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The Effect of Chronic Pain Intensity on the Stability Limits in Patients With Low Back Pain
THE EFFECT OF CHRONIC PAIN INTENSITY ON THE STABILITY LIMITS IN PATIENTS WITH LOW BACK PAIN Tomasz Sipko, PhD, a and Michal Kuczyn´ski, PhD, DSc b /
ABSTRACT Objective: The purpose of this study was to evaluate if the intensity of recurrent chronic pain would modify postural performance in reaching the functional limits of stability (LOS) in chronic low back pain (CLBP) patients. Methods: Three groups of subjects were investigated. Healthy persons comprised the asymptomatic group (n = 32) while CLBP patients (n = 36) were divided into 2 subgroups, according to the reported intensity of resting pain on a numerical rating scale: patients with low (LP) and high pain (HP) levels. The maximal displacement of the center of pressure (COP) indexing the LOS magnitude and the COP mean velocity indexing the performance in reaching LOS were calculated on a Kistler force plate during forward and backward voluntary body lean with eyes open (EO) or closed (EC). Results: The forward LOS was lower in both the LP (P b .01) and HP (P b .01) subgroups than in the asymptomatic under EO and EC conditions, while no differences between the LP and HP groups were found. The backward LOS was lower in the HP group than in asymptomatic but only with EC (P = .01). Eye closure caused an increase in forward (P = .02) and backward (P = .001) COP velocity in the LP group and forward COP velocity in the asymptomatic (P = .04) only. With EC, the only intergroup difference was lower forward COP velocity in the HP than LP group (P = .04). Conclusion: Subjects with CLBP had reduced forward LOS regardless the pain level. However, the higher level of pain was associated with slower execution of voluntary leaning tasks, with EC only. (J Manipulative Physiol Ther 2013;36:612-618) Key Indexing Terms: Balance; Postural Equilibrium; Posture; Range of Motion; Articular
he functional limits of stability (LOS) are defined as the area which is much smaller than the base of support and over which individuals can move their center of mass and maintain equilibrium without adjusting their base of support. 1 The center of pressure (COP) as the center of distribution of the total force applied to the base of support regulates the position a passive variable—the center of mass. 2 Few studies have analyzed dynamic balance in chronic low back pain (CLBP) patients by assessing the limits of postural stability. 3-6 There is evidence based on an experiment with a movable platform that the alteration of
T
a
PT, Senior lecture, Faculty of Physiotherapy, Academy of Physical Education, Wrocław, Poland. b Professor, Academy of Physical Education, Wrocław and University of Technology, Opole, Poland. Submit requests for reprints to: Tomasz Sipko, PhD, PT, Senior lecture, Faculty of Physiotherapy, Academy of Physical Education, Wrocław, Poland (e-mail: [email protected]). Paper submitted February 8, 2013; in revised form August 27, 2013; accepted August 27, 2013. 0161-4754/$36.00 Copyright © 2013 by National University of Health Sciences. http://dx.doi.org/10.1016/j.jmpt.2013.08.005
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neuromuscular control found in patients with CLBP may contribute to their LOS, making these patients more at risk for loss of equilibrium. 4 However, falling is not the largest problem related to decreased balance in individuals with CLBP, but a modified postural strategy which might sustain chronic symptoms and alter active daily living. Stiff postural strategy in automatic postural responses was especially evidenced in the postural tasks of CLBP patients, 6,7 as well as decreased voluntary forward lean 5 and even loss of variability in strategy selection. 8 CLBP patients reduced hip control strategy during quiet standing, 9 favoring the ankle strategy, 4,10 which was not effective in demanding conditions. 3,5 Most of the studies 3,4,6,7 investigated automatic postural responses in dynamic balance of CLBP, but only Mientjes and Frank 5 estimated LOS in voluntary body lean, and only in the forward direction. All of these studies 3-7 excluded CLBP patients with increased pain; however, a characteristic CLBP symptom is recurrent pain with differing locations and intensity. It is known that in CLBP patients standing quietly, velocity of COP increased linearly with an increase in perceived pain intensity greater than 4 on a numerical pain scale, 11 but there is no explanation as to the effect intensity of recurrent chronic pain on dynamic balance.
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The aim of this study was to examine the forward and backward limits of stability during voluntary body lean, and the COP velocity as a measure of reaching LOS in subjects with low pain levels (LP) and with high pain levels (HP). The purpose of this study was to evaluate if the intensity of chronic pain for CLBP patients would modify the postural performance in the voluntary LOS tasks. We hypothesized that pain intensity would modify performance in the following ways: (1) decrease the LOS magnitude and (2) decrease the velocity of task execution. It was expected that disturbances in the postural performance of the HP group were more likely to occur when the subjects’ eyes were closed because of deficits in the sensorimotor processing associated with the pain level.
METHODS Subjects Thirty-six people with CLBP aged 30 to 65 years, undergoing a non-invasive treatment in Cieplice Zdrój health resort participated in the study (for details see Ref. 12). Thirty-two healthy persons served as an asymptomatic group. All participants were screened by physiotherapist and provided completed pain and status health questionnaire packages. The inclusion criteria encompassed patients with low back pain due to a herniated disc confirmed by magnetic resonance imaging who have had persistent or recurrent chronic pain for at least 3 months. The exclusion criteria included the presence of neurological diseases, orthopedic conditions, stenosis of the spinal canal, and surgical treatment of the herniated disc. The level of resting pain on the day of investigation was determined on the basis of a numerical rating scale of 0 to 10 (NRS) by marking the current intensity of pain. 13 The CLBP group was stratified in terms of pain intensity into a subgroup with low pain level (LP group; NRS = 0-3) and a subgroup with high pain level (HP group; NRS = 4-10). A cut-off point was found in the results of Corbeil et al, 14 as weak pain intensity minimally affected control of posture but the moderate and extreme pain intensities gradually deteriorated the postural system. A convenience sample of 32 asymptomatic volunteers, who were not involved in any regular physical activity, was recruited from the Faculty of Physiotherapy. The inclusion criteria encompassed asymptomatic subjects (NRS = 0), with no history of CLBP, aged 30–65. The exclusion criteria were as follows: neurological diseases, orthopedic problems of spine, hip, knee or foot, low back pain at the time of testing, and bad physical and mental state on the day of the study. All subjects gave informed consent prior to their participation in the study, and the procedures were approved by the Academy of Physical Education Bioethics Committee.
