59

Evaluation of Postural Stability during Quiet Standing, Step-Up

and Step-Up with Lateral Perturbation in Subjects with and

without Low Back Pain

(Publication History: Evaluation of postural stability during quiet standing, step-up and

step-up with lateral perturbation in subjects with and without low back pain. Ram Prasad,

M., Shweta Shenoy, D., Jaspal Singh Sandhu,. Sankara, N,. Sukdeb Mahanto. The South

African Journal of Physiotherapy. Editorially accepted for publication – Final revision for

publication in progress).

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 aim of this study was

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

significant different anterior-posterior (p=0 .01) as well as medio- lateral (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. Sub-group analysis in CLBP patients (p=0.005)

shows significant differences only in medio-lateral COP excursions during normal

standing. As the task difficulty increases CLBP patients exhibited significantly different

postural stability characteristics compared to healthy participants. Subgroup analysis of

CLBP population revealed no significant differences in tasks with higher difficulty such as

step up and step up task with lateral perturbation conversely significant difference was

observed during quiet standing in this study.

Keywords: Postural Balance; Posturography; Chronic Back Pain; Step Up Task

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4.1 Introduction

Musculoskeletal disorders have significant influence on balance performance (Byl

and Sinnott 1991; Wegener et al. 1997) and limit the use of corrective movement strategies

during balance perturbations (Shumway-Cook 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 compared to healthy participants.

Mok et al (2004) suggested that people with low back pain demonstrated an inability to

control hip strategy for balance recovery in response to an anterior- posterior balance

challenge.

Postural stability is achieved as the result of the interaction between the sensory

and motor systems, for a given task and environment, which ultimately produces postural

forces that result in movement of the center of pressure (COP) to maintain balance or

posture and it is totally independent of the center of mass (COM). According to Winter

(1995) the COP always lies at the stance foot, but in case of bilateral stance the COP lies

somewhere between the two feet, depending on the relative weight taken by each foot.

COP displacements are commonly recorded using force platforms and gives major

information about the postural control for a given task, performed on the forceplate

(Nardone and Schieppati 1988; Noe et al. 2003; Slijper and Latash 2000).

A variety of tasks and test situations used to examine postural stability in CLBP

patients have been reported in literature. They were obtained from subject induced

perturbations (voluntarily generated) or external force induced perturbations that

challenged postural control. Some reported methods were stepping (Burleigh et al. 1994;

Chang et al. 1999; Mann et al. 1979), bipedal to unipedal support (Polcyn et al. 1998),

slow and fast extremity movements (Rogers and Pai 1993), sit to stand (Magnan et al.

1996; Shum et al. 2005, Shum et al. 2009), reaching and pick up tasks (Silfies et al. 2005;

Shum et al. 2007), surface translations (Hitt et al. 2006), limb loading (Leinonen et al.

2007) and calibrated force transducers-induced perturbations (Radebold et al. 2000; Stokes

et al. 2006).

Recent studies have reported problems in the domain of functional activities of the

CLBP patients on the task specific perspective of the motor control rehabilitation i.e.,

impaired thorax, lumbar and pelvis segmental coordination and poor erector spinae activity

patterns during walking (Lamoth et al. 2002; Lamoth et al. 2006(a); Lamoth et al.

2006(b)) and decreased power flow of the pelvic and lower limb segments and altered load

61

sharing strategies during the sit-to-stand and stand-to-sit activities (Shum et al. 2005; Shum

et al. 2009).

On the other end, numerous studies have demonstrated dysfunction in the muscular

domain i.e., altered neuromuscular recruitment patterns, temporal and amplitude

insufficiencies in the activity of deep abdominal and para-spinal muscles of CLBP patients

compared to healthy individuals during activities such as bending and spinal loading

(Hodges and Richardson 1996; Hodges 2001; Radebold et al. 2000; Watson et al. 1997).

In many studies quiet standing was commonly used for postural stability

assessment despite the fact that most onset of back pain reported during dynamic

functional activities. 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.

Conversely these kinds of simple tasks particularly voluntarily generated tasks can be used

as a training modality in the early functional back rehabilitation or along with other active

exercise interventions such as walking and bicycling (Kerr et al. 2007; Smeets et al. 2006).

