The effect of cervical protrusion and retraction on postural stability in healthy adults.
Protrusion and retraction reflect two extreme head postures. This
study aimed to investigate balance performance in these head postures
when compared to the neutral posture. Balance was tested in healthy
males aged 18 to 30 years (n=25) using the equilibrium score (ES)
generated by the EquiTest Balance Manager[TM]. The test re-test
reliability of the ES for six test conditions in three head postures
(neutral, protrusion and retraction) was determined for participants
(subset, n=16) using intraclass correlation coefficients (ICC,3k). The
differences in ES values between the three head postures in the six
testing conditions were examined using repeated measures analysis of
variance (ANOVA). The level of significance was determined to be p [less
than or equal to] 0.05. The results showed ICCs for neutral, protrusion
and retraction ranged from 0.63 (-0.03- 0.87 CI)--0.86 (0.62-0.95 CI)
with good to excellent reliability. No significant difference in ES for
any of the balance tasks was detected. These results indicate that in
healthy adult males the postural stability is unaltered in extreme
simulated head postures even when balance is challenged across a range
of different sensory testing conditions.
Sivayogam A, Johnson GM, Skinner MA (2011): The effect of cervical protrusion and retraction on postural stability in healthy adults. New Zealand Journal of Physiotherapy 39(3) 110-115.
Key words: Balance, Head posture, Postural stability, Protrusion, Retraction
Therapeutics, Physiological (Analysis)
Johnson, Gillian M.
Skinner, Margot A.
|Publication:||Name: New Zealand Journal of Physiotherapy Publisher: New Zealand Society of Physiotherapists Audience: Academic Format: Magazine/Journal Subject: Health Copyright: COPYRIGHT 2011 New Zealand Society of Physiotherapists ISSN: 0303-7193|
|Issue:||Date: Nov, 2011 Source Volume: 39 Source Issue: 3|
In standing, postural stability or balance is defined as the ability to maintain the projected centre of mass (COM) within the limits of the base of support, otherwise referred to as the stability limits (Cook and Woollacott 2001). Stability for stable upright posture and co-ordinated head and eye movement is reliant on afferent information from a combination of the vestibular, visual and somatosensory systems, which converge in several areas of the central nervous system (CNS) (Treleaven 2008). The somatosensory system provides proprioceptive information from the peripheral mechanoreceptors to the CNS in order to maintain balance (Treleaven 2008). In particular, most of the proprioceptive information is supplied from the upper cervical region and is reflected in the abundance of cervical mechanoreceptors, especially from the y-muscle spindles in the deep segmental upper cervical muscles (Boyd-Clark et al 2002, Kulkarni et al 2001). These cervical mechanoreceptors provide the CNS with information regarding the orientation of the head with respect to the rest of the body via direct neurophysiological connections to the vestibular and visual systems (Kulkarni et al 2001, Treleaven et al 2006).
From the literature it is suggested that any change in the afferent input from the cervical spine will impact on postural stability (Kristjansson and Treleaven 2009, Treleaven et al 2006). This theory is supported by observations of healthy individuals experiencing alterations in postural stability when the head is positioned in the fully extended position (Jackson and Epstein 1991). Patients with chronic neck pain have also been shown to exhibit symptoms of dizziness, unsteadiness, visual disturbances and signs of altered postural stability (Kristjansson and Treleaven 2009). Further support for this theory was demonstrated by Kogler et. al. who showed that differences in postural stability in both normal subjects and patients with neck pain were elicited when the head was held in extension but not in the neutral and flexion head postures (Kogler et al 2000).
It is postulated that the observations of altered postural stability seen when the head is positioned in full extension may be due to change in the position of the utricular otolith organs of the vestibular system beyond their normal working range (Jackson and Epstein 1991, Kogler et al 2000). Another theory is that altered postural stability may be due to the compression of vertebral artery causing intermittent ischaemia of the brainstem and/or the inner ear (Kogler et al 2000). Furthermore, it is known that any positional changes in head posture will alter its COM. The increase in load on the core anterior or posterior stabilising neck muscles is dependant on the respective head position, which in turn, determines the alteration in postural stability (Danis et al 1998).
