Evaluation of postural stability during quiet standing , step-up and step-up with lateral perturbation in subjects with and without low back pain

Correspondence to: Ramprasad M Srinivas College of Physiotherapy and Research Center, Pandeswara, Mangalore, Karnataka, India. E­mail: mramprasad@rediffmail.com AbSTrAcT: The evaluation of postural stability during quiet stance, step up and step up task with perturbation using posturography could be useful in treatment and outcome monitoring in chronic low back pain rehabilitation (CLBP). The aims of this study were twofold and investigating 1) differences of postural stability measures between CLBP patients and healthy participants during above mentioned tasks. 2) postural stability characteristics between control and movement impairment groups of CLBP patients on above tasks. Fourteen CLBP and fifteen normal individuals participated and posturography outcome variables were obtained during above tasks. The low back pain subjects showed significantly different anterior-posterior (p=0 .01) as well as mediolateral (p=0.05) postural stability characteristics during the step up task with external perturbation, whereas quiet standing and simple step up task did not show any differences. In addition to these values, in CLBP population, the maximum COP excursion (p=0.01), standard stability (p=0.02) and the stability scores (p=0.02) were also found significant in step up with perturbation task compared to healthy participants. As the task difficulty increases CLBP patients exhibited significantly different postural stability characteristics compared to healthy participants. Conversely, sub-group analysis in CLBP patients revealed significant differences only in medio-lateral COP excursions during normal standing (p=0.005). No significant differences were observed in tasks of higher difficulties such as step up and step up task with lateral perturbation in-between patients with movement and control impairment groups of CLBP. These findings have implications for assessment and optimizing postural control interventions on functional back pain rehabilitation.


Evaluation of postural stability during quiet standing, step-up and step-up
with lateral perturbation in subjects with and without low back pain.
COP displacements are commonly recorded using force platforms and gives major information about the postural stability characteristics of a given task, performed on the forceplate.In many studies quiet standing was commonly used for postural stability assessment despite the fact that most onset of back pain reported during dynamic activities such as daytoday functional activi ties.These assessments may be helpful in evaluating and screening back pain but the clinical use of these results in back pain rehabilitation was found to be limited.On the other hand, these kinds of simple tasks particularly voluntarily generated tasks can be used as a train ing modality in the early functional back

INTRODuCTION
Musculoskeletal disorders have signifi cant influence on balance performance (Byl and Sinnott 1991;Wegener et al 1997) and limit the use of corrective movement strategies during balance perturbations (ShumwayCook 1996).Byl and Sinnott (1991) reported that low back pain patients had a greater degree of sway, a greater use of hip strategy and a more posterior center of pressure, in erect stance when com pared to healthy participants.Mok et al (2004) suggested that people with low back pain demonstrated an inability to control hip strategy for balance recov ery in response to an anterior posterior balance challenge.rehabilitation or along with other active exercise interventions such as walking and bicycling (Kerr et al 2007).The recent surge of interest in motor control issues has prompted the development and inclusion of postural stability training along with concurrent muscle (strength and endurance) training for comprehen sive back rehabilitation and successful functional back restoration program.
However postural stability variables for these functional tasks and their pro cesses are not well understood in back pain rehabilitation, despite its potential as a window into functional back rehabi litation.Hence detailed kinetics, kinema tics of postural stability characteristics need to be determined before applying into clinical practice.Postural control fundamentally relies on two domains, i.e., ability in maintaining a given pos ture and ensuring equilibrium in position change, hence in this study step up task was used to examine the postural sta bility.Further Sims and Brauer (2000) reported that the step up task provided a greater challenge to medio lateral (ml) postural stability than step forward.
A subgrouping approach i.e., classi fying CLBP patients into homogenous groups i.e., movement and control impairment, was performed to diffe rentially analyze postural stability characteristics of complex heteroge neous CLBP population.An external perturbation during mid of stepup task was introduced to examine the effect of external perturbation on stepup medi ated postural control responses in CLBP and healthy participants.The direction of perturbation was kept to the lateral side to examine the influence of late rally induced postural adjustments dur ing stepup rather than sagittal fashion commonly used in many studies.The primary aim of this study was to inves tigate differences in postural stability characteristics of patients with and with out low back pain during quiet stand ing, voluntary step up and step up with externally induced lateral perturbation.Several studies have reported larger COP displacements (Dellavolpe et al 2006;Popa et al 2007) with narrow and self selected natural stance widths.We there fore hypothesize that wider stance width may reduce the likelihood of greater resultant COP displacements in CLBP population.Further as stated above an attempt was made to investigate whether a difference exist between movement and control impairment groups of CLBP subjects (O'Sullivan 2005) on postural stability characteristics.