Sipko and Kuczyński Stability Limits in Chronic Back Pain Patients
Postural Balance Examination The limits of stability were assessed according to a simplified protocol based on the traditional evaluation of LOS. 1 Subjects performed two 20-second trials on a hard surface, separated by 1-minute breaks. They performed the first trial with their eyes open (EO) and the second with their eyes closed (EC). The subjects were instructed to stand barefoot on a Kistler force plate (type 9286; http://www.kistler.com), with their feet parallel and 5 cm apart, and their gaze focused on a marker placed at eye level at a distance of 1.5 m. A practice trial was allowed prior to the test to ensure that the subjects were able to complete the task. They were asked to maintain full contact between the soles of their feet and the force plate during a voluntary body leaning. The maximal displacement of the center of pressure (COP) was calculated duringa forward, followed by a backward, voluntary body lean, with EO or EC, relative to the baseline data computed as an average COP value from the first five seconds of standing in the resting position. The sampling rate was 100 Hz and the sampling time was 20 seconds, resulting in 2000 sampling segments for each recorded COP time series in the sagittal plane. The dependent variables included forward (LOS.For) and backward (LOS.Back) limits of stability in the sagittal plane and the mean velocity of the COP as a measure of reaching LOS in forward (LOS.VS.for) and backward directions (LOS.VS.back). For the between-subjects comparison, the COP data were normalized according to the subjects’ foot lengths: normalized LOS = (LOS/foot length) × 100. 4,5
Statistical Methods Biometric data were compared statistically using oneway analysis of variance, followed by the Tukey post hoc test. As the postural measures had non-normal distribution, the results were presented as medians, mean, and SD. The statistical significance of the differences between groups was assessed with the Mann-Whitney U test. For the intragroup comparisons, to evaluate the effect of vision, a Wilcoxon matched pair test was applied. Spearman rank correlation was used to evaluate the relationships between LOS parameters and intensity of pain. The level of significance was set at P b .05. The effect size measure of Cohen’s d was obtained and used to quantify the differences in outcome variables.
RESULTS Demographic Data Both subgroups were alike in terms of age and height except for body weight and pain intensity. Asymptomatic group had significantly lesser weight than LP group (P = .0004) and lesser body mass index (BMI) than both subgroups (P b .05). The
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Table 1. Characteristics of participants Asymptomatic group n = 32, mean (SD) Age (y) Weight (kg) Height (m) BMI NRS (0-10)
44.6(8.6) 68.4(13.8) 1.7(0.09) 23.46(2.72) 0(0)
LP group n = 16, mean (SD)
HP group n = 20, mean (SD)
Tukey’s post hoc test Asym-LP/Asym-HP/LP-HP
51.4(8.0) 85.7(16.0) 1.75(0.01) 27.6(3.7) 1.47(1.23)
50.2(7.8) 74.1(11.5) 1.69(0.09) 26.1(4.5) 5.31(1.37)
0.02a/0.051/0.90 0.0004a/0.31/0.03a 0.14/0.92/0.10 0.0008a/0.026a/0.42 0.0001a
BMI, body mass index; HP, high pain; LP, low pain; NRS, numerical rating scale; SD, standard deviation. Significance of differences: aP b .05. NRS = t test.
experimental groups were overweight while the asymptomatic group was not (Table 1).
Secondly, eye closure resulted in altered performance of leaning tasks in the LP and the asymptomatic group as demonstrated by higher values of the velocity of COP. The higher velocity of COP might represent the generation of a Limits of Stability Normalized data were presented in Figures 1a and b, and 2a normal active sway to find a stable solution to the postural 2 and b, and Table 2. Cohen’s d effect size was shown in Table 3. challenge. The lack of visual information makes subjects 16 The forward LOS were significantly lower in both the LP (P b rely more on proprioception. In this context, the lack of .01) and HP (P = .004) subgroups than in the asymptomatic increase in the COP velocity in the HP group may account group under EO and EC conditions, while no differences for deficits in sensorimotor processing and suggests that between the LP and HP groups were found (Fig 1a), accounting there may be some pain threshold level beyond which EC for the overall effect of chronic back pain on LOS regardless of deficits are more common. Thirdly, there were no differences in the LOS between the pain level. Eye closure decreased the forward LOS in the LP (P = .004) and in the asymptomatic group (P = .04), with no the LP and HP groups. Also, no correlation was found between the level of pain and LOS in either group. This effect on the HP group (Fig 1a). The only intergroup difference in the backward LOS was the indicated no direct relationship between the resting pain lower LOS value in the HP group compared to the asymptomatic intensity and the magnitude of LOS in the CLBP. Our LP and HP groups presented a similar LOS group (P = .01) when performing with EC (Fig 1b), suggesting that high pain restricted backward LOS under EC conditions only. magnitude which reflected no effect of resting pain intensity The only intergroup difference in forward velocity of COP on postural stability limits. However, the lower LOS in showed lower values in the HP than the LP group (P = .04) patients compared with asymptomatic subjects and the with EC (Fig 2a). Eye closure caused a significant increase in differences in performance during EC leaning tasks forward (P = .02) and backward (P = .001) velocity of COP in between the LP and HP groups account for two reasons the LP group and forward velocity of COP in the for the underlying postural disorders. The former comparison seems to suggest reduced limits of stability in CLBP, asymptomatic group only (P = .04) (Fig 2a and b). There was no significant relationship between pain while the latter may imply a deficit in sensorimotor intensity and magnitude of LOS under EO and EC conditions processing. Several factors may contribute to reduced forward LOS in CLBP, including stiffening postural in any group (Table 4). strategy, 6,7 an alteration in the neuromuscular synergy of the trunk extensor muscles in standing reach, 17 and DISCUSSION sensitized and painful trunk extensor muscles. 12,18 In 19 6 The aim of this study was to examine the limits of addition, young persons with LBP and chronic LBP stability in CLBP patients during voluntary body leaning as have adapted forward inclined posture and positioned their well as the velocity of COP. Three novel findings with mean COP more to the anterior when vision was occluded. This corresponded well with decreased forward LOS in our clinical relevance are presented. Firstly, both subgroups of patients had a markedly lower CLBP subgroups under EC conditions. In contrast to the forward LOS than the asymptomatic group subjects while the forward direction, stability limits in the backward direction restricted backward LOS was found only in the HP in trials were not significantly different between asymptomatic with EC. A decrease in stability limits causes an increase in group and both CLBP subgroups under EO condition. the margin of error, which has been suggested to provide the These results could be due to “floor” effect on maximum central nervous system with more time for making the backward inclination common to both the asymptomatic necessary postural corrections. 15 In the same line, Popa et al 6 group and CLBP due to biomechanical constraints to indicated that in CLBP the process of internal estimation must leaning backward. Similar findings were reported by 20 take into account and compensate for a loss of sensory Mancini et al in an investigation of the functional LOS in Parkinson’s disease. precision during postural tasks.