Protrusion and retraction are two extreme head postures in which movements primarily take place in the upper cervical spine with protrusion tending to occur predominantly as an extension movement at the segmental levels of C0-C1 and C1-C2 (Ordway et al 1999). Likewise, retraction takes place principally as a flexion movement at the segmental level C0C1 and C1-C2 (Ordway et al 1999). The lordotic curve of the cervical spine becomes accentuated during head protrusion, and vice versa during head retraction (Penning 1992, Ordway et al 1999).
Hanten et al (1991) found, across the age groups, that males have a greater excursion distance from head retraction to protrusion, and hold their heads farther from the vertical while standing than women do. Hanten et al (1991) concluded that normal head and neck posture is different for men and women and therefore should not be judged by the same standard.
The upper cervical spine is the region attributed with the role of contributing to postural stability due to the abundance of cervical mechanoreceptors in its sub-occipital muscles (BoydClark et al 2002, Kulkarni et al 2001, Peterson 2004). So hypothetically, positioning the spine in protrusion or retraction will challenge this proprioceptive system accordingly. An added advantage of evaluating postural stability with the cervical spine positioned in protrusion or retraction is that the change in head orientation is limited to the sagittal plane during testing conditions with the body in a standing upright position. Such positional changes are likely to minimise the compromise to the vestibular system that has been demonstrated when the head is positioned in extreme extension (Kogler et al 2000).
From a clinical perspective, protrusion is frequently observed as a feature of forward head posture (FHP) and retraction is the therapeutic exercise commonly used to treat FHP (Paris 1990). Further insight into understanding the relationships between extreme head posture and postural stability in healthy subjects will assist our understanding of the relative contribution of the upper cervical spine in maintaining postural stability.
The aim of this study was to use a set of standardised balance tasks to examine the influence of head posture on postural stability in healthy male adults whilst positioned in protrusion and retraction. It was hypothesised that adoption of extreme head postures is associated with concomitant changes in postural stability.
The study was an observational one with repeated measures undertaken for a subset of the participants to ascertain the reliability of the measures undertaken in the experimental set-up.
In this study 25 healthy young male adults, age 25.1 [+ or -] 3.4 years (mean [+ or -] SD), mass 76.0 [+ or -] 8.2 kg, height 175.6 [+ or -] 5.1 cm were recruited from a university student population. Females were excluded from the study as it is known that normal head and neck posture is different for men and women (Hanten et al 1991).
Based on the results from a previous study in which the effects of head position on balance in normal subjects using equilibrium score (ES) were demonstrated (Jackson and Epstein 1991), a sample size of n=25 was deemed to be an adequate number in order to establish a statistical difference. To be included in the study, participants were required to be free from any history of neck pain or neck injury in the 12 months preceding the time of data collection. Exclusion criteria were a history of vertigo, neuro-musculoskeletal problems or spinal pathologies either acquired or congenital, cervicogenic headache, dizziness during head and neck movements, diabetes, vertebrobasilar insufficiency, visual problems and/or balance disturbances. Confirmation of the exclusion criteria was established using a self-reported screening profile after written informed consent was gained from the volunteers. Approval for the study was obtained from the University Human Ethics Committee.
Postural stability was evaluated using a computerized system, NeuroCom[R] Balance Manager[TM]* (NeuroCom[R] International, Inc, Clackamas, Oregon, USA) and accompanying EquiTest software (SMART EquiTest CRS/8 channel EMG/invision/HSSOT (8.4.0)). The design of the system enables the standing platform to be programmed to have a fixed or moving surface and the frame to be programmed to a fixed or moving visual environment, thereby enabling a range of conditions to be included in the software protocols. Postural stability was measured for three different head postures (neutral, protrusion and retraction) using a standardised protocol. The protocol comprised three consecutive trials carried out in each of the six sensory organisation test (SOT) conditions (Figure 1), with a total of 3 x 6 = 18 trials conducted for each respective head position. The trials were all undertaken in the standing position and each participant was instructed to ignore any surface or visual surround motion and to remain upright and as steady as possible.