METHODOLOgy
Selection of the participants: Chronic low back pain participants were recruited from the affiliated hospitals and rehabilitation centers of SCPTRC Mangalore, Karnataka, India.Informed consent was obtained from all the sub jects, which was approved by the univer sity ethical committee.
Patients with chronic localized low back pain lasting more than 6 months and radiating no further than the but tock with normal neurological exami nation were included in this study; of these none had neurological disorders (sciatica or radicular involvement), major musculoskeletal disorders, or previous lumbar or abdominal surgery.An orthopedic surgeon performed the examination.All CLBP subjects were instructed to avoid medication 24 hours before the test.Prior to the experiment, the CLBP patients completed visual ana log scale for pain (VAS), Ronald Morris Disability Questionnaire (RMDQ) and Fear Avoidance Belief Questionnaire (FABQ).
A musculoskeletal assessment to identify movement impairment or con trol impairment based on guidelines provided by O'Sullivan (2005) was performed by a sports physiotherapist trained from Curtin University, Australia and had 6 years of clinical experience in back rehabilitation.This classification system was based on set of substantially reliable essential characteristics pro posed by Dankaerts and O' Sullivan et al (2006).The control impairment group were identified by the presence of "pain with minimal radiation and absence of impaired movement of the sympto matic segment in the painful direction of movement or loading (based on clinical joint motion palpation exami nation)".If hypomobility or the pres ence of impaired movement was found in involved segment, the subject was categorised into movement impairment group.
A BERTEC force plate with Balance Screener Setup (Columbus, U.S.A.) was used to record the COP displacements during normal quiet standing, voluntary stepup, and stepup with perturbation as described below.For stepup with lateral perturbation task, initially Digital Acquire setup of forceplate was used to determine the weight shift on the stepping leg.The following outcome variables were computed: COPmedial lateral and anteriorposterior excursions, Maximum and minimum COP excur sions, Percentages of maximum standard stability and stability scores, Minimum/ maximum COP excursion ratio and Minimum stability.Based on the COP displacements postural stability outcomes variables were computed using screener setup and their calibration procedures reported in Annexure 1 (Parker 1973; http://bertec.com/uploads/pdfs/manuals/ BalanceCheck% 20Screener.pdf).

Normal Quiet Standing:
A marked foot chart with the inter malleolar distance of 25cm placed on the force plate was used as a reference.While standing 30 seconds on the foot chart, participants were instructed to fix their gaze at a point on the wall to their eye level to minimize head tilting.

Voluntary Step-up:
The subjects were asked to stand 10 cm away from the force plate, which height was kept at 10 cm.The subjects were informed to stepup on the force plate using natural speed.A metronome was used to coordinate the stepup task for 5 consequent beeps to complete the entire stepup task.The entire step up task was completed within 10 seconds and data was stored.
Step-up Task with Lateral Perturbation: All participants were informed to achieve and maintain half of their body weight on the force plate monitor using their stepping leg while maintaining the stance foot on the ground.Once the participants achieved the neces sary weight level on the force plate, an external perturbation was provided at the stepping leg's side through pendu lum setup.COP excursion of above 2 standard deviations for 50 milliseconds obtained from the quiet standing posi tion in mediolateral direction was kept as minimal requirement of perturbation and weight on the pendulum was calcu lated as reference weight.This was determined by 'Digital Acquire' setup of the force plate.Perturbations which triggered stumbling reactions were excluded and weights on the pendulum were readjusted to identify the exact reference weight through maximum of three trial tasks.
The pendulum weights were adjusted to produce similar perturbation on the Bertec screener setup.To minimize the amount of measurement error, particu larly to achieve 50% body weight on force plate, up to three trials were pro vided to become fully comfortable and familiar with the testing protocol.The pendulum setup was suspended from the ceiling and the resting position of pendulum was positioned at midpoint of base of support on the foot chart on the force plate.The perturbation was given at shoulder level by moving the pendulum laterally and released manu ally by the operator.Their weights were adjusted based on above minimal COP ML shift excursion criteria.Mean values of three trials of each task were taken for statistical analysis.Up to four trials were performed to achieve valid recordings from the force plate dur ing stepup with lateral perturbation.Independent ttest was used to analyze the difference between CLBP and normal participants.A pvalue of less than 0.05 was used to determine significance.

