J Rehabil Med. 2003 (Sep); 35 (5): 229–235 ~ FULL TEXT
P. Michaelson, M. Michaelson, S. Jaric, M .L. Latash,
P. Sjölander, M. Djupsjöbacka
Southern Lapland Research Department,
OBJECTIVE: To evaluate postural performance and head stabilization of patients with chronic neck pain.
DESIGN: A single-blind comparative group study.
SUBJECTS: Patients with work-related chronic neck pain (n = 9), with chronic whiplash associated disorders (n = 9) and healthy subjects (n = 16).
METHODS: During quiet standing in different conditions (e.g. 1 and 2 feet standing, tandem standing, and open and closed eyes) the sway areas and the ability to maintain the postures were measured. The maximal peak-to-peak displacement of the centre of pressure and the head translation were analysed during predictable and unpredictable postural perturbations.
RESULTS: Patients with chronic neck pain, in particular those with whiplash-associated disorders, showed larger sway areas and reduced ability to successfully execute more challenging balance tasks. They also displayed larger sway areas and reduced head stability during perturbations.
CONCLUSION: The results show that disturbances of postural control in chronic neck pain are dependent on the aetiology, and that it is possible to quantify characteristic postural disturbances in different neck pain conditions. It is suggested that the dissimilarities in postural performance are a reflection of different degrees of disturbances of the proprioceptive input to the central nervous system and/or of the central processing of such input.
From the FULL TEXT Article:
Several studies have reported significant disturbances of vertical
posture during various standing and walking conditions in
patients with chronic neck pain. [1-4] There are however
conflicting reports on the characteristics of postural sway during
quiet standing in patients with chronic neck pain. McPartland et
al.  reported similar body sway in patients suffering from
chronic neck pain to that seen in healthy subjects. In contrast,
poor balance and increased sway have been reported in several
studies of patients with chronic musculoskeletal pain of various
aetiologies including patients with whiplash injuries. [1-4, 6] In
studies of the migration of the centre of pressure (CoP),
increased sway has been indicated for patients with cervical
dizziness and vertigo. [7, 8] Patients with chronic neck pain have
been reported to develop significantly larger mean torque during
simple balance test, compared with healthy control subjects, but
not during more challenging tests such as one-foot and tandem
standing.  However, it has also been shown that the ability to
execute different postural tasks is reduced with increasing task
complexity in patients with whiplash associated disorders
One purpose of the current study was to resolve the controversy about the effects of chronic neck pain on postural sway during quiet standing in various conditions.
Two hypotheses were suggested.
Hypothesis-1: patients with chronic neck pain, compared with
healthy controls, are expected to show increased postural sway and reduced ability to successfully complete various balance tests.
Hypothesis-2: the differences in the sway and in the inability
to complete tests are expected to become more pronounced in more challenging postural tasks.
The maintenance of balance during quiet standing is only one
component of everyday tasks faced by the system for postural
stabilization. Equally, or even more, important is the system’s
ability to generate appropriate corrective signals in the presence
of expected and unexpected postural perturbations (for reviews,
see [9, 10]). A disturbed control of back muscles that stabilize the
trunk has been demonstrated in patients with low back pain
during rapid arm movements.  In patients with chronic neck
pain, postural perturbations that cause perturbation of the head
posture with respect to the trunk appear to be particularly
important since they directly influence the painful region. Thus,
an additional hypothesis was suggested. Patients with chronic
neck pain are expected to show increased sway and poor head
stabilization in the presence of postural perturbations (Hypothesis-3).
