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Computer Methods in Biomechanics and BiomedicalEngineering

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Effectiveness and recovery action of aperturbation balance test – a comparison ofsingle-leg and bipedal stances

Bálint Petró, Judit T Nagy & Rita M Kiss

To cite this article: Bálint Petró, Judit T Nagy & Rita M Kiss (2018) Effectiveness and recoveryaction of a perturbation balance test – a comparison of single-leg and bipedal stances,Computer Methods in Biomechanics and Biomedical Engineering, 21:10, 593-600, DOI:10.1080/10255842.2018.1502278

To link to this article: https://doi.org/10.1080/10255842.2018.1502278

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ARTICLE

Effectiveness and recovery action of a perturbation balance test – acomparison of single-leg and bipedal stances

B�alint Petr�oa , Judit T Nagyb and Rita M Kissa

aDepartment of Mechatronics, Optics and Mechanical Engineering Informatics, Budapest University of Technology and Economics,Budapest, Hungary; bDoctoral School of Mathematical and Computational Sciences, University of Debrecen, Debrecen, Hungary

ABSTRACTDynamic balancing abilities can be assessed with perturbation tests. The present study examinedthe effectiveness of balancing (damping ratio) and the recovery action (directional ratio) in bipedaland dominant single-leg stance in the young population. Twenty-four healthy young adult partici-pants completed unidirectional lateral perturbations successfully using a Posturomed# platform(Haider Bioswing, Germany). Single-leg stances had similar damping scores (p¼ 0.551) to and lowerdirectional ratio values (p¼ 0.002) than bipedal recoveries. This shows that different recoveryactions can achieve similar effectiveness in the two stances. A test evaluation methodology thatsimultaneously utilises performance and motion characteristic parameters was demonstrated.

ARTICLE HISTORYReceived 23 November 2017Accepted 16 July 2018

KEYWORDSBipedal stance; Dynamicbalancing; Posturomed;single-leg balancing

Introduction

The maintenance of balance, particularly in bipedssuch as humans, is an intricate process carried out byan integrated neuro-musculoskeletal system. Staticbalancing is examined in the field of posturography,whereas dynamic balancing corresponds to the studyof all movements (Winter 2009). Postural balancerequires non-linear control with time delays, wherethe central nervous system has to ensure stabilityand accuracy in a fraction of a second (Chagdes et al.2013). To solve this challenging task, the central ner-vous system creates synergic muscle strategies throughlearning and recalls these strategies when differentbalancing tasks have to be carried out (Giboin et al.2015). These strategies have been extensively studied,both theoretically and experimentally (Rietdyk et al.1999; Terry et al. 2011; Shirota et al. 2014).

Various methods have been developed for the assess-ment of balancing abilities; for a review of dynamicstanding balance assessment see (Petr�o et al. 2017). Afree oscillatory platform utilises sudden lateral pertur-bations, after which the participants have to regain theirbalance, thereby damping the oscillation and stoppingthe motion of the platform. The Posturomed# device(Haider Bioswing, Weiden, Germany) is an example ofsuch platforms that are widely used in Europe to train

athletes, as well as to provide rehabilitation, therapeutic,and evaluation methods (M€uller et al. 2004). Steelsprings allow the platform to freely move along thehorizontal plane, and a fastening apparatus allows theplatform to be locked outside its resting position. Theperturbation occurs when the fastening apparatus isreleased, resulting in a sudden lateral disturbance. Afterthe disturbance, the platform starts to oscillate in orderto return to its original, stable state. Using compensa-tory recovery actions, the participant has to counterbal-ance this sudden disturbance and damp the subsequentoscillation (M€uller et al. 2004; Kiss 2011). This meansthe oscillating platform with an individual standing onit is a damped system (M€uller et al. 2004; Kiss 2011).Results of the sudden perturbation test and thus the bal-ancing ability of the participant are characterised byvarious measures. Most notable measures are:

� Balance index: number of recorded oscillations fol-lowing ML and AP perturbation (M€uller et al. 2004)

� The time of successful balancing: the time whenthe motion of the platform stays within ±2mm ofthe resting position during a set trial period, e.g.20 s (Giboin et al. 2015).