RESuLTS
Data were collected from fourteen indi viduals with chronic nonspecific low back pain and fifteen healthy indivi duals.CLBP subjects had a mean age of 36.8(2.8(SD))years, mean height of 165.7(8.8)centimeters, mean body mass index (BMI) of 22.3(3.3)and healthy participants had a mean age of (SD) 32.7(1.2) years, mean height of 163.8(9.0)centimeters and BMI of 20.9(3.6).CLBP patients had a mean(SD) score of 4.72(2.5)for actual pain intensity (0= no pain, 10= most severe pain), a dis ability level of 7.7(4.7)measured by the RMDQ (0= no disabilities, 24= severe disabilities), Fear Avoidance Belief for Work score component of 19.8(1.2) (0 = minimal score, 42 = maximum score) and Fear Avoidance Belief for Physical activity score component of 14(4.5)(0 = minimal score, 24 = maximum score), as measured by the FABQ.
Our study revealed significantly differ ent postural sway characteristics in the directions of mediolateral and anterior posterior COP excursions, maximal COP amplitudes, percentages of maximum standard stability, and stability scores only in the stepup with lateral displace ment task between CLBP and healthy participants (p<0.05)(Figure 1,2,3,4 and 5).CLBP and healthy participants did not demonstrated significant differ ence in quiet standing as well as stepup task.Further significant differences were observed between groups of movement and control impairment CLBP patients only during quiet standing on COP (Mediolateral) excursions (p<0.05),however no significant COP (Medio lateral) excursions observed during step up and stepup with lateral perturbation tasks (Figure 6).

Analysis of quiet standing and step-up task:
This study found no differences in COP excursions on mediolateral and anteriorposterior directions, maximal COP excursions and maximum standard stability scores during stepup and quiet standing between healthy participants and CLBP subjects (Figure 1, 2 and 3).In our study CLBP patients reported COP sway characteristics particularly excursion amplitudes similar to healthy participants contrary to smaller or larger postural sway commonly reported in CLBP population while comparing to healthy participants during usual stand ing and sitting tasks (Byl and Sinnott 1991; Van Dieen 2010; Van Daele 2010).These nonsignificant changes in COP excursions (Mediolateral, anteriorpos terior), maximum COP excursions and standard stability scores during quiet standing and stepping up task of CLBP patients might have resulted from wider base of support used in the study.Hence we postulate that with an optimal wider base of support such as used in this study, abnormal postural strategies can be minimized in CLBP population.
The results support the hypothesis that abnormal propensities of COP oscil lations can be reduced by widen the stance of foot in CLBP population.
Nonsignificant larger stability score also support this notion, 93.3% and 94.3% respectively in CLBP and healthy participants indicates that the patient population was also able to main perfect stillness as close to healthy participants in wider stance width (Fig 4).Some aspects of our methodology warrant attention.Stepup task and the resultant nonsignificant COP excursions between CLBP and healthy participants could have been affected by the height and length of stepup (10cm) used in this study.This height was relatively lower compared to exigencies of daytoday activities.Hence, step height altera tions can be varied in future studies to evaluate the postural stability and COP displacements in CLBP patients during the stepup task.