Studies have also advocated concurrent muscle (strength) training as well as postural

stability training for comprehensive back rehabilitation (Kollmitzer et al. 2000; Luoto et al.

1998) and successful functional restoration programe (Pfingsten 2001; Poiraudeau et al.

2007)

However postural stability variables for these functional tasks and their processes

are not well understood in back pain rehabilitation, despite its potential as a window into

functional back rehabilitation. Hence detailed kinetics, kinematics and postural stability

characteristics need to be determined before applying into clinical practice. As postural

control fundamentally relies on two domains and in view of the goal of postural control,

such task should represent and concern ability in both maintaining a given posture

and ensuring equilibrium in position change (Massion 1994; Horak 2006).

Hence in this study step up task was used to examine the postural stability in CLBP

patients compared to healthy participants. Further Sims and Brauer (2000) reported that the

step up task provided a greater challenge to medio-lateral (m-l) postural stability than step

forward. An external perturbation during mid of step-up task was introduced to examine

the effect of external perturbation on step-up mediated postural control responses in CLBP

and healthy participants. The direction of perturbation was kept to the lateral side to

examine the influence of laterally induced postural adjustments during step-up rather than

sagittal fashion commonly used in many studies. Hence, the primary aim of this study was

to investigate differences in postural stability or sway characteristics between groups with

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and without low back pain during quiet standing, voluntary step up and step up with

external lateral perturbation.

On the other hand studies reported larger COP displacements (Della Volpe et al.

2006; Popa et al. 2007) with narrow and self-selected natural stance widths. Hence we

hypothesized with wider stance width CLBP population demonstrate reduced likelihood of

greater resultant COP displacements. Further an attempt was made to investigate whether a

difference exist between movement and control impairment groups of CLBP subjects

(O‟Sullivan 2005) on medio-lateral postural responses. This study also aimed to test this

hypothesis. This sub-grouping approach was used to differentially analyze postural

stability characteristics in complex heterogeneous CLBP population.

4.2 Methodology

4.2.1 Selection and Description of 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 subjects, which was approved by the university ethical committee.

Data were collected from fourteen individuals with chronic non-specific low back pain and

fifteen healthy individuals.

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).

Patients with chronic localized low back pain lasting more than 6 months and radiating no

further than the buttock with normal neurological examination were included in this study.

Among these LBP subjects, none of them reported any history of neurological disorders or

major musculoskeletal disorders. Any history of sciatica or radicular involvement, previous

lumbar or abdominal surgery was also excluded. 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 analog scale (VAS) [(Mean

(SD)) 4.72(2.5)] for actual pain intensity ratings (0= no pain, 10= most severe pain), had a

disability level of 7.7(4.7) measured by the Ronald Morris Disability Questionnaire

(RMDQ) (0= no disabilities, 24= severe disabilities) and also had the Fear Avoidance

Belief for Work score component of 19.8(1.2) (0 = minimal score, 42 = maximum score)

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and Physical activity score component of 14(4.5) (0 = minimal score, 24 = maximum

score) measured by the Fear Avoidance Belief Questionnaire (FABQ).

A musculoskeletal assessment to identify movement impairment or control

impairment based on guidelines provided by O‟Sullivan (2005) was performed by a sports

physiotherapist with 6 years of clinical experience in back rehabilitation trained under

Curtin University, Australia. This classification system was based on set of substantially

reliable essential characteristics proposed 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 symptomatic segment in the painful direction of

movement or loading (based on clinical joint motion palpation examination)”. If

hypomobility or the presence of impaired movement was judged at involved segment, the

subject was categorised into movement impairment group

BERTEC forceplate, (Balance Screener Setup) Columbus, Ohio 43229, U.S.A. was

used to record the COP displacements during normal quiet standing, voluntary step-up, and

step-up with perturbation. The force plate‟s COP measurements were found valid and

reliable in determining postural stability parameters (Goldie et al. 1989; Spradley et al.