� Lehr’s damping ratio (D): ratio of actual dampingof the system and the critical damping thatprevents the system from oscillating (Kiss 2011).

CONTACT Rita M Kiss rikiss@mail.bme.hu Department of Mechatronics, Optics and Mechanical Engineering Informatics, Budapest University ofTechnology and Economics, M}uegyetem rkp 3., 1111 Budapest, Hungary.

Supplemental data for this article can be accessed on the publisher’s website

� 2018 Informa UK Limited, trading as Taylor & Francis Group

COMPUTER METHODS IN BIOMECHANICS AND BIOMEDICAL ENGINEERING2018, VOL. 21, NO. 10, 593–600https://doi.org/10.1080/10255842.2018.1502278

Lehr’s damping ratio has been used with goodresults in the field of clinical orthopaedics (Kiss 2012;Holnapy and Kiss 2013; Pethes et al. 2015). An inter-esting finding is that for groups of elderly people,Lehr’s damping ratio did not differ significantly forbipedal and single-leg stances on the laterally domin-ant leg (Kiss 2012). This was true regardless of health,the difference in age or gender (healthy and moderateor severe osteoarthritis, ages 65–69 and 70–74, malesand females) (Kiss 2012). It has not been establishedwhether this stands for other populations as well.

Already existing measurement protocols in clinicalpractice obtain data by tracking the platform motionto calculate measures of balancing performance.However, platform trajectories can show variousshapes (Figure 1). In bipedal stance recovery, themotion frequently stays mostly linear and in the dir-ection of the initial perturbation (Figure 1(a)). In asingle-leg stance, different circular and ellipticalshapes can frequently be noted (Figure 1(b,c)). Insome cases, the medial-lateral perturbation is turnedinto an anterior-posterior motion by the participant(Figure 1(d)). It could be supposed that the shape ofthe platform trajectory (the execution of regainingbalance) contains information regarding the balancingabilities of participants. For this reason, a new vari-able was introduced to measure the path of balancerecovery called the directional ratio (R) (Petr�o and

Kiss 2017), which is the quotient of the distancetravelled parallel and perpendicular to perturbation.The interaction between the platform trajectory andthe associated balancing performance still needs to befurther explored.

The objective of the present study was to determinewhether balance recovery performance levels, as meas-ured by Lehr’s damping ratio, and platform motion tra-jectory, as characterised by the directional ratio, differbetween dominant single-leg and bipedal balance recov-eries for the young, healthy population. Our hypothesiswas that similarly to the elderly (Kiss 2012), the differ-ence in Lehr’s damping ratio is not significant and thusbalance recovery performance is similar in the two stan-ces. We hypothesised that for the recovery performanceto be similar, different recovery motions are neededin the two stances, which is indicated by a significantdifference in the directional ratio.

Methods

Participants

The investigation included 33 healthy young collegiatevolunteers, of which 24 participants (19 male, 5 female,age: 22.8 ± 1.3 yrs, height: 175.3 ± 6.6 cm, body mass:73.0 ± 11.4 kg, BMI: 23.6 ± 2.8) performed the suddenperturbation balance measurements successfully.

Figure 1. Observed balance recovery paths: (a) linear, (b) circular, (c) elliptical, (d) forward rotating. Coordinates are in mm.

594 B. PETR�O ET AL.

All anthropometric data are reported in Supportingmaterials (S Table 1). Candidate participants with anyneurological alterations (e.g. cerebral apoplexy), uncon-trolled hypertonia, unstable angina, vision correctiongreater than ±5.0 dioptres, musculoskeletal alterationsor the presence of muscle or joint pain were notincluded in this investigation. The tests were authorisedby the Science and Research Ethics Committee ofSemmelweis University (174/2005). Each volunteer pro-vided informed written consent before taking part inthe tests.