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Sipko and Kuczyński Stability Limits in Chronic Back Pain Patients
a) Limit of stability in forward
a) Velocity of COP in forward
70
50 a
a
a
40 30 20
a a
10
LP
a
HP LOS.For.eo
Normalized velocity of COP
Normalized range
60 50
45
a
Asymptomatic
40 35 30 25
a
20 15 10
LOS.For.ec
a
5
b) Limit of stability in backward
LP
HP
60
LOS.VS.For.eo
Asymptomatic LOS.VS.For.ec
b) Velocity of COP in backward
50
40
40
30
20
10
0
a LP
HP LOS.Back.eo
Asymptomatic LOS.Back.ec
Fig 1. Normalized values median, lower, and upper quartile 25%75%, min-max of limit of stability in forward (a) (LOS.For); backward (b) (LOS.Back) in the LP (low pain), HP (high pain), and asymptomatic groups; trials with EO and EC; and significance of differences ( aP b .05).
The deficit in sensorimotor processing was revealed in the HP subgroup only during performances with EC, indicating that proprioception in individuals with high pain might be disturbed. Our observation supports the findings of other researchers who have claimed that postural deterioration occurs due to lumbar proprioceptive deficits caused by pain. 3,16,21,22 Indeed, in a study by Mientjes and Frank, 5 the forward leaning task with eyes closed showed the largest differences between CLBP and the control group. The reduction in stability limits during EC trials suggests that these limits are not purely biomechanical, but are affected by the availability of sensory information. 23 In the LP and the asymptomatic group, velocity increased with EC as the subjects relied mainly on proprioceptive input. This was most likely a normal postural strategy during voluntary body
Normalized Velocity of COP
Normalized range
45
35
a
30 25 20 15 10 5 LP
HP LOS.VS.back.eo
Asymptomatic LOS.VS.back.ec
Fig 2. Normalized values median, coefficient 25%-75%, min-max of velocity of COP in forward (a) (LOS.VS.For) and backward (b) (LOS.VS.Back) in the LP (low pain), HP (high pain), and asymptomatic groups; trials with EO and EC; and significance of differences ( aP b .05). leaning to reach the LOS. However, the HP group did not follow the same pattern of postural compensation, providing further evidence of differences in motor control strategies for people with low and high pain. This might be explained by the pain-adaptation model, 24 as the control strategy reduced velocity and voluntary movement excursions to prevent provocation of pain. 25 Clear evidence in support of this model was found in a study by Graven-Nielsen et al 26 in which muscle pain seems to cause the general protection of painful muscles during both static and dynamic contractions.
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Table 2. Mean, SD, and median LOS and velocity of COP (VS) in forward (LOS.For) and backward (LOS.Back) in the asymptomatic, LP and HP groups; trials with EO and EC Asymptomatic n = 32
Los.For.eo Los.Back.eo VS.For.eo VS.Back.eo Los.For.ec Los.Back.ec VS.For.ec VS.Back.ec
LP group n = 16
HP group n = 20
Mean(SD)
Median
Mean(SD)
Median
Mean(SD)
Median
40.64(7.45) 27.02(5.58) 15.39(5.16) 17.32(4.94) 37.80(6.78) 27.08(5.14) 18.91(9.12) 18.60(4.5)
40.27 27.86 15.07 16.63 38.91 26.74 16.71 18.33
34.49(6.96) 24.90(7.99) 14.75(3.30) 14.82(4.22) 29.39(7.81) 27.07(11.30) 17.55(3.06) 20.16(6.68)
35.14 25.84 14.36 14.22 31.29 25.32 17.38 18.50
31.92(9.86) 25.07(8.37) 14.16(7.63) 15.66(4.09) 31.73(5.55) 22.57(9.53) 15.36(4.80) 18.82(6.33)
35.30 23.74 11.90 16.21 32.89 22.72 14.48 17.46
HP, high pain; Los.Back.ec, backward limit of stability with eyes closed; Los.Back.eo, backward limit of stability with eyes open; Los.For.ec, forward limit of stability with eyes closed; Los.For.eo, forward limit of stability with eyes open; LP, low pain; NRS, numerical rating scale; SD, standard deviation; VS.Back.ec, backward velocity of COP with eyes closed; VS.Back.eo, backward velocity of COP with eyes open; VS.For.ec, forward velocity of COP with eyes closed; VS.For.eo, forward velocity of COP with eyes open. Note: the magnitude of peak displacement has been normalized and therefore has no units.