1996)

For step-up with lateral perturbation task, initially Digital Acquire setup was used to

determine the weight shift on the stepping leg. Based on the COP displacements postural

stability outcomes were measured using screener setup and their calibration procedures

were reported in Annexure 1 (Parker 1973; http://bertec.com/uploads/pdfs/manuals/

BalanceCheck% 20Screener.pdf).

4.2.2 Normal Quiet Standing

A marked foot chart with the inter-malleolar distance of 25-cm placed on the

forceplate was used as a reference. While standing on the foot chart participants were

instructed to fix their gaze at a point on the wall with visual gaze at their eye level to

minimize head tilting and to stand for 30 second on the forceplate.

4.2.3 Voluntary Step- Up

The subjects were asked to stand 10 cm in front of the forceplate which height was

adjusted to 10 cm. In order to minimize the abnormal postural strategies the step height

was kept low in this study. The subjects were informed to step-up on the forceplate using

natural speed. A metronome was used to coordinate the step-up task for 5 consequent

beeps to complete the entire step-up task. The entire step up task was completed within 10

seconds and data was stored.

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4.2.4 Step- Up Task with Lateral Perturbation

All participants were informed to achieve and maintain half of their body weight on

the forceplate monitor using their stepping leg while maintaining the stance foot on the

ground. Once the participants achieved the necessary weight level on the forceplate, an

external perturbation was provided at the stepping leg‟s side through pendulum setup. COP

excursion of above 2 standard deviations for 50milliseconds from quiet standing in Medio-

Lateral direction was kept as minimal requirement of perturbation and weight on the

pendulum was calculated as reference weight. This was determined by „Digital Acquire‟

setup of the forceplate. 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, particularly to achieve 50%

body weight on forceplate, up to three trials were provided 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 forceplate. The perturbation was given at shoulder level by moving the

pendulum laterally and released manually by the operator. Their weights were adjusted

based on above minimal COP M-L 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 forceplate during step-up with lateral perturbation. Independent

t-test was used to analyze the difference between CLBP and normal participants. A p-value

of less than 0.05 was used to determine significance (Appendix E, Figure V to VII)

4.3 Results

Postural sway characteristics in medio-lateral, anterior-posterior (a-p) direction,

maximal COP amplitudes, percentages of maximum standard stability, and stability scores

were significantly different in step-up with lateral displacement task between CLBP and

healthy participants (p<0.05). Quiet standing and step-up task did not demonstrated

significant difference between both the population (p<0.05).

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4.4 Discussion

4.4.1. Analysis of Quiet Standing and Step-Up Task

This study found no differences in COP excursions on medio-lateral and anterior-

posterior directions, maximal COP excursions and maximum standard stability scores

during step-up and quiet standing between healthy participants and CLBP subjects (Figure

1,2,3). In our study CLBP patients reported COP sway characteristics (amplitude) similar

to healthy participants contrary to smaller postural sway commonly reported in CLBP

population during usual standing and sitting tasks (Van dieen 2010; Van daele 2010;

Bouche 2006; Luoto 1998) or larger postural sway (Byl and Sinnott 1991) in literatures.

These non significant changes in COP (m-l) excursions, COP (a-p) excursions,

maximum COP excursions and standard stability scores during quiet standing and stepping

up tasks found in CLBP patients compared to healthy participants clearly indicating that

with a wider base of support abnormal postural strategies can be minimized in CLBP

population.

The results support the hypothesis that abnormal propensity of COP oscillations can be

reduced by altering the stance width from narrower to wider in CLBP population. Non

significant larger stability score also support this notion, 93.3% and 94.3% respectively in

CLBP and healthy participants indicates that the patient population was able to main

perfect stillness as close to healthy participants in wider stance width (Fig 4).

Some aspects of our methodology warrant attention particularly step-up task and

their non significant COP excursions in CLBP versus healthy participants. The height and

length of step-up (10cm) used in our study was relatively lower compared to exigencies of

day-to-day activities. Hence step height alterations can be varied in future studies to

determine the relationship between postural stability and COP displacements in CLBP

patients.

4.4.2. Analysis of Step-Up with Lateral Perturbation

During step-up with lateral destabilization postural responses CLBP subjects

exhibited significant increase in COP excursions on medio-lateral as well as anterior-

posterior directions (Fig 1,2).