Procedure

The measurement procedure was similar to the onepreviously described in (Kiss 2011). Briefly, a suddenperturbation provocation test was utilized withthe free oscillating platform Posturomed# (HaiderBioswing, Weiden, Germany). The platform is a solidmetal plate (12 kg, 60 cm � 60 cm) manufacturedwith an anti-slip surface that is suspended on eightidentical 15 cm long steel springs, which allow theplatform to shift freely along the horizontal plane(Figure 2). In the present study, the device was set toits easiest level with four unlocked springs.

The fastening apparatus allows the platform to belocked outside its resting position by approximately20mm. First, the test participant adopts either a bipedalor single-leg stance on the platform (Figure 2(b) andFigure 2(c)). Then, the perturbation occurs when thefastening apparatus is released, resulting in a suddenlateral disturbance. One oscillation takes approximately0.4 s with the four-spring setting. Next, the participantinstinctively attempts to regain their postural balance.Since the device lacks significant damping and wouldfreely oscillate, the balancing ability of the participant

must act as a damping agent to decrease and eventuallystop the oscillation.

The standardised bipedal stance position involvesstanding barefoot at shoulder width above the middleof the measurement platform as indicated by the reflect-ive markers and surface pattern of the platform; armsare hanging freely. The standardised single-leg stanceposition involves standing barefoot on the supportingleg in the middle of the measurement platform witharms hanging freely. The participant is instructed toraise the heel of the non-supporting leg until the calf isparallel to the ground and is told that the non-support-ing leg is not allowed to come into contact with the sup-porting leg or the ground. The appropriate position andtouching of the legs were self-monitored and continu-ously monitored by staff as well. The participants wereinstructed not to watch their motion but instead to lookstraight ahead. In addition, they were told they can holdon to the handrails only to prevent falling off the plat-form. Participants were allowed to briefly familiarisethemselves with the unstable platform while listening tofurther instructions. One or two unrecorded perturb-ation trials were allowed to alleviate participants’ fear ofthe procedure. Exclusion conditions that result in therejection of a recorded trial included touching theguardrails with any body part or shifting the position ofthe supporting foot (feet).

The objective was to obtain measurement data forthree successful recoveries of bipedal stance and three ofthe single-leg stance that achieved superior performancefor the given participant (the dominant leg). Volunteerswere allowed up to five trials using bipedal stance, rightand left single-leg stance, in this order. Nine volunteerswho did not perform three successful trials in either leftor right stance were at this point excluded from thestudy. Thirteen participants met the required number inonly one of the (left or right) single-leg stances. For

Figure 2 Measurement setup. The Posturomed# device with unlocked provocation unit and reflective markers (a). The participantis positioned in a bipedal stance (b) and in a single-leg stance (c).

COMPUTER METHODS IN BIOMECHANICS AND BIOMEDICAL ENGINEERING 595

them, the more frequently successful side was selectedfor analysis. Eleven participants met the three trials inboth single-leg stances. For them, data of the side achiev-ing a higher average damping ratio were selected. Withregards to bipedal stance, all participants met therequired number.

Data collection

The motion of the platform was recorded using anOptiTrack (NaturalPoint Inc., Oregon, USA) infra-red18 camera motion capture system with passive reflectivemarkers at a measurement frequency of 120Hz. Theaccuracy of this measuring system is sub-millimetre.The infrared markers were rigidly attached on top ofthe platform (Figure 2(a)). Data were collected with thefactory-built OptiTrack Motive:Body software. The sys-tem recorded the platform position along the x- and y-axes, where x is parallel to the perturbation (positive inthe direction of perturbation) and y is perpendicular tothe perturbation in the horizontal plane (positive in theanterior direction) (Figure 2). The measurements wereperformed during the day at the Department ofMechatronics at Budapest University of Technologyand Economics, Hungary.

Data analyses

The recorded platform position data were processedusing a custom MATLAB routine (The MathWorks,Inc., Natick, MA, USA). Personal, test sequence andstance identifiers were also noted.