Table 3. Effect sizes for variables of interest Cohen’s d effect size Asymptomatic-LP Los.For.eo Los.Back.eo VS.For.eo VS.Back.eo Los.For.ec Los.Back.ec VS.For.ec VS.Back.ec
0.85 a 0.30 0.14 0.54 1.14 a 0.001 0.19 − 0.27
Asymptomatic-HP 1.20 a 0.27 0.18 0.36 0.97 a 0.58 0.48 − 0.04
LP-HP 0.30 − 0.02 0.10 − 0.20 − 0.34 0.43 0.54 0.20
HP, high pain; Los.Back.ec, backward limit of stability with eyes closed; Los.Back.eo, backward limit of stability with eyes open; Los.For.ec, forward limit of stability with eyes closed; Los.For.eo, forward limit of stability with eyes open; LP, low pain; NRS, numerical rating scale; VS.Back.ec, backward velocity of COP with eyes closed; VS.Back.eo, backward velocity of COP with eyes open; VS.For.ec, forward velocity of COP with eyes closed; VS.For.eo, forward velocity of COP with eyes open. Interpretation: effect sizes small, d = 0 .2, medium, d = 0 .5 and large. a d = 0.8.
In this context, chronic and high intensity pain was a significant factor for differing postural performance in our CLBP. The similar context was discovered in a study by Tétreau et al, 27 expectations of low or high pain during trunk movement translate into different neuromuscular changes in trunk muscles, although pain-related adaptations may be appropriate in acute LBP, their persistence longer may contribute to pain chronicity.
Limitations The subjective nature of pain perception and therefore of pain rating may have influenced the results of both groups. The cut-off point on the NRS, 3 or 4, in the LP/HP group is of concern 11,14 particularly because of the possible overlap between the characteristics of subjects who were close to this cut-off point. The confounding effect of different BMIs cannot be excluded either. Although limited anterior
Table 4. The Spearman correlations between LOS, velocity of COP (VS) and the level of rest pain (NRS) in forward (LOS.For) and backward (LOS.Back) in the asymptomatic, LP, and HP groups, trials with EO and EC Asymptomatic n = 32 NRS Los.For.eo Los.For.ec Los.Back.eo Los.Back.ec VS.For.eo VS.For.ec VS.Back.eo VS.Back.ec
− 0.28 0.16 0.30 0.04 0.06 − 0.14 0.04 0.06
LP group n = 16 NRS 0.06 0.17 − 0.04 0.01 0.36 − 0.24 − 0.24 − 0.07
HP group n = 20 NRS − 0.26 − 0.31 − 0.05 − 0.01 − 0.26 − 0.31 − 0.30 − 0.12
HP, high pain; Los.Back.ec, backward limit of stability with eyes closed; Los.Back.eo, backward limit of stability with eyes open; Los.For.ec, forward limit of stability with eyes closed; Los.For.eo, forward limit of stability with eyes open; LP, low pain; NRS, numerical rating scale; VS.Back.ec, backward velocity of COP with eyes closed; VS.Back.eo, backward velocity of COP with eyes closed; VS.For.ec, forward velocity of COP with eyes closed; VS.For.eo, forward velocity of COP with eyes open. Significance of differences: P b .05.
LOS have been reported in individuals with grade III obesity only, 28 the presence of back pain could give rise to this deficit in less obese subjects as well. In a study by Cruz-Gómez et al 29 obese subjects showed a longer length and a larger area of sway than lean and overweight subjects, but in epidemiology study (n = 829 791 adolescents) higher BMI was significantly associated with low back pain in overweight as well in obese, both male and female. 30 Thus, some interaction between overweight and pain may have had an effect and matching groups on BMI may be useful in future studies. Also, the documented varieties of postural protective strategies used by CLBP patients may mask their actual behavior with simple statistics, unless attempts are made to investigate homogenous groups. Further, we did not formally estimate the sample size in this study which could have led to a type II error, that is, with a larger sample the differences in
Journal of Manipulative and Physiological Therapeutics Volume 36, Number 9
LOS between the two patients groups might become significant. Finally, there is little consensus about the criteria for clinical significance in posturography. Depending on the subjects’ characteristic and the purpose of the study, even small improvement in postural control may significantly contribute to the actual functional performance being examined. On the other hand, relatively large changes in postural indices may be not sufficient to differentiate the overall inter-group performance. In view of these limitations this study should be considered a preliminary study being helpful for designing future research.
Sipko and Kuczyński Stability Limits in Chronic Back Pain Patients
Data collection/processing (responsible for experiments, patient management, organization, or reporting data): TS, KM. Analysis/interpretation (responsible for statistical analysis, evaluation, and presentation of the results): TS, KM. Literature search (performed the literature search): TS, KM. Writing (responsible for writing a substantive part of the manuscript): TS, KM. Critical review (revised manuscript for intellectual content, this does not relate to spelling and grammar checking): TS, KM.
CONCLUSIONS The results of this study suggest differences in the postural performance of patients with low and high levels of pain. However, this apparently protective strategy suffers from its own, probably disadvantageous, effects on the limits of stability as well as its consequences for motor and postural control. Subjects with CLBP had reduced forward LOS regardless the pain level. However, the higher level of pain was associated with slower execution of voluntary leaning tasks, with EC only. The recurrent episodes of low back pain with higher levels of pain modify postural performance in dynamic balance. Alterations in postural performance in dynamic balance might be one of the factors contributing to pain chronicity in CLBP.
Practical Applications • Chronic and high intensity pain was a significant factor for differing postural performance in CLBP patients. • Alterations in postural performance in dynamic balance might be one of the factors contributing to pain chronicity.
FUNDING SOURCES AND POTENTIAL CONFLICTS OF INTEREST No funding sources or conflicts of interest were reported for this study.
CONTRIBUTORSHIP INFORMATION Concept development (provided idea for the research): TS, KM. Design (planned the methods to generate the results): TS, KM. Supervision (provided oversight, responsible for organization and implementation, writing of the manuscript): TS, KM.
