Abstract
Background and aims:
This study investigated whether young university students with neck pain (NP) have postural control deficits when compared to sex and age-matched asymptomatic subjects.
Methods:
Centre of pressure (COP) sway area, velocity, anterior-posterior and mediolateral distances were measured in participants with (n=27) and without (n=27) neck pain for different combinations of static standing (narrow stance, tandem stance and single leg stance) and measurement time (90, 60, 30 and 15 s) with eyes closed using a force plate. Additionally, static and dynamic clinical tests of postural control were used.
Results:
No significant between group differences were found for the COP measurements (p>0.05). However, individuals with subclinical NP were more likely to fail the 90 s tandem test (p<0.05) in the force plate and univariate comparisons revealed significant between group differences in the tandem and single leg stance clinical test measurements.
Conclusions:
Taken together, the inconsistent results might suggest an emerging postural control deficit in university students with low disability and low intensity chronic idiopathic NP.
1 Introduction
Neck pain (NP) is common in the general population with a mean lifetime prevalence of 48.5% [1]. In particular, the university student population is highly affected by neck pain, with 1 year prevalence estimates of 46.0% and one-third of students reporting persistent and disabling neck pain [2], [3]. Furthermore, there is some indication that neck pain may even be more common than low back pain in student populations [4], [5]. The high prevalence of neck pain and the indication of previous episodes of neck pain being a risk factor for developing future episodes [6], [7] suggest that it is of major relevance to study neck pain at early stages of neck pain, i.e. subclinical neck pain, as a means to inform interventions aiming at preventing chronicity and disability.
A systematic review of studies comparing postural control between participants with and without neck pain has shown that participants with neck pain have impaired postural control when compared with asymptomatic participants [8]. The cervical spine seems to have an important role in postural control due to the abundant cervical mechanoreceptors and their extensive connections with the vestibular, the visual and the central nervous system [9], [10]. Several mechanisms may contribute to the altered cervical somatosensory input and integration to the postural control system in patients with neck pain, including: the effects of pain itself on both nociceptor and mechanoreceptor activity locally, at the spinal cord and within the central nervous system [11], [12], disturbed sensitivity of the cervical joint and muscles receptors [13] or by inflammatory mediators [14].
Existing studies comparing postural control between individuals with and without neck pain mainly use participants aged 40 years old or more, with moderate levels of pain and that actively sought treatment for their neck pain [8]. Whether young university students with low levels of neck pain intensity and disability have postural control deficits is unknown. In addition, studies comparing participants with and without neck pain differ in terms of methodology [8]. For example, there is wide variation in the participant position (e.g. bipedal standing, single leg stance, tandem standing) or in the time period during which the participant is required to maintain the position (e.g. 30 s, 60 s, 90 s). However, the procedures used are likely to affect the reliability of the measurements [15] and, eventually, the outcome of the comparison. Therefore, the main aims of this study are: (i) to compare postural control between university students with subclinical neck pain and asymptomatic participants matched for age and sex and (ii) to assess the influence of measurement procedures and type of measurement (laboratorial vs. clinical) on that comparison. A secondary aim was to explore the association between pain characteristics and postural control.
2 Methods
Data was collected at the School of Health Sciences, Aveiro University. Measurements were taken in one session only, between November 2014 and February 2015. The study was approved by the Ethics Committee of the Social and Health Sciences Department, Faculty of Medicine, Porto, Portugal. Before data collection, all participants gave their written informed consent.
2.1 Participants
Participants were students from Aveiro University. To be included in the subclinical neck pain group, participants had to: (i) have idiopathic neck pain and report feeling neck pain at least once a week in the last 3 months, (ii) have no other pain complaint in the last 6 months and (iii) have never received any treatment for their neck pain (except occasional analgesics). Neck pain was defined as pain felt dorsally between the inferior margin of the occiput and the first thoracic spinous process [16] and it was considered subclinical if it was a pain of mild intensity for which no treatment was received. To be included in the control group, participants had to: (i) report no previous history of neck pain and no pain in any other body region in the last 6 months. Participants in both groups were matched for sex and age (±1 year), had to be ≥18 years-old and report no neurological, rheumatic, vestibular or orthopaedic pathology and alcohol consumption in the 24 h period before data collection.
2.2 Procedures
Demographic, anthropometric and postural control data were collected for both groups. Additionally, neck pain characteristics were assessed for participants with neck pain. Measurement procedures are specified below.