During step-up with perturbation task, CLBP patients further demonstrated

significant increase in maximum COP excursions (p=0.01) and maximum standard

stability (p=0.02) (Fig 4). Maximum COP excursion indicates the magnitude of the

movement in the direction of maximum movement. The smaller value in healthy

66

participants indicated better postural adjustments during step-up 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%) compared to the scope of the healthy participants (28%) indicated a larger

standard limit of stability used by CLBP patents during step-up with perturbation task. This

indicates the inability of the CLBP population to prepare and resist the pre-informed lateral

displacement applied in this study and tendency to lean larger in m-l direction for lateral

displacement predisposing them to fall laterally. However healthy participants were well

prepared and resisted the suddenly applied lateral displacement and demonstrated

significantly smaller lean in m-l direction.

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 limit of stability during the test. The obtained stability scores

for 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 step-up with lateral perturbation.

Further, CLBP subjects demonstrated no significant changes compared to healthy

participants in minimum COP excursion, minimum/maximum COP excursion ratio,

minimum stability, and direction of instability parameters during step-up with perturbation

task, step-up and quiet standing. The above results clearly indicate that frontal and sagittal

plane control dysfunction in CLBP subjects while encountering demanding postural task

during this study.

The findings of this study support the literatures reporting relation between COP

displacements and stance width. Larger medial-lateral sway and COP oscillations were

reported with narrow stance width in healthy participants (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 perturbations. They further

reported during wide stance, equilibrium control relied on passive stiffness resulting from

changes in limb geometry where as narrow stance relied on active postural strategy

regulating loading and unloading of the limbs.

Further studies have reported increased stiffness of legs-pelvis and the hip-ankle

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

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sagittal plane control dysfunction found in our study may be attributed to dysfunction in

hip strategy (Mok et al. 2004) and corrosion of postural control of above-mentioned

mechanisms during exigent situations in CLBP patients.

To test the other hypothesis, i.e., to identify the differences between movement and

control impairment in CLBP population, the values of COP excursions on medio-lateral

direction was analyzed as this was the direction of the perturbation in our study. Statistical

analysis revealed significant differences between movement and control impairment

groups only during quiet standing (p<0.05) and did not reveal any difference during step-

up task and step-up with lateral perturbation task (Figure 6). Control impairment group

(n=6) demonstrated significantly higher mean COP (m-l) oscillations than the movement

impairment group (n = 8). These results provide preliminary evidence for the importance

of sub-grouping of CLBP patients for their specific interventions. However sub-grouping

them further into larger group of „flexion pattern‟, „active extension pattern‟ and „multiple

pattern‟ could have provided more distinct information on postural control characteristics

rather than generally classifying them into movement and control impairment group while

examining task specific postural control responses.

4.5 Implications

These simple functional tasks, if practiced repeatedly and cyclical in manner,

specific muscle training can be achieved that invariably facilitate the desired functional

task with minimal abnormal postural strategies in CLBP patients (for e.g. recumbent

cycling for the sit-to-stand and step-up tasks). Further the use of these robust, highly

flexible cyclic movements such as stepping and step-up 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 findings suggest that use of wider stance width during exercise sessions of early

functional and motor/postural control specific back rehabilitation can be helpful in

68

reducing abnormal postural strategies commonly reported with patients selected or narrow

stance width.

4.6. Limitations

Perturbation was induced by manual method and adjusted accordingly with the

postural adjustments responses produced during familiarization trials. It may be possible

that some participants might have developed rapid adaptation to the test situations. 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 sub-grouping of CLBP

patients could have resulted in significant different postural responses during tested tasks

in this study. Larger sub-group sample size with improved research methods are needed to

substantiate the results.

4.7 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 encountering difficult postural

adjustments. Using wider stance width and adequate monitoring 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.

69

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Figure Captions

Figure 1: COP (m-l) excursions during step-up with perturbation, step-up and quiet

standing task in CLBP and healthy participants with independent „t‟ test

results.