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23. Forth KE, Fiedler MJ, Paloski WH. Estimating functional stability boundaries for bipedal stance. Gait Posture 2011;33: 715-7. 24. Lund JP, Donga R, Widmer CG, Stohler CS. The painadaptation model: a discussion of the relationship between chronic musculoskeletal pain and motor activity. Can J Physiol Pharmacol 1991;69:683-94. 25. Van Dieën JH, Selen LP, Cholewicki J. Trunk muscle activation in low-back pain patients, an analysis of the literature. J Electromyogr Kinesiol 2003;13:333-51. 26. Graven-Nielsen T, Svensson P, Arendt-Nielsen L. Effects of experimental muscle pain on muscle activity and co-ordination during static and dynamic motor function. Electroencephalogr Clin Neurophysiol 1997; 105:156-64. 27. Tétreau C, Dubois JD, Piché M, Descarreaux M. Modulation of pain-induced neuromuscular trunk responses by pain expectations: a single group study. J Manipulative Physiol Ther 2012;35:636-44, http://dx.doi.org/10.1016/j.jmpt.2012. 06.008. 28. Błaszczyk JW, Cieślinska-Swider J, Plewa M, ZahorskaMarkiewicz B, Markiewicz A. Effects of excessive body weight on postural control. J Biomech 2009;19:1295-300. 29. Cruz-Gómez NS, Plascencia G, Villanueva-Padrón LA, Jáuregui-Renaud K. Influence of obesity and gender on the postural stability during upright stance. Obes Facts 2011;4: 212-7, http://dx.doi.org/10.1159/000329408. 30. Hershkovich O, Friedlander A, Gordon B, et al. Associations of body mass index and body height with low back pain in 829,791 adolescents. Am J Epidemiol 2013;178:603-9.
ABSTRACT Objective: The purpose of this study was to evaluate if the intensity of recurrent chronic pain would modify postural performance in reaching the functional limits of stability (LOS) in chronic low back pain (CLBP) patients. Methods: Three groups of subjects were investigated. Healthy persons comprised the asymptomatic group (n = 32) while CLBP patients (n = 36) were divided into 2 subgroups, according to the reported intensity of resting pain on a numerical rating scale: patients with low (LP) and high pain (HP) levels. The maximal displacement of the center of pressure (COP) indexing the LOS magnitude and the COP mean velocity indexing the performance in reaching LOS were calculated on a Kistler force plate during forward and backward voluntary body lean with eyes open (EO) or closed (EC). Results: The forward LOS was lower in both the LP (P b .01) and HP (P b .01) subgroups than in the asymptomatic under EO and EC conditions, while no differences between the LP and HP groups were found. The backward LOS was lower in the HP group than in asymptomatic but only with EC (P = .01). Eye closure caused an increase in forward (P = .02) and backward (P = .001) COP velocity in the LP group and forward COP velocity in the asymptomatic (P = .04) only. With EC, the only intergroup difference was lower forward COP velocity in the HP than LP group (P = .04). Conclusion: Subjects with CLBP had reduced forward LOS regardless the pain level. However, the higher level of pain was associated with slower execution of voluntary leaning tasks, with EC only. (J Manipulative Physiol Ther 2013;36:612-618) Key Indexing Terms: Balance; Postural Equilibrium; Posture; Range of Motion; Articular
he functional limits of stability (LOS) are defined as the area which is much smaller than the base of support and over which individuals can move their center of mass and maintain equilibrium without adjusting their base of support. 1 The center of pressure (COP) as the center of distribution of the total force applied to the base of support regulates the position a passive variable—the center of mass. 2 Few studies have analyzed dynamic balance in chronic low back pain (CLBP) patients by assessing the limits of postural stability. 3-6 There is evidence based on an experiment with a movable platform that the alteration of
T
a
PT, Senior lecture, Faculty of Physiotherapy, Academy of Physical Education, Wrocław, Poland. b Professor, Academy of Physical Education, Wrocław and University of Technology, Opole, Poland. Submit requests for reprints to: Tomasz Sipko, PhD, PT, Senior lecture, Faculty of Physiotherapy, Academy of Physical Education, Wrocław, Poland (e-mail: [email protected]). Paper submitted February 8, 2013; in revised form August 27, 2013; accepted August 27, 2013. 0161-4754/$36.00 Copyright © 2013 by National University of Health Sciences. http://dx.doi.org/10.1016/j.jmpt.2013.08.005
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neuromuscular control found in patients with CLBP may contribute to their LOS, making these patients more at risk for loss of equilibrium. 4 However, falling is not the largest problem related to decreased balance in individuals with CLBP, but a modified postural strategy which might sustain chronic symptoms and alter active daily living. Stiff postural strategy in automatic postural responses was especially evidenced in the postural tasks of CLBP patients, 6,7 as well as decreased voluntary forward lean 5 and even loss of variability in strategy selection. 8 CLBP patients reduced hip control strategy during quiet standing, 9 favoring the ankle strategy, 4,10 which was not effective in demanding conditions. 3,5 Most of the studies 3,4,6,7 investigated automatic postural responses in dynamic balance of CLBP, but only Mientjes and Frank 5 estimated LOS in voluntary body lean, and only in the forward direction. All of these studies 3-7 excluded CLBP patients with increased pain; however, a characteristic CLBP symptom is recurrent pain with differing locations and intensity. It is known that in CLBP patients standing quietly, velocity of COP increased linearly with an increase in perceived pain intensity greater than 4 on a numerical pain scale, 11 but there is no explanation as to the effect intensity of recurrent chronic pain on dynamic balance.
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The aim of this study was to examine the forward and backward limits of stability during voluntary body lean, and the COP velocity as a measure of reaching LOS in subjects with low pain levels (LP) and with high pain levels (HP). The purpose of this study was to evaluate if the intensity of chronic pain for CLBP patients would modify the postural performance in the voluntary LOS tasks. We hypothesized that pain intensity would modify performance in the following ways: (1) decrease the LOS magnitude and (2) decrease the velocity of task execution. It was expected that disturbances in the postural performance of the HP group were more likely to occur when the subjects’ eyes were closed because of deficits in the sensorimotor processing associated with the pain level.