2.2.1 Neck pain
Neck pain intensity at the moment, frequency in the previous week, duration and associated disability were assessed. Pain intensity was measured using a 10 cm visual analogue scale (VAS) anchored with “no pain” and “worst pain imaginable”. Pain frequency was assessed by asking participants to choose one of the following options: (1) seldom (once a week), (2) occasionally (2–3 times a week), (3) often (more than 3 times a week) or (4) always (all days). Pain duration (How long have you been feeling neck pain?) and associated disability (Does your neck pain interfere with your daily activities? If yes, please specify the activities) were assessed using open questions.
2.2.2 Postural control
Postural control was assessed using both a force plate and clinical tests. For the first, a computerised, stable force platform (AMTI BP400600-2000, AMTI, Watertown, MA, USA) was used to measure postural sway at a frequency of 1,000 Hz. A MatLab macro (MathWorks, Madrid, Spain) was then used to compute: total sway area, anteroposterior and mediolateral centre of pressure (COP) displacement and mean COP velocity. Participants were asked to stand upright with their eyes closed, arms by their sides and barefoot on the top of the platform for three different positions: narrow stance (feet together), tandem stance and single leg stance. Participants were required to maintain narrow and tandem stance for 30, 60 and 90 s and single leg stance for 15 s (Fig. 1). Between measurements, participants were asked to walk around the room for approximately 15 s. The order of assessment (for position and time) was randomized for each participant using online software (www.randomizer.org). We aimed to perform three measurements for each combination of position and time period of assessment. Each participant was allowed three trials with loss of balance before the end of the test being classified as failed. Success or failure to complete the stance tests was registered. Data collection procedures were in line with recommendations aiming to enhance reliability [15].
Static postural control: (A) feet together; (B) tandem stance and (C) unipedal stance.
Four clinical tests adapted from previous authors [7], [17] were performed: three assessed static postural control and one assessed dynamic postural control. For the static postural control tests, participants were asked to step onto 10 cm thick soft foam and maintain an upright posture in narrow stance (Fig. 2) and tandem stance with eyes closed for a maximum of 90 s and single leg stance with eyes closed for a maximum of 15 s. The tests finished when the participant was no longer able to maintain the required position or reached the maximum time specified. Three measurements were performed for each position and the order of assessment was randomized as for the force plate measurements. Participants were asked to walk around the room for 15 s between measurements. The dynamic postural control was assessed using a step test. The step was placed on the ground and near a wall and participants were just behind a strip of white tape marked on the floor and were asked to step on and off a 15 cm height step with their dominant foot (defined as the foot used to kick a ball) as quickly as possible while maintaining the opposite foot in the initial position (Fig. 3). The number of steps performed were counted and used in the analysis.
Clinical test: static balance with feet together.
Clinical test: step test for dynamic balance.
All measurements and tests were performed by a blind assessor, i.e. the assessor did not know whether participants had neck pain or were asymptomatic. Additionally, participants were asked not to comment on their status with the assessor.
2.3 Data analysis
Statistical analysis was performed using SPSS version 22. Not all participants were able to do three repetitions for the same position and measurement time. Therefore, we used data from one measurement only (the first complete measurement for each position and time) in the analysis. For the clinical tests, we used the mean of the three measurements in the analysis. Differences between groups for weight and height were investigated using an independent t-test. For measurements in the force plate, two different analyses were performed to investigate differences between groups: a chi-square to investigate differences in the number of participants able to complete the three repetitions of each combination of position and time and a multivariate analysis of variance (MANOVA) to investigate differences in the COP parameters. For the latter, COP area, mediolateral and anteroposterior COP distance and COP velocity were the dependent variables. A MANOVA was also used to explore between group differences in the performance of clinical tests. The dependent variables were time of tandem and single leg stance and number of steps. Group (neck pain vs. asymptomatic) was the fixed factor for all MANOVA analysis. There was homogeneity of between group variance for the dependent variables (Levene’s test, p>0.05). Some dependent variables did not follow a normal distribution (as tested by the Shapiro-Wilk test; p<0.05), but MANOVA is claimed to be robust to minor violations of normality [18]. Associations between variables (postural control and pain characteristics) were explored using a Spearman correlation coefficient. Significance was set at p<0.05.