[FIGURE 1 OMITTED]
The Cervical Range of Motion Device (CROM) (Performance Attainment Associates, Minnesota, USA) was used to establish zero degrees of sagittal rotation at the start of each trial in each of the three head postures in order to quantitatively standardise head postures between individuals. Neutral head posture was registered by instructing the participants to look ahead at an imaginary point on the visual surround in front whilst in a relaxed standing position with arms by the side. (Ordway et al 1999) (Figure 2 A). The posture of head protrusion was established by instructing the participant to undertake maximal forward glide or maximal anterior translation of the head while maintaining the jaw parallel to the ground to maintain zero sagittal rotation (Ordway et al 1999) (Figure 2 B). Head retraction was defined as maximal backward glide or maximal posterior translation of the head whilst also maintaining the jaw parallel to the ground to maintain zero sagittal rotation (Ordway et al 1999) (Figure 2 C).
The participant was instructed to maintain each of the respective head postures for 20 seconds in each of the trials in the six test conditions. A five minute rest period was given between each change of head posture and a five second rest between each trial. The order of the three head postures was randomised, and the six SOT conditions were sequenced according to the standard testing protocol for the NeuroCom[R] Balance Manager[TMs].
[FIGURE 2 OMITTED]
The test protocol and the use of the safety harness was demonstrated to each participant prior to undertaking the test procedure. The participant was then instructed to fasten the safety harness and step onto the force platform, standing upright and facing the visual surround. The foot position was standardised according to manufacturer's instructions by placing each calcaneus or lateral border of the foot on the appropriate "T" line and the medial malleolus to the centre horizontal line of the platform (Jacobson et al 1997).
A randomly selected subset of 16 participants was followed up, in order to determine the reliability of the data obtained for each of the head postures. To overcome any learning experience each test was carried out approximately one week after the first testing session The same procedure was repeated on the second occasion and the order of the testing was randomised as for the first day.
For the purposes of this study, the standardized protocol referred to as the SOT was followed (Jacobson et al 1997). The ES generated by the EquiTest software was used as the main outcome measurement variable to evaluate postural stability based on the previous studies (Jackson and Epstein 1991, Kogler et al 2000). The ES represents the angular difference between the participant's measured anterior to posterior COM displacement to the theoretical sway stability limit of 12.5[degrees] (Jacobson et al 1997) based on the formula,
Equilibriums = [12.5[degrees]--([theta] max - [theta] min)/12.5[degrees]]x100
The ES ranged from 0 to 100%, where 0% represents sway that exceeded the theoretical limits of stability and 100% denotes perfect stability (Jacobson et al 1997).
The anthropometric data were analysed using descriptive statistics to derive the mean, standard deviation (SD) and range. Test re-test reliability for the six test conditions in each of the three head postures was determined using intraclass correlation coefficients (ICC,3k) and their 95% confidence intervals (Shrout and Fleiss 1979). The ICC criteria used in this study to evaluate the ICC values were as follows; > 0.75 = excellent reliability, 0.4 to 0.75 fair to good reliability and < 0.4 = poor reliability (Fleiss 1999).
The mean values obtained from the three consecutive trials within each of the test conditions were used for the analysis. Statistical differences in the mean ES for head postures neutral, protrusion and retraction in each test condition were examined using repeated measures of analysis of variance (ANOVA). A probability level of p [less than or equal to] 0.05 was considered to be statistically significant. All data were analysed using the Statistical Package of Social Science (SPSS) software for Windows (Version 16.0).
Test re-test reliability
The test re-test reliability of the ES for the six balance conditions in three head postures was determined for a subset of the participants (n=16). The mean [+ or -] SD duration between the repeat testing sessions was 6.56 [+ or -] 1.99 days. The ICC values and their 95% confidence intervals are given in Table 1. In accordance with the criteria of Fleiss (1999) the ICC values for neutral, protrusion and retraction head posture showed good to excellent reliability. In the neutral head posture the ICC values ranged from 0.61 to 0.86; in head protrusion from 0.69 to 0.86 and in the retracted head posture from 0.70 to 0.85 (Table 1).
Repeated measures of analysis of variance (ANOVA)
There was no significant difference noted in ES in any support surface conditions (stable and sway-referenced) or visual input conditions (eyes open and eyes closed) between the head postures when the participants were positioned in neutral, protrusion or retraction. The means [+ or -] SD and the p values for ES compared between the different head postures are given in Table 2. The mean values for ES across different head postures (neutral, protrusion and retraction) for each condition (SOT, one to six) are depicted in Figure 3. From Table 2 and Figure 3 it can be seen that conditions five and six were the most challenging in terms of maintaining balance, yet head posture did not impact significantly on the ES scores.