METHODS Subjects Thirty-six people with CLBP aged 30 to 65 years, undergoing a non-invasive treatment in Cieplice Zdrój health resort participated in the study (for details see Ref. 12). Thirty-two healthy persons served as an asymptomatic group. All participants were screened by physiotherapist and provided completed pain and status health questionnaire packages. The inclusion criteria encompassed patients with low back pain due to a herniated disc confirmed by magnetic resonance imaging who have had persistent or recurrent chronic pain for at least 3 months. The exclusion criteria included the presence of neurological diseases, orthopedic conditions, stenosis of the spinal canal, and surgical treatment of the herniated disc. The level of resting pain on the day of investigation was determined on the basis of a numerical rating scale of 0 to 10 (NRS) by marking the current intensity of pain. 13 The CLBP group was stratified in terms of pain intensity into a subgroup with low pain level (LP group; NRS = 0-3) and a subgroup with high pain level (HP group; NRS = 4-10). A cut-off point was found in the results of Corbeil et al, 14 as weak pain intensity minimally affected control of posture but the moderate and extreme pain intensities gradually deteriorated the postural system. A convenience sample of 32 asymptomatic volunteers, who were not involved in any regular physical activity, was recruited from the Faculty of Physiotherapy. The inclusion criteria encompassed asymptomatic subjects (NRS = 0), with no history of CLBP, aged 30–65. The exclusion criteria were as follows: neurological diseases, orthopedic problems of spine, hip, knee or foot, low back pain at the time of testing, and bad physical and mental state on the day of the study. All subjects gave informed consent prior to their participation in the study, and the procedures were approved by the Academy of Physical Education Bioethics Committee.
Sipko and Kuczyński Stability Limits in Chronic Back Pain Patients
Postural Balance Examination The limits of stability were assessed according to a simplified protocol based on the traditional evaluation of LOS. 1 Subjects performed two 20-second trials on a hard surface, separated by 1-minute breaks. They performed the first trial with their eyes open (EO) and the second with their eyes closed (EC). The subjects were instructed to stand barefoot on a Kistler force plate (type 9286; http://www.kistler.com), with their feet parallel and 5 cm apart, and their gaze focused on a marker placed at eye level at a distance of 1.5 m. A practice trial was allowed prior to the test to ensure that the subjects were able to complete the task. They were asked to maintain full contact between the soles of their feet and the force plate during a voluntary body leaning. The maximal displacement of the center of pressure (COP) was calculated duringa forward, followed by a backward, voluntary body lean, with EO or EC, relative to the baseline data computed as an average COP value from the first five seconds of standing in the resting position. The sampling rate was 100 Hz and the sampling time was 20 seconds, resulting in 2000 sampling segments for each recorded COP time series in the sagittal plane. The dependent variables included forward (LOS.For) and backward (LOS.Back) limits of stability in the sagittal plane and the mean velocity of the COP as a measure of reaching LOS in forward (LOS.VS.for) and backward directions (LOS.VS.back). For the between-subjects comparison, the COP data were normalized according to the subjects’ foot lengths: normalized LOS = (LOS/foot length) × 100. 4,5
Statistical Methods Biometric data were compared statistically using oneway analysis of variance, followed by the Tukey post hoc test. As the postural measures had non-normal distribution, the results were presented as medians, mean, and SD. The statistical significance of the differences between groups was assessed with the Mann-Whitney U test. For the intragroup comparisons, to evaluate the effect of vision, a Wilcoxon matched pair test was applied. Spearman rank correlation was used to evaluate the relationships between LOS parameters and intensity of pain. The level of significance was set at P b .05. The effect size measure of Cohen’s d was obtained and used to quantify the differences in outcome variables.
RESULTS Demographic Data Both subgroups were alike in terms of age and height except for body weight and pain intensity. Asymptomatic group had significantly lesser weight than LP group (P = .0004) and lesser body mass index (BMI) than both subgroups (P b .05). The
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Table 1. Characteristics of participants Asymptomatic group n = 32, mean (SD) Age (y) Weight (kg) Height (m) BMI NRS (0-10)
44.6(8.6) 68.4(13.8) 1.7(0.09) 23.46(2.72) 0(0)
LP group n = 16, mean (SD)
HP group n = 20, mean (SD)
Tukey’s post hoc test Asym-LP/Asym-HP/LP-HP
51.4(8.0) 85.7(16.0) 1.75(0.01) 27.6(3.7) 1.47(1.23)
50.2(7.8) 74.1(11.5) 1.69(0.09) 26.1(4.5) 5.31(1.37)
0.02a/0.051/0.90 0.0004a/0.31/0.03a 0.14/0.92/0.10 0.0008a/0.026a/0.42 0.0001a
BMI, body mass index; HP, high pain; LP, low pain; NRS, numerical rating scale; SD, standard deviation. Significance of differences: aP b .05. NRS = t test.
experimental groups were overweight while the asymptomatic group was not (Table 1).