3 Results
A total of 54 participants entered the study: 27 participants with subclinical neck pain and 27 asymptomatic participants matched for sex and age (±1 year) to the subclinical neck pain participants. No significant differences (p>0.05) were found between groups for age, weight and height (Table 1).
Mean±standard deviation values for age, weight and height and p-value for between group comparisons.
| Variable | Neck pain |
Asymptomatic |
p-Value | ||
|---|---|---|---|---|---|
| n | Mean±SD | n | Mean±SD | ||
| Age (year) | 27 | 21.30±2.20 | 27 | 21.10±1.90 | 0.70 |
| Weight (Kg) | 27 | 64.10±14.30 | 27 | 62.80±12.50 | 0.72 |
| Height (cm) | 27 | 165.90±9.30 | 27 | 165.00±7.30 | 0.58 |
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Y=years; Kg=kilograms; cm=centimeters; SD=standard deviation.
3.1 Neck pain characteristics
All participants reported to have neck pain at the time of data collection and mean (±SD) neck pain intensity was 1.58±1.41; mean (±SD) neck pain duration was 1.81±0.92 years (minimum – 3 months; maximum – 5 years). In terms of neck pain frequency in the week preceding data collection: seven (26.9%) participants reported pain rarely (once a week), 14 (53.8%) reported pain occasionally (2–3 times a week), four (15.4%) participants reported to have felt neck pain several times (more than 3 times a week) and one participant (3.8%) reported pain always. Data on pain frequency was missing for one participant. When questioned about neck pain associated disability, only eight (29.6%) participants reported their neck pain to interfere with their day-to-day activities, which included studying and using the computer.
3.2 Postural control
When analyzing the number of participants able to complete all measurements in all combinations of position and time, results show that all participants in both groups were able to maintain narrow stance for 30, 60 and 90 s. In contrast, only five (18.5%) participants in the subclinical neck pain group and 12 (44.0%) in the asymptomatic group were able to complete three measurements of 90 s in tandem stance (Table 2). This difference was statistically significant [X2(1, n=54)=4.21, p=0.04] and approached significance for the 60 s time period [X2(1, n=54)=3.82, p=0.05]. No significant differences were found for 30 s tandem stance [X2(1, n=54)=2.67, p=0.10] or single leg stance [X2(1, n=54)=0.08, p=0.78].
Number of trials completed (out of a maximum of three) in tandem stance for 90, 60 and 30 s, and single leg stance for participants with and without neck pain.
| Position |
Tandem stance |
Single leg stance |
||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Time | 90 s | 60 s | 30 s | 15 s | ||||||||||||
| Number of trials completed | 3 | 2 | 1 | 0 | 3 | 2 | 1 | 0 | 3 | 2 | 1 | 0 | 3 | 2 | 1 | 0 |
| Neck pain | 5 (18%) | 8 (30%) | 6 (22%) | 8 (30%) | 7 (26%) | 7 (26%) | 9 (33%) | 4 (15%) | 10 (37%) | 9 (33%) | 6 (22%) | 2 (8%) | 11 (40%) | 8 (30%) | 5 (18%) | 3 (12%) |
| Asymptomatic | 12 (44%) | 8 (30%) | 2 (8%) | 5 (18%) | 14 (52%) | 8 (30%) | 4 (15%) | 1 (3%) | 16 (59%) | 8 (30%) | 2 (8%) | 1 (3%) | 12 (44%) | 7 (26%) | 5 (18%) | 3 (12%) |
When comparing COP area, COP velocity and COP anteroposterior and mediolateral displacement between groups, no significant main effect was found for any of the combinations of position and time period of assessment (Pillai’s trace>0.05), suggesting that there are no differences in these parameters between participants with subclinical neck pain and asymptomatic participants (Table 3). In addition, no significant main effect was found for the clinical tests (Pillai’s trace>0.05). Nevertheless, the univariate comparisons were significant for the tandem stance test [F(1,52)=4.05, p=0.049] and for the single leg stance test [F(1,52)=4.40, p=0.041], but not for the step test [F(1,52)=1.97, p=0.167], suggesting that asymptomatic participants were able to maintain static positions for longer than participants with subclinical neck pain. No significant correlations were found between pain characteristics and measurements of postural control (p<0.05).
