiece helmet strapped firmly to his/her head with a laser pointer fixed to the top of the helmet aimed at a target 90 cm in front of the subject. The subject was asked to sit in a comfortable position while their arms are hanging by their side, and the feet are on the floor. The eyes and vision were blocked by a sleeping mask and the subject is asked to assume a neutral position of the head and memorize it, and to be able to return to it after the completion of any movement. This point was considered as a reference point for each subject and after that the subject should actively perform maximum neck flexion and then return the head in the previous reference point (Fig. 3). The difference between the starting reference point and the returning point as determined by the laser beam on the target was recorded in cm as the error measure [26]. Three trials were performed for each direction (flexion, extension, lateral flexion), and the mean of the error difference was used in the statistical analysis.
Cervical ROM (C-ROM)
Active cervical range-of-motion (ROM) was measured in the transverse (rotation), sagittal (flexion–extension), and frontal (lateral-bending) planes with a cervical ROM device. The test was performed by an occupational therapist with > 10 years of work experience. The C-ROM device was placed on the subject’s head while they were in an upright sitting position with both feet on the floor [26]. The difference between the start and end position in each direction from the natural posture was recorded as the ROM. Three repetitions were measured, and the average used for statistical analysis.
Forward head posture (FHP)
A digital camera (Canon IXY 12, MP, Japan) was used to take a lateral view picture of the subject in standing and then calculate the relevant angles to assess the FHP [27]. The camera was fixed on a tripod at 1.5 m from the subject at the level of their shoulder [28]. The subject adopted a natural neck posture before the photography and for this purpose was asked to move their neck into flexion and extension and gradually reduced its range until the head and neck was placed in a natural self-selected comfortable position [27]. Three markers were attached: over the seventh cervical spinous process (C7); the tragus of the ear; and the acromion process. These were used to define the position of the head and neck in the sagittal plane. After taking the photograph, it was used for calculating the craniovertebral angle (CA), shoulder angle (SA), sagittal head angle (SHA) and forward head distance (FHD) as described below (Fig. 4) [29].
Craniovertebral angle (CVA) is the angle between the line joining the tragus of the ear to C7, and the horizontal line at C7, and has high reliability (r = 0.88) [30, 31]. A higher CVA indicated a lower FHP [32].
Shoulder angle (SA) is the angle between the line joining the C7 to the acromion process, and the horizontal line at the acromion process. A smaller SA represented a greater FHP and kyphotic posture [32].
Sagittal head angle (SHA) is the angle between the line joining the tragus of the ear to the canthus of the eye, and the horizontal line at the tragus. A lower value of this angle represented a lower FHP [32, 33].
Forward head distance (FHD) is the horizontal distance between the tragus, and the C7 vertebra. A higher distance indicated a higher FHP [27].
Smartphone Addiction Scale-Short Version (SAS-SV)
The long form of the SAS is a 33-item questionnaire originally developed by Kwon et al. in 2013 [34]. Subsequently, a short version (SAS-SV) was presented with 10-items [35]. We used the validated Persian SAS-SV in this study to identify the level of the smartphone addiction risk and to distinguish high-risk participants. The psychometric properties of the Persian version were determined by Mokhtarinia et al. in 2020 [36] and demonstrated high reliability and validity. The cut-off values to detect the ‘Addicted’ person are: male = 31 and female = 33.
Statistical analysis
All statistical analyses were performed using the SPSS statistical software version 16.0 (IBM SPSS Statistics for Windows). The descriptive results for quantitative variables were calculated as mean ± standard deviation (SD), and for categorical data as frequency and percentages. The normality of measurable data was evaluated using the Kolmogorov–Smirnov test. The sample did not follow a normal distribution; therefore, non-parametric statistical testing was performed. A Mann–Whitney test was used to determine whether there was a significant difference in measured variables between ‘Addicted’ and ‘Non-Addicted’ persons smartphone use. In controlling for the Type I error rate for the hypothesis testing, a Bonferroni correction was applied by dividing the significance level by the number of tested hypotheses (0.05:11 = 0.0045). The Spearman correlation coefficient was used to assess the associations and correlations between the variables.