Figure 2: COP (a-p) excursions during step-up with perturbation, step-up and quiet

standing task in CLBP and healthy participants with independent 't' test

results

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 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 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 6: Sub-group analysis of COP(m-l) 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

77

Figure 1: COP (m-l) excursions during step-up with perturbation, step-up and quiet

standing task in CLBP and healthy participants with independent ‘t’ test results

1.02

8.7

0.410.74

10.4

0.36

0

2

4

6

8

10

12

Stepup perturbed(p<0.05) Stepup(p>0.05) Standing(p>0.05)

cm

COP(m-l)CLBP COP(m-l)Normals

Figure 2: COP (a-p) excursions during step-up with perturbation, step-up and quiet

standing task in CLBP and healthy participants with independent 't' test results

3.92 4.05

0.58

2.62

4.17

0.48

0

1

2

3

4

5

Stepup perturbed(p<0.05) Stepup(p>0.05) Standing (p>0.05)

cm

COP(a-p)CLBP COP(a-p)Normals

78

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

4.1

9.2

0.6

2.7

10.3

0.5

0

2

4

6

8

10

12

Stepup perturbed(p<0.05) Stepup(p>0.05) Standing(p>0.05)

cm

Max COPex CLBP Max COPex Normals

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

41

92.3

6.5

27.8

107.7

5.5

0

20

40

60

80

100

120

Stepup perturbed(p<0.05) Stepup(p>0.05) Standing(p>0.05)

% p

erc

en

tag

e

Mx. Stability CLBP Mx.Stability Normals

79

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

58.5

17

93.3

71.8

9.8

94.3

0

10

20

30

40

50

60

70

80

90

100

Stepup perturbed(p<0.05) Stepup(p>0.05) Stand(p>0.05)

% p

erc

en

tag

e

Stability Score CLBP Stability Score Normals

Figure 6: Sub-group analysis of COP(m-l) 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

4.1

7.8

0.3

3.6

9.8

0.5

0

2

4

6

8

10

12

Stepup perturbed(p>0.05) Stepup(p>0.05) Standing(p<0.05)

cm

COP(m-l)MovImp COP(m-l)ConImp

80

Annexure-1

COP (m-l) Excursions: The amount of movement of the Center of Pressure in the lateral

plane. It is calculated as the projection of the 95% confidence ellipse on the lateral axis.

(95% Confidence Ellipse - The ellipse containing 95% of the Center of Pressure points. It

is determined by multiplying the standard deviation of the coordinates of the Center of

Pressure points by 1.96).

COP (a-p) Excursions: The amount of movement of the Center of Pressure in the sagittal

plane. It is calculated as the projection of the 95% confidence ellipse on the sagittal axis.

Maximum COP Excursions: The maximum movement of the Center of Pressure in the

Direction of Maximum Instability. (Direction of Max Instability - The direction in which

the patient is less stable, and therefore most likely to fall. It corresponds to the angle

between the patient‟s posterior-anterior (forward) direction and the major axis of the

ellipse. Angles to the left are indicated as negative numbers)

Maximum Standard Stability%: How much of the Standard Limits of Stability was used

in the patient‟s Direction of Maximum Instability.

Stability Scores%: is a score of the patient's ability to maintain balance during the test. It

is calculated as percentage of S standard – A max / S standard, where A max is the major semi-axis

of the 95% confidence ellipse and S standard represents the Standard Limits of Stability,

calculated as S standard = 0.55 H sin 6.250. H is the patient's height.

Minimum COP Excursion: The maximum movement of the Center of Pressure in the

direction of minimum instability (Direction of Min Instability - The direction in which the

patient is more stable, and therefore less likely to fall).

Minimum/maximum COP Excursion Ratio: Min/Max CoP Excursion Ratio - The ratio

between the Minimum CoP Excursion and the Maximum CoP Excursion.

Minimum Stability: This is an evaluation of the patient's ability to maintain balance. It is

calculated as min [RNS-EO / RLoS ] % where RNS-EO is the distance from the origin of any

point of the 95% confidence ellipse for the normal stability - Eyes Open test and RLoS is

the corresponding distance on the ellipse representing the patient's Limits of Stability.