COP measurement values for neck pain and asymptomatic participants.
| Condition (time) | Variables | Neck pain | n | Asymptomatic | n |
|---|---|---|---|---|---|
| Narrow stance (90 s) | COP AP distance (cm) | 3.86±1.04 | 27 | 3.43±0.67 | 27 |
| COP ML distance (cm) | 3.58±1.08 | 3.39±0.82 | |||
| COP sway area (cm2) | 7.00±3.80 | 6.00±1.90 | |||
| COP velocity (cm/s) | 1.95±0.57 | 1.91±0.77 | |||
| Narrow stance (60 s) | COP AP distance (cm) | 3.71±0.97 | 27 | 3.28±0.71 | 27 |
| COP ML distance (cm) | 3.34±0.84 | 3.13±0.74 | |||
| COP sway area (cm2) | 7.00±3.20 | 6.00±3.6 | |||
| COP velocity (cm/s) | 2.00±0.47 | 2.00±0.83 | |||
| Narrow stance (30 s) | COP AP distance (cm) | 3.37±0.80 | 27 | 3.08±0.65 | 27 |
| COP ML distance (cm) | 2.78±0.65 | 2.69±0.65 | |||
| COP sway area (cm2) | 6.00±3.10 | 6.00±4.20 | |||
| COP velocity (cm/s) | 2.18±0.53 | 2.20±0.96 | |||
| Tandem stance (90 s) | COP AP distance (cm) | 5.21±1.04 | 19a | 5.29±1.05 | 22a |
| COP ML distance (cm) | 6.18±2.33 | 6.87±2.59 | |||
| COP sway area (cm2) | 22.12±18.41 | 18.33±11.04 | |||
| COP velocity (cm/s) | 5.00±1.11 | 5.90±3.54 | |||
| Tandem stance (60 s) | COP AP distance (cm) | 5.33±1.19 | 23a | 5.18±0.93 | 26a |
| COP ML distance (cm) | 5.79±2.04 | 6.51±2.78 | |||
| COP sway area (cm2) | 14.72±5.94 | 18.03±11.32 | |||
| COP velocity (cm/s) | 5.65±1.59 | 6.24±3.96 | |||
| Tandem stance (30 s) | COP AP distance (cm) | 4.89±1.10 | 25a | 4.79±1.04 | 26a |
| COP ML distance (cm) | 5.15±2.49 | 5.39±2.91 | |||
| COP sway area (cm2) | 14.33±8.91 | 14.70±8.99 | |||
| COP velocity (cm/s) | 5.99±1.74 | 6.39±4.06 | |||
| Single leg stance (15 s) | COP AP distance (cm) | 5.33±5.01 | 24a | 4.39±0.71 | 24a |
| COP ML distance (cm) | 6.41±3.25 | 6.48±2.43 | |||
| COP sway area (cm2) | 34.82±39.77 | 33.65±41.90 | |||
| COP velocity (cm/s) | 10.29±2.95 | 10.79±4.69 | |||
| Clinical tests | Tandem (s) | 45.27±31.65 | 27 | 61.49±28.63 | 27 |
| Unilateral (s) | 17.27±12.11 | 27 | 25.68±16.94 | 27 | |
| Steps (number) | 21.81±2.47 | 27 | 21.81±2.46 | 27 |
-
aThe total number of participants is less than 27 as some participants were unable to maintain the position.
4 Discussion
This study compared postural control between university students with chronic idiopathic subclinical neck pain and a matched group of university students without neck pain. University students with subclinical neck pain seem to have more difficulty assuming tandem stance for 90 s and tendency to perform worse in static clinical tests, but no between group differences were found in the COP parameters when postural control was assessed using a force plate. These inconsistent results might suggest that a population with subclinical neck pain show an emerging deficit in postural control. In addition, they might also suggest that measurement procedures (force plate vs. clinical tests) and analysis (COP parameters vs. failure rates) might influence the outcome of the comparison. Furthermore, this study results seem to add to the assumption that postural control deficits are secondary to neck pain.