The present study investigated the influence of change in head posture, namely retraction and protrusion on postural stability.
The results demonstrate that in healthy young adult males, no significant difference in the ES was detected when the head was positioned in protrusion or retraction compared with the neutral head posture in any of the six different test conditions for balance.
In previous studies that have identified significant influences of head extension on postural stability, the subjects have been positioned with the cervical spine in full extension. Resultant changes in head orientation with respect to the horizontal ground surface therefore also directly challenge the vestibular systems (Buckley et al 2005, Jackson and Epstein 1991, Kogler et al 2000). However, it has been argued that the relative change in head posture will not impact on postural stability because the otolith signals generated in different head postures are integrated with other balance-related sensory inputs to maintain adequate postural stability (Hamid 1994). Active muscle contraction of the deep neck flexors in retraction and, the sub-occipital muscles in protrusion are known to influence muscle spindle activity within the neck muscles (Peterson 2004). Although it is reasonable, at least in theory, to argue that a change in head on neck posture will alter postural stability, our results showed, the postural control system was independent of head postural change in our testing conditions.
[FIGURE 3 OMITTED]
The vestibular system plays a central role in the maintenance of equilibrium and gaze stability (Kristjansson and Treleaven 2009).
The vestibular sensory organs particularly the utricular otoliths are responsible for linear acceleration and have an optimal working range for head position in order to maintain postural stability (Brandt et al 1981). Any alteration in head posture with respect to horizontal ground surface will affect this optimal working range and hence affect the postural stability (Brandt et al 1981). It is evident from the literature that there is a 40% decrease in utricular sensitivity with 30[degrees] of neck extension and 15% increase in utricular sensitivity with static neck flexion (Brandt et al 1981). However, the results from our study suggest that despite protrusion and retraction of the head, postural stability was able to be maintained. The likely explanation is that in protrusion and retraction the head orientation with respect to the horizontal ground surface was unaltered; hence the vestibular system was in a good state to assist with controlling postural stability and furthermore the compression effect on the vertebral artery that can be identified during head extension is not evident in head protrusion and retraction (Danek 1992, Kogler et al 2000).
In head protrusion, the COM of the head is located anterior to a vertical axis. This will increase the load on posterior core stabilising neck muscles, in particular where the movement is repeated or the static position prolonged (Danis et al 1998). Theoretically this may alter the afferent input from the proprioceptors in the sub-occipital muscles which in turn could alter postural stability (Danis et al 1998). However, in our study no significant alteration in the ES was found when the head protrusion and retraction were maintained for a brief period of time (20 seconds) in a group of healthy young adult males. These results suggest that a healthy individual is able to control the whole body COM amplitude of displacement despite the posture of the head relative to the neck position in protrusion and retraction.
Clinically, the results of this study give an insight to clinicians that postural stability is well maintained by the postural control system during extremes of head postures in healthy individuals, at least in protrusion and retraction. Even in the absence of visual feedback (eyes closed--Conditions 2 and 5), altered sway referenced visual conditions (Conditions 3 and 6) and in sway referenced support surface conditions ( Conditions 4, 5 and 6), we found postural stability was not compromised. These results would suggest any pathology of the cervical spine that in the short term results in a more retracted or forward head posture is unlikely to impact on postural stability. It is also known that individuals with whiplash associated disorders, even at least 3 months post injury, have altered postural stability (Treleaven et al 2005). Whilst such changes are attributed to disturbed afferent input from the cervical receptors (Treleaven et al 2005) the underlying mechanisms for altered stability in individuals with whiplash are not clear. In certain clinical populations, particularly the elderly, extreme forward head postures are accompanied by alteration in spinal curvature (Nemmers and Miller 2008), and in such individuals, postural stability may well be compromised due to a variety of other age-related changes.
The key limitation in our study was that positional changes in protrusion and retraction could not be totally confined to the upper cervical spine as some degree of movement is known to take place in the lower cervical spine as well (Ordway et al 1999). Also in the sway-referenced support surface conditions (Conditions 4, 5 and 6) of the SOT, there were minor accompanying adjustments in head position consequent to the changes in angulation of the foot plate thereby moving the head slightly out of zero degrees of sagittal rotation.