Analysis of step-up with lateral perturbation:
During stepup with lateral destabiliza tion postural responses, CLBP subjects exhibited significant increase in COP excursions on mediolateral as well as anteriorposterior directions (Fig 1 and 2).
During stepup with perturbation task, CLBP patients further demonstrated significant increase in maximum COP excursions (p=0.01) and maximum stan dard stability (p=0.02)(Fig 4).Maxi mum COP excursion indicates the magnitude of the movement in the direction of maxi mum movement.The smaller value in healthy participants clearly demonstrated the better postural adjustments during stepup with perturbation compared to CLBP population.
Maximum standard stability scores represented how much of the standard limit of stability was used during the test in the direction of maximum movement.A higher score of CLBP (41%) com pared to the group of the healthy partici pants (28%) indicated a larger standard limit of stability used by CLBP patents during stepup with perturbation task.This indicates the inability of the CLBP population to prepare and resist the pre informed lateral displacement applied and tendency to lean larger in medio lateral direction for lateral displacement, predisposing them to fall laterally in this study.However healthy partici pants were well prepared to counter the suddenly applied lateral displacement and demonstrated significantly smaller lean in mediolateral direction during stepup with lateral perturbation.
Stability scores represent the ability to maintain balance during the test.100% indicates that the patient was able to maintain perfect stillness.0% indicates that the patient used all the standard efforts to maintain the stability during the test.The obtained stability scores of CLBP patients (58%) compared to healthy participants (71%) during step up with lateral perturbation task, was significantly lower (p<0.02, Figure 5) indicating CLBP patients were unable to maintain balance during the stepup with lateral perturbation.However, CLBP subjects demonstrated no significant changes compared to healthy participants in minimum COP excursion, minimum/ maximum COP excursion ratio, mini mum stability, and direction of instability parameters during stepup with pertur bation task, stepup and quiet standing.The above results clearly revealed the frontal and sagittal plane movement execution dysfunction in CLBP subjects, while encountering demanding postural task during this study.
The findings of this study support the previous literatures reporting relation between COP displacements and stance width.Larger mediallateral sway and COP oscillations were reported with narrow stance width in healthy par ticipants (Kirby et al 1987;Henry et al 2001).Henry et al ( 2001) also reported more trunk displacements in narrow stance due to larger changes in COP oscillations in response to lateral pertur bations.They further reported during wide stance, equilibrium control relied on passive stiffness resulting from changes in limb geometry, whereas narrow stance relied on active postural strategy regulating loading and unload ing of the limbs.
Further studies have reported increased stiffness of legspelvis and the hipankle coupling (Day et al 1993), and hip abductor/ adductor muscles mediated stiffness control for frontal plane motion with wider stance width (Winter et al. 1996;1998).The frontal and sagittal plane control execution dysfunction found in our study may be   attributed to dysfunction in hip stra tegy (Mok et al 2004) and corrosion of postural control of abovementioned mechanisms during exigent situations in CLBP patients.
The value of COP excursions on mediolateral direction was analyzed to study the differences in movement and control impairment groups of CLBP participants.Statistical analysis revealed significant differences between groups of movement and control impairment during quiet standing (p<0.05),but not during the stepup and stepup with lateral perturbation tasks (Figure 6).The control impairment group (n=6) demonstrated significantly higher mean COP (Mediolateral) oscillations than the movement impairment group (n = 8).These results provide preliminary evi dence for the importance of subgroup ing in CLBP patients for functional specific exercise interventions.Inclusion of more subgroups as specified by O'Sullivan (2005) such as 'flexion pat tern', 'active extension pattern' and 'multiple pattern' could have provided more distinct information on postural control characteristics pertaining to the groups during perturbation, rather than generally classifying them into move ment and control impairment.

Implications:
Specific muscle training can be achieved through simple functional tasks such as stepping, if these tasks practiced repeat edly and cyclical in manner for func tional specific back rehabilitation.This may facilitate the desired functional task specific outcome with minimal abnormal postural strategies in CLBP patients (for e.g.recumbent cycling for the sittostand and stepup tasks).Further, the use of these robust, highly flexible cyclic movements such as step ping and stepup can benefit from the advantage of sequentially stretching and shortening of the muscles involved to produce more work (force) and use of spinal neural oscillators that optimize the postural control strategies related to locomotion (Kerr et al 2007;Smits engelsman et al 2006).The assessment of postural stability characteristics of these simple functional tasks may help clinicians to quantify the impairments   associated with these tasks, may provide effective intervention strategies aimed at optimizing abnormal postural control variables and may help in assessing the efficacy of treatment strategies for the training of the particular task.Our find ings suggest that use of optimal wider stance width during exercise sessions of early functional and motor/postural control specific back rehabilitation can be helpful in reducing abnormal postural sways contrary to commonly reported patients selected or narrow stance width and associated abnormal postural sways during functional tasks.