Previous studies have shown participants with chronic idiopathic neck pain to have postural control deficits when compared to asymptomatic participants [8]. However, studies on young individuals are scarce. Cheng et al. [19] compared postural sway between young individuals with neck pain (mean age±SD=24.7±3.6 years) and an asymptomatic group (mean age±SD=22.1±2.2) and found that the neck pain group showed significantly increased anteroposterior and mediolateral displacement, sway area, and mean velocity of the COP (all p<0.05) compared with the control group. However, neck pain participants in Cheng et al. [19] study were required to have pain for a longer period (6 months) and of higher intensity [≥3 in a 10 cm visual analogue scale (VAS)]. The different neck pain characteristics might explain the contrasting results. This argument finds support in previous studies that reported pain intensity to affect postural control [20], [21], [22]. In particular, increased postural COP velocity [23] and impaired movement detection [20] was only reported for pain intensities greater than 4 in an 11 points numerical rating scale and in a VAS scale, respectively. In our study only two participants reported a VAS score ≥4 and 14 participants reported a VAS score ≤1. In line with our study findings, Cheng et al. [19] did not find significant correlations between postural control measurements and pain characteristics.
It is believed that postural control deficits in neck pain individuals are due to altered cervical somatosensory input and integration to the postural control system which might be a consequence of the effects of pain itself on both nociceptor and mechanoreceptor activity locally, at the spinal cord and within the central nervous system [11], [12]. As reported in the previous paragraph, evidence suggests that only pain of considerable intensity might be able to consistently alter somatosensory input [20], [21], [22]. It is also possible that the area of neck pain distribution in this study participants’ was insufficient to disturb postural control when assessed using COP parameters. For example, Matre et al. [20] reported that experimental pain induced to the ankle disturbed detection of movement only when induced in two muscles simultaneously, at relatively high intensity (VAS=4.5), suggesting that the central nervous system receives sufficient information from other sources when only few muscle spindles are disturbed. In the present study differences between participants with and without neck pain are more apparent for more challenging tasks such as standing in unstable surfaces (foam) or maintaining tandem stance in the force plate for 90 s, suggesting that the nervous system might not be able to compensate the inaccuracy of information originating from the cervical spine with information from other sources in more challenging tasks. Furthermore, it is also possible that changes in the central nervous system that mediate postural control deficits in patients with neck pain are less extensive in subclinical neck pain. For example, gray matter structural changes in low back patients seem to be dependent on pain characteristics (e.g. duration, intensity) [24].
For the force plate measurements, position and time did not interfere with the outcome of the between group comparisons. Previous research has suggested that COP data should be collected for 90 s and measures repeated 3 times in order to increase measurements reliability [15]. However, the present study shows that a considerable percentage of participants with neck pain are unable to assume demanding positions for long periods in line with previous research. Jørgensen et al. [25] found that a higher percentage (81.0%) of participants with neck pain failed to maintain unilateral stance for 30 s compared with participants without neck pain (61%) (p<0.01). Also Field et al. [26] reported that 36.7% of participants with chronic idiopathic neck pain failed a 30 s tandem stance test against 10.0% of asymptomatic participants.
4.1 Limitations and future research
The present findings need to be seen in light of the study limitations. The use of university students while of great relevance to a population where neck pain is highly prevalent and increasing, limits the generalizability of results. The group of participants in this study is heterogeneous in terms of pain characteristics what might have impacted results. For example, Lee et al. [27] classified “subclinical pain” in three different groups according to the frequency of their pain, and found significant between group differences for muscle strength and endurance. Future studies should consider exploring differences in postural control between subgroups of university students with chronic neck pain and further explore the impact of using different procedures for the assessment of postural control. Whether targeting subclinical neck pain has the potential to improve complete recovery rates and decrease the risk of neck pain progression also deserves to be considered in future research. Despite its limitations, this study also has strengths. In particular, participants were excluded if they had pain in any body site other than the neck. This is seldom reported in previous studies [8], however it might influence results. For example, Jørgensen et al. [25] found that the failure rates to maintain a 30 s unilateral stance was higher for participants with concurrent neck and back pain compared to participants with neck as the only complaint (asymptomatic=56.8%, neck pain only=56.3%, neck pain and low back pain=85.7%).
5 Conclusion
University students with subclinical neck pain seem to have more difficulty assuming tandem stance for 90 s and a tendency to perform worse in static clinical tests, but no between group differences were found in the COP parameters when postural control was assessed using a force plate. These inconsistent results might suggest that university students with low intensity and low disability neck pain show an emerging deficit in postural control.
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Authors’ statements
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Research funding: This work received no funding.
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Conflict of interest: Authors declare no conflicts of interest.
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Informed consent: Before data collection, all participants gave their written informed consent.
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Ethical approval: The study was approved by the Ethics Committee of the Social and Health Sciences Department, Faculty of Medicine, Porto, Portugal.
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