The results of the current study indicate that postural stability is unaltered in extreme head postures maintained for a limited period of time, even when balance is challenged across a range of different testing conditions. These results add to the body of knowledge regarding change in head posture and postural stability at least in healthy young male adults. Positioning the head in full protrusion or retraction with zero degrees of sagittal rotation allows balance to be evaluated without the confounding influence of the vestibular system. This knowledge may be of benefit in the differential diagnosis of balance disorders, particularly those of cervical origin.
It is recommended that research is undertaken in both male and female adults who have a range of cervical disorders (whiplash associated disorders, chronic neck pain, cervicogenic headache) in order to fully understand the contribution the cervical spine makes to functional postural responses associated with the disorders. It will also be important to explore the relationship between postural stability and head posture (protrusion and retraction) in both male and female adults across the age spectrum as the results of this study were confined to healthy young adult males.
* The literature suggests that any change in the afferent input from the cervical spine will impact on postural stability; hence, the study was developed to investigate balance performance in the extreme head postures (protrusion and retraction) when compared to the neutral posture.
* The results indicate that the postural stability is unaltered in extreme simulated head postures of protrusion and retraction even when balance is challenged across a range of different sensory testing conditions.
* The most likely explanation for the results is that in protrusion and retraction the head orientation with respect to the horizontal ground surface was unaltered; and the vestibular system is not comprised in its ability to assist with controlling postural stability.
CONFLICT OF INTEREST STATEMENT
Anand Sivayogam, Gillian Johnson and Margot Skinner report no actual or potential conflict of interest including any financial, personal or other relationships with other people or organisations that could inappropriately influence or bias their work, within three years of beginning the work submitted.
Permission from Neurcom[c] to use their illustration (Figure 1) for the purposes of this manuscript publication is acknowledged.
Dr Gillian Johnson, PhD, MSc, Dip Phty, FNZCP, School of Physiotherapy, University of Otago, PO Box 56, Dunedin 9054, New Zealand. Ph +64 3 4795424, Fax +64 3 4798414, Email firstname.lastname@example.org
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Anand Sivayogam BPT, MPHTY
Gillian M Johnson PhD, MSc, FNZCP
Margot A Skinner PhD, MPhEd, FNZCP
Centre for Physiotherapy Research, School of Physiotherapy, University of Otago, Dunedin, New Zealand
Table 1: Intraclass correlation coefficient (ICC,3k) and 95% confidence intervals in three head postures for each of the six SOT conditions (n=16) SOT Neutral Protrusion Condition 1 0.63 (-0.03-0.87) 0.69 (0.12-0.89) 2 0.79 (0.41-0.92) 0.72 (0.19-0.90) 3 0.77 (0.34-0.92) 0.84 (0.56-0.94) 4 0.86 (0.60-0.95) 0.85 (0.57-0.94) 5 0.61(-0.11-0.86) 0.86 (0.62-0.95) 6 0.83 (0.51-0.94) 0.83 (0.52-0.94) SOT Retraction Condition 1 0.71 (0.17-0.90) 2 0.84 (0.56-0.94) 3 0.71 (0.19-0.90) 4 0.82 (0.49-0.93) 5 0.78 (0.39-0.92) 6 0.69 (0.13-0.89) SOT: Sensory Organisation Test Table 2: Mean ([+ or -]SD) of ES in the neutral, protruded and retracted head positions along with levels of statistical significance (p values) in the six SOT conditions (n=25) SOT Head neutral Head protrusion Conditions ES Mean [+ or -]SD ES Mean [+ or -]SD 1 94.25[+ or -]1.90 94.74[+ or -]1.92 2 91.82[+ or -]2.33 91.69[+ or -]2.24 3 92.88[+ or -]2.39 92.54[+ or -]2.28 4 77.20[+ or -]15.11 74.53[+ or -]16.49 5 55.13[+ or -]16.97 54.32[+ or -]20.12 6 60.14[+ or -]20.11 58.20[+ or -]20.70 SOT Head retraction Conditions Es Mean [+ or -]SD P value 1 94.53[+ or -]2.01 0.604 2 91.68[+ or -]2.19 0.449 3 92.34[+ or -]2.93 0.739 4 76.86[+ or -]13.39 0.394 5 54.28[+ or -]15.29 0.170 6 57.00[+ or -]19.16 0.760 SD: Standard Deviation, ES: Equilibrium Score, SOT: Sensory Organisation Test
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