Secondly, eye closure resulted in altered performance of leaning tasks in the LP and the asymptomatic group as demonstrated by higher values of the velocity of COP. The higher velocity of COP might represent the generation of a Limits of Stability Normalized data were presented in Figures 1a and b, and 2a normal active sway to find a stable solution to the postural 2 and b, and Table 2. Cohen’s d effect size was shown in Table 3. challenge. The lack of visual information makes subjects 16 The forward LOS were significantly lower in both the LP (P b rely more on proprioception. In this context, the lack of .01) and HP (P = .004) subgroups than in the asymptomatic increase in the COP velocity in the HP group may account group under EO and EC conditions, while no differences for deficits in sensorimotor processing and suggests that between the LP and HP groups were found (Fig 1a), accounting there may be some pain threshold level beyond which EC for the overall effect of chronic back pain on LOS regardless of deficits are more common. Thirdly, there were no differences in the LOS between the pain level. Eye closure decreased the forward LOS in the LP (P = .004) and in the asymptomatic group (P = .04), with no the LP and HP groups. Also, no correlation was found between the level of pain and LOS in either group. This effect on the HP group (Fig 1a). The only intergroup difference in the backward LOS was the indicated no direct relationship between the resting pain lower LOS value in the HP group compared to the asymptomatic intensity and the magnitude of LOS in the CLBP. Our LP and HP groups presented a similar LOS group (P = .01) when performing with EC (Fig 1b), suggesting that high pain restricted backward LOS under EC conditions only. magnitude which reflected no effect of resting pain intensity The only intergroup difference in forward velocity of COP on postural stability limits. However, the lower LOS in showed lower values in the HP than the LP group (P = .04) patients compared with asymptomatic subjects and the with EC (Fig 2a). Eye closure caused a significant increase in differences in performance during EC leaning tasks forward (P = .02) and backward (P = .001) velocity of COP in between the LP and HP groups account for two reasons the LP group and forward velocity of COP in the for the underlying postural disorders. The former comparison seems to suggest reduced limits of stability in CLBP, asymptomatic group only (P = .04) (Fig 2a and b). There was no significant relationship between pain while the latter may imply a deficit in sensorimotor intensity and magnitude of LOS under EO and EC conditions processing. Several factors may contribute to reduced forward LOS in CLBP, including stiffening postural in any group (Table 4). strategy, 6,7 an alteration in the neuromuscular synergy of the trunk extensor muscles in standing reach, 17 and DISCUSSION sensitized and painful trunk extensor muscles. 12,18 In 19 6 The aim of this study was to examine the limits of addition, young persons with LBP and chronic LBP stability in CLBP patients during voluntary body leaning as have adapted forward inclined posture and positioned their well as the velocity of COP. Three novel findings with mean COP more to the anterior when vision was occluded. This corresponded well with decreased forward LOS in our clinical relevance are presented. Firstly, both subgroups of patients had a markedly lower CLBP subgroups under EC conditions. In contrast to the forward LOS than the asymptomatic group subjects while the forward direction, stability limits in the backward direction restricted backward LOS was found only in the HP in trials were not significantly different between asymptomatic with EC. A decrease in stability limits causes an increase in group and both CLBP subgroups under EO condition. the margin of error, which has been suggested to provide the These results could be due to “floor” effect on maximum central nervous system with more time for making the backward inclination common to both the asymptomatic necessary postural corrections. 15 In the same line, Popa et al 6 group and CLBP due to biomechanical constraints to indicated that in CLBP the process of internal estimation must leaning backward. Similar findings were reported by 20 take into account and compensate for a loss of sensory Mancini et al in an investigation of the functional LOS in Parkinson’s disease. precision during postural tasks.
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a) Limit of stability in forward
a) Velocity of COP in forward
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Fig 1. Normalized values median, lower, and upper quartile 25%75%, min-max of limit of stability in forward (a) (LOS.For); backward (b) (LOS.Back) in the LP (low pain), HP (high pain), and asymptomatic groups; trials with EO and EC; and significance of differences ( aP b .05).
The deficit in sensorimotor processing was revealed in the HP subgroup only during performances with EC, indicating that proprioception in individuals with high pain might be disturbed. Our observation supports the findings of other researchers who have claimed that postural deterioration occurs due to lumbar proprioceptive deficits caused by pain. 3,16,21,22 Indeed, in a study by Mientjes and Frank, 5 the forward leaning task with eyes closed showed the largest differences between CLBP and the control group. The reduction in stability limits during EC trials suggests that these limits are not purely biomechanical, but are affected by the availability of sensory information. 23 In the LP and the asymptomatic group, velocity increased with EC as the subjects relied mainly on proprioceptive input. This was most likely a normal postural strategy during voluntary body
Normalized Velocity of COP
Normalized range
45
35
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30 25 20 15 10 5 LP
HP LOS.VS.back.eo
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Fig 2. Normalized values median, coefficient 25%-75%, min-max of velocity of COP in forward (a) (LOS.VS.For) and backward (b) (LOS.VS.Back) in the LP (low pain), HP (high pain), and asymptomatic groups; trials with EO and EC; and significance of differences ( aP b .05). leaning to reach the LOS. However, the HP group did not follow the same pattern of postural compensation, providing further evidence of differences in motor control strategies for people with low and high pain. This might be explained by the pain-adaptation model, 24 as the control strategy reduced velocity and voluntary movement excursions to prevent provocation of pain. 25 Clear evidence in support of this model was found in a study by Graven-Nielsen et al 26 in which muscle pain seems to cause the general protection of painful muscles during both static and dynamic contractions.
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Table 2. Mean, SD, and median LOS and velocity of COP (VS) in forward (LOS.For) and backward (LOS.Back) in the asymptomatic, LP and HP groups; trials with EO and EC Asymptomatic n = 32
Los.For.eo Los.Back.eo VS.For.eo VS.Back.eo Los.For.ec Los.Back.ec VS.For.ec VS.Back.ec
LP group n = 16
HP group n = 20
Mean(SD)
Median
Mean(SD)
Median
Mean(SD)
Median
40.64(7.45) 27.02(5.58) 15.39(5.16) 17.32(4.94) 37.80(6.78) 27.08(5.14) 18.91(9.12) 18.60(4.5)
40.27 27.86 15.07 16.63 38.91 26.74 16.71 18.33
34.49(6.96) 24.90(7.99) 14.75(3.30) 14.82(4.22) 29.39(7.81) 27.07(11.30) 17.55(3.06) 20.16(6.68)
35.14 25.84 14.36 14.22 31.29 25.32 17.38 18.50
31.92(9.86) 25.07(8.37) 14.16(7.63) 15.66(4.09) 31.73(5.55) 22.57(9.53) 15.36(4.80) 18.82(6.33)
35.30 23.74 11.90 16.21 32.89 22.72 14.48 17.46
HP, high pain; Los.Back.ec, backward limit of stability with eyes closed; Los.Back.eo, backward limit of stability with eyes open; Los.For.ec, forward limit of stability with eyes closed; Los.For.eo, forward limit of stability with eyes open; LP, low pain; NRS, numerical rating scale; SD, standard deviation; VS.Back.ec, backward velocity of COP with eyes closed; VS.Back.eo, backward velocity of COP with eyes open; VS.For.ec, forward velocity of COP with eyes closed; VS.For.eo, forward velocity of COP with eyes open. Note: the magnitude of peak displacement has been normalized and therefore has no units.