Cervical muscles in patients with chronic neck pain have been
shown to be morphologically different from non-painful neck muscles (for references, see ). It has also been reported that
vibration of neck muscles in patients with chronic neck pain
induces exaggerated perturbing effects of the vertical posture , indicating that their balance disorders might be related to
altered sensitivity of proprioceptors within the neck muscles. [2, 13] This is supported by animal studies showing that the
output from muscle spindles in neck muscles is significantly
changed during activation of nociceptors in neck muscles and
cervical facet joints. [14, 15] Studies of patients with WAD
indicate that they have more severe balance disturbances
compared with patients with work-related chronic neck pain [4, 6], suggesting that a whiplash trauma imposes disturbances
of the proprioceptive output from the neck region, and/or causes
motor control dysfunction that is not present in chronic neck
pain without a traumatic origin. To test possible effects of
aetiology on sway characteristics and on the ability successfully
execute various balance tests, we investigated 2 groups of
patients, those who suffered from work-related chronic neck
pain and those who had developed chronic neck pain following a
Despite the fact that a rather small number of patients and
subjects were included in the present study, a number of
significant differences in balance and head stabilization were
observed between patients with chronic neck pain and healthy
subjects. These differences indicated a strong dependence on the
aetiology of the neck pain, i.e. larger differences between the
patients with whiplash associated disorder (WAD) and the control subjects than between the
patients with work-related pain (WRP) and the control group. The design of the
study allow us to address the hypotheses on postural sway and
head stabilization in chronic neck pain (see Introduction), and
the results indicate that it is possible to quantify characteristic
postural disturbances in neck pain of various origin.
Changes in postural stability with task complexity
Making the task of vertical standing more challenging, by
closing the eyes and by moving from standing in Romberg
position to Tandem standing and one foot standing, resulted in
increased sway and decreased ability to complete the task across
all subjects. The differences between the WAD group and the
other 2 groups increased with increasing difficulty of the
postural task (Table II). The WAD patients were particularly
likely to fail in the more challenging tasks. It seems conceivable
that the rate of success in a particular postural task is related to
the ability to keep the postural sway below a given threshold.
These observations support both Hypothesis-1 and 2, and are in
line with earlier reports (4, 6). Similar effects have also been
described on patients with chronic low back pain (24).
Average sway area (in cm2) with standard deviation (SD)
in different conditions of quite standing, separately shown for the
control subjects, patients with work-related pain (WRP) and patients
with whiplash associated disorders (WAD).
F- and p-values of one-way ANOVAs
Postural adjustments to perturbations
The most pronounced differences between the WAD and the
WRP group were observed when external perturbations were
added to a quite standing task. We used 2 tests to assess how
well the subjects maintained vertical posture and head stability
during self-imposed mechanical perturbations. Voluntary bilateral
arm movements have been used in many studies of postural
adjustments to perturbations. [25, 26] Arm movement imposes
torque on the trunk and other body segments, and is accompanied
by postural corrections leading to shifts of the centre of
mass.  The impact of such perturbations seems to be related
to the speed of arm movement. In our study, the control subjects
moved their arms faster than both the WAD and WRP patients,
although the differences were significant only in comparison to
the WRP group (Table III). Hence, one would expect transient
mechanical perturbations associated with the movement to be
larger in the control subjects. However, the amplitude of the
anterioposterior CoP-displacement immediately following the
perturbation was larger in patients with WAD, in spite of the
smaller perturbations (Table III).
Range of movement (ROMSH) and average peak velocity (VELSH)
in the Arm-Lift test, and average anterioposterior peak-to-peak shift of CoP
(CoPpp) during perturbation tests (Arm-Lift, Drop-Self and Drop-Exp tests).
Mean values with standard deviation (SD) are separately shown for the
control subjects, patients with work-related pain (WRP) and patients
with whiplash associated disorders (WAD).
Fand p-values of one-way ANOVAs
The load-release tests, which were independent of the
subjects’ ability to perform fast movements , also showed
a significant difference between the groups. In both the self-triggered
and experimenter-triggered load release tests, the
patients with WAD demonstrated increased displacement of the
CoP immediately following the perturbation, as compared to the
control subjects and the patients with WRP. Taken together,
these findings suggest a major impairment of the ability to maintain body posture during perturbations in patients with WAD, which is in support of Hypothesis-3.