Limitations:
Perturbation was induced by manual method and adjusted accordingly with the postural responses produced during familiarization trials.It may be pos sible that some participants might have devel oped rapid adaptation to the test situa tions.Larger step length, step height, maximum foot width and foot length with narrow to wider base of support combinations should be considered in future studies to examine the postural stability related parameters in back pain patients.More precise subgrouping of CLBP patients could have resulted in significant different postural responses during tested tasks in this study.Larger subgroup sample size with improved research methods are needed to substan tiate the results.

CONCLuSION
CLBP population demonstrated frontal and sagittal plane control dysfunction while encountering demanding postural task during this study.No significant difference was observed in subgroups of CLBP population while encounter ing difficult postural adjustments.Using wider stance width and adequate moni toring of postural stability responses during early functional specific back rehabilitation can curtail the problem of inducing abnormal postural strategies in CLBP patients as poor stability and control may influence abnormal spinal loading and sustain the production of peripheral nociception.

ACkNOWLEDgEMENTS
Our thanks to Selvamani K, Joseley SP, Narayanan V of SCPTRC and Srinivas Hospital, Raguveera KMC, Manipal for their assistance in force plate data analysis process; Trupti Metha, Abhisk, Pranav and Sruthi for countless assis tance in mining data and assisting in each part of the overall study; Ramprabhu, Balasubramanian, John Varghese for their assistance during the revision of this manuscript; and A Shama Rao Foundation office and participated hospitals and our research center staffs for their overall assistance.This project was funded in part with an internal grant from SCPTRC, Mangalore, Karnataka, India.

Figure 2 :
Figure 2: coP (Antero-posterior) excursions during step-up with perturbation, step-up and quiet standing task in clBP and healthy participants with independent 't' test results.

Figure 1 :
Figure 1: coP (Medio-lateral) excursions during step-up with perturbation, step-up and quiet standing task in clBP and healthy participants with independent 't' test results.

Figure 3 :
Figure 3: Maximum coP excursions during step-up with perturbation, step-up and quiet standing task in clBP and healthy participants with independent 't' test results.

Figure 5 :
Figure 5: Stability scores (%) during step-up with perturbation, step-up and quiet standing task in clBP and healthy participants with independent 't' test results.

Figure 4 :
Figure 4: Maximum Standard Stability % during step-up with perturbation, step-up and quiet standing task in clBP and healthy participants with independent 't' test results.

Figure 6 :
Figure 6: Sub-group analysis of coP (Medio-lateral) excursions during step-up with perturbation, step-up and quiet standing task between movement impairment and control impairment clBP groups with paired 't' test results.
Byl N , Sinnott P 1991 Variations in balance and body sway in middle aged adults: subjects with healthy backs compared with subjects with low back dysfunction.Spine 16: 32530.Dankaerts W, O'Sullivan PB, Straker LM, Burnett AF, Skouen JS 2006 The interexaminer reliability of a classification method for non specific chronic low back pain patients with motor control impairment.Manual therapy 111:2839.Day BL, Steiger MJ, Thompson PD, Marsden CD 1993 Effect of vision and stance width on human body motion when standing: Implications for afferent control of lateral sway.Journal of physio logy 469: 479-499.Della Volpe R , Popa T, Ginanneschi F, Spida lieri R, Mazzocchio R , Rossi A 2006 Changes in coordination of postural control during dynamic stance in chronic low back pain patients.Gait & posture 24:349355.Henry SM, Fung J, Horak FB 2001 Effect of stance width on multidirectional postural responses.Journal of neurophysiology 85:559-570.