Table 3. Effect sizes for variables of interest Cohen’s d effect size Asymptomatic-LP Los.For.eo Los.Back.eo VS.For.eo VS.Back.eo Los.For.ec Los.Back.ec VS.For.ec VS.Back.ec
0.85 a 0.30 0.14 0.54 1.14 a 0.001 0.19 − 0.27
Asymptomatic-HP 1.20 a 0.27 0.18 0.36 0.97 a 0.58 0.48 − 0.04
LP-HP 0.30 − 0.02 0.10 − 0.20 − 0.34 0.43 0.54 0.20
HP, high pain; Los.Back.ec, backward limit of stability with eyes closed; Los.Back.eo, backward limit of stability with eyes open; Los.For.ec, forward limit of stability with eyes closed; Los.For.eo, forward limit of stability with eyes open; LP, low pain; NRS, numerical rating scale; VS.Back.ec, backward velocity of COP with eyes closed; VS.Back.eo, backward velocity of COP with eyes open; VS.For.ec, forward velocity of COP with eyes closed; VS.For.eo, forward velocity of COP with eyes open. Interpretation: effect sizes small, d = 0 .2, medium, d = 0 .5 and large. a d = 0.8.
In this context, chronic and high intensity pain was a significant factor for differing postural performance in our CLBP. The similar context was discovered in a study by Tétreau et al, 27 expectations of low or high pain during trunk movement translate into different neuromuscular changes in trunk muscles, although pain-related adaptations may be appropriate in acute LBP, their persistence longer may contribute to pain chronicity.
Limitations The subjective nature of pain perception and therefore of pain rating may have influenced the results of both groups. The cut-off point on the NRS, 3 or 4, in the LP/HP group is of concern 11,14 particularly because of the possible overlap between the characteristics of subjects who were close to this cut-off point. The confounding effect of different BMIs cannot be excluded either. Although limited anterior
Table 4. The Spearman correlations between LOS, velocity of COP (VS) and the level of rest pain (NRS) in forward (LOS.For) and backward (LOS.Back) in the asymptomatic, LP, and HP groups, trials with EO and EC Asymptomatic n = 32 NRS Los.For.eo Los.For.ec Los.Back.eo Los.Back.ec VS.For.eo VS.For.ec VS.Back.eo VS.Back.ec
− 0.28 0.16 0.30 0.04 0.06 − 0.14 0.04 0.06
LP group n = 16 NRS 0.06 0.17 − 0.04 0.01 0.36 − 0.24 − 0.24 − 0.07
HP group n = 20 NRS − 0.26 − 0.31 − 0.05 − 0.01 − 0.26 − 0.31 − 0.30 − 0.12
HP, high pain; Los.Back.ec, backward limit of stability with eyes closed; Los.Back.eo, backward limit of stability with eyes open; Los.For.ec, forward limit of stability with eyes closed; Los.For.eo, forward limit of stability with eyes open; LP, low pain; NRS, numerical rating scale; VS.Back.ec, backward velocity of COP with eyes closed; VS.Back.eo, backward velocity of COP with eyes closed; VS.For.ec, forward velocity of COP with eyes closed; VS.For.eo, forward velocity of COP with eyes open. Significance of differences: P b .05.
LOS have been reported in individuals with grade III obesity only, 28 the presence of back pain could give rise to this deficit in less obese subjects as well. In a study by Cruz-Gómez et al 29 obese subjects showed a longer length and a larger area of sway than lean and overweight subjects, but in epidemiology study (n = 829 791 adolescents) higher BMI was significantly associated with low back pain in overweight as well in obese, both male and female. 30 Thus, some interaction between overweight and pain may have had an effect and matching groups on BMI may be useful in future studies. Also, the documented varieties of postural protective strategies used by CLBP patients may mask their actual behavior with simple statistics, unless attempts are made to investigate homogenous groups. Further, we did not formally estimate the sample size in this study which could have led to a type II error, that is, with a larger sample the differences in
Journal of Manipulative and Physiological Therapeutics Volume 36, Number 9
LOS between the two patients groups might become significant. Finally, there is little consensus about the criteria for clinical significance in posturography. Depending on the subjects’ characteristic and the purpose of the study, even small improvement in postural control may significantly contribute to the actual functional performance being examined. On the other hand, relatively large changes in postural indices may be not sufficient to differentiate the overall inter-group performance. In view of these limitations this study should be considered a preliminary study being helpful for designing future research.
Sipko and Kuczyński Stability Limits in Chronic Back Pain Patients
Data collection/processing (responsible for experiments, patient management, organization, or reporting data): TS, KM. Analysis/interpretation (responsible for statistical analysis, evaluation, and presentation of the results): TS, KM. Literature search (performed the literature search): TS, KM. Writing (responsible for writing a substantive part of the manuscript): TS, KM. Critical review (revised manuscript for intellectual content, this does not relate to spelling and grammar checking): TS, KM.
CONCLUSIONS The results of this study suggest differences in the postural performance of patients with low and high levels of pain. However, this apparently protective strategy suffers from its own, probably disadvantageous, effects on the limits of stability as well as its consequences for motor and postural control. Subjects with CLBP had reduced forward LOS regardless the pain level. However, the higher level of pain was associated with slower execution of voluntary leaning tasks, with EC only. The recurrent episodes of low back pain with higher levels of pain modify postural performance in dynamic balance. Alterations in postural performance in dynamic balance might be one of the factors contributing to pain chronicity in CLBP.
Practical Applications • Chronic and high intensity pain was a significant factor for differing postural performance in CLBP patients. • Alterations in postural performance in dynamic balance might be one of the factors contributing to pain chronicity.
FUNDING SOURCES AND POTENTIAL CONFLICTS OF INTEREST No funding sources or conflicts of interest were reported for this study.
CONTRIBUTORSHIP INFORMATION Concept development (provided idea for the research): TS, KM. Design (planned the methods to generate the results): TS, KM. Supervision (provided oversight, responsible for organization and implementation, writing of the manuscript): TS, KM.
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