Chronic neck pain and head stabilization
Fast arm movements and load releases unavoidably induce
perturbations on both the vertical posture and the head stability,
because of the mechanical coupling of different body segments.
Our observations of head translation occurring immediately
after a perturbation revealed that the patients with WAD
demonstrated larger neck motion as a result of both selftriggered
and experimenter-triggered perturbations (Figure 3),
which is in agreement with Hypothesis-3. The larger neck
motion could be a result of an inadequate ability to co-ordinate
the neck muscles, potentially leading to larger perturbing torques acting on the head. The latter is supported by Jull , who showed increased activity in superficial neck muscles
in patients with in combination with a decreased ability to
perform controlled neck flexion, implying a disturbed control of
the neck muscles involved in neck stabilization.
Average anterioposterior peak-to-peak head translation (Headpp)
and standard deviation, separately shown for the control, whiplash associated
disorders group (WAD) and work-related pain group (WRP), in a rapid arm lift
test (Arm-Lift), in load-drop tests initiated by the subjects (Drop-Self) and
by the experimenter (Drop-Exp). In all tests the subjects were standing in
the Romberg position with closed eyes
(control, n = 16; WRP, n = 9; WAD,
n = 6).
Possible explanations for the observed differences between
WAD and WRP
There is a risk that the differences found between WAD and
WRP to some extent could have been due to a selection bias
inferred by the small number of patients included in the study.
Although a multifactorial inclusion/exclusion procedure was
applied, it can not be ruled out that partly different results would
have occurred if other criteria had been used. Yet, since all
patients showed typical characteristics of chronic neck pain
syndromes, and outliers were identified through stringent
criteria, it seems more likely that the observed differences had
Motor control disturbances found in patients with chronic
neck pain are thought to be due largely to changed proprioceptive
signals from neck muscles (e.g. [2, 13]). It has been shown
that activation of nociceptors in muscles and joints excite
fusimotor neurones, which alter the sensitivity of the muscle
spindle afferents. [14, 15, 29–30] A disturbed sensitivity of the
fusimotor system could be triggered by long-lasting exposure to
awkward postures or static/repetitive work, such as in WRP, or
by a massive, transient afferent input onto the fusimotor
neurones from nociceptors and mechanoreceptors in muscles,
tendons, ligaments and joint capsules [15, 29], which is likely to
occur during a whiplash trauma. Thus, the differences in
postural performance found between the 2 patient groups might
reflect different degrees of disturbance of the fusimotor system,
causing differences in the proprioceptive precision [29, 30],
perhaps in combination with permanent damage to cervical soft
tissue, the spinal cord and/or the brain stem in some patients
with WAD. [31-33]
Another possibility, which does not exclude effects of
changed proprioception, is that the observed differences to some extent may be related to adaptive adjustments of postural
control strategies developed by patients with chronic neck pain.
Adaptive changes of posture and movement have been reported
in subjects with atypical movement patterns (for review, see ).
The slower arm movements of the patients with neck pain,
together with their increased head motion and sway area during
perturbations, could be a reflection of control strategies adopted
to minimize the risk of repeated neck injuries. However, it
remains to be elucidated whether or not motor control strategies
with a protective purpose are developed in chronic neck pain.
Our experiments have confirmed earlier reports on increased
postural sway and decreased ability to maintain more demanding
standing posture in patients with chronic neck pain, and, in
addition, showed major differences in vertical postural and head
stability to perturbations. The differences between the patients
with WAD and those suffering from WRP suggest that deficits in
proprioception and motor control, rather than the chronic pain
itself, may be the main factors defining the clinical picture in
different chronic neck pain conditions. Consequently, qualitative
and quantitative measures of postural performance and head
stabilization could be used to increase the precision and
efficiency of diagnosis and rehabilitation of chronic neck pain
of different aetiology.