A recorded balance recovery attempt starts at therelease of the provocation unit and ends when theplatform reaches equilibrium, i.e. it remains in the2mm range of the stable position. The followingparameters were calculated from position dataobtained from the motion capture system:

� Lehr’s damping ratio (D, in %): the actual damp-ing value normalised by the critical damping value,to be calculated from the logarithmic decrement(Kiss 2011):

D ¼ KffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiK2 þ 4p2

p � 100% (1)

where K is the logarithmic decrement, calculated as

Ki ¼ 1ilnK0

Ki(2)

where K0 is the platform amplitude at time t¼ t0and Ki is the amplitude at time t¼ t0þ i�period.

It can be noted that if D¼ 100%, the dampingreached critical damping, so the system will returnto its stable equilibrium position without oscilla-tion; if D¼ 0%, the result is undamped oscillation,and if 0% <D< 100%, then the motion is damped.

� Directional ratio (R): the quotient of the distancetravelled parallel and perpendicular to the initialperturbation, respectively. To calculate travelled dis-tances, the position time series along the respectiveaxis is numerically differentiated to obtain velocity,then the absolute value of velocity is integrated.

Statistical analyses

A within-subject study design was adopted. Statisticalmethods were applied using IBM SPSS 23 (IBM, NewYork, USA). The unusual case finder tool was used toidentify potentially anomalous data points. First, cal-culated variables (D, R) were examined by descriptivemeasures and normality of distributions was assessedwith the Shapiro–Wilk test. In order to compare sin-gle-leg and bipedal stance balance recoveries withinsubjects, a Friedman test was conducted with a sig-nificance level of a¼ 0.05 applied. This was followedby a post-hoc Dunn test with Bonferroni correction toexamine pairwise comparisons.

Results

The required number of successful recoveries (threein the same single-leg and three in bipedal stance)was achieved by 24 participants and these recordswere selected for analysis (72 single-leg, 72 bipedalstances). Note that henceforth, a Xyz notation is used,where X denotes the parameter (D or R); y denotesstance type (1 for single-leg, 2 for bipedal stance); zdenotes sequence number (1, 2, or 3). Descriptive sta-tistics for all variables are given in Supporting mater-ial (S Table 2). Boxplots grouped by stance type andsequence number for D and R are displayed in Figure3 and Figure 4, respectively. For the group studied,interquartile ranges of D overlap notably (Figure 3)while interquartile ranges of R1z and R2z aremore disjoint (Figure 4). This is even more promin-ent in confidence interval values (Supporting material,S Table 2).

Visualising the results on an R-D scatter plot(Figure 5), each data point represents a successful bal-ance recovery test. The horizontal position of a datapoint corresponds to the trajectory shape as a lowerdirectional ratio indicates a trajectory with moremotion perpendicular to the perturbation. The vertical

596 B. PETR�O ET AL.

position of a data point corresponds to the effective-ness of balancing as a lower Lehr’s damping ratioindicates weaker balancing capacity.

The Shapiro–Wilk test indicated that 2 of the total12 variables are of non-normal distribution(Supporting material, S Table 3), which suggests theuse of a non-parametric test for the comparison ofdistributions. The Friedman test showed that compar-ing balancing effectiveness, there was no significantdifference between values of D (v2(5)¼ 3.987, p¼ 0.551). Comparing platform trajectories, there was asignificant difference between variables of R(v2(5)¼ 68.635, p< 0.001). Post-hoc Dunn tests

Figure 4. Boxplot of directional ratio values.

Figure 5. Scatter plot of successful balance recoveries on adirectional ratio by Lehr’s damping ratio plane, grouped bystance. Data points may overlap.

Figure 3. Boxplot of Lehr’s damping ratio values.

COMPUTER METHODS IN BIOMECHANICS AND BIOMEDICAL ENGINEERING 597

revealed that in a pairwise comparison, there was nosignificant difference amongst single-leg (R1z) oramongst bipedal (R2z) recoveries (Table 1). Meanranks are presented in Supporting material (S Table4). All pairwise comparisons of directional ratiobetween single-leg and bipedal recoveries showed asignificant (p� 0.002) difference (Table 1). The box-plot (Figure 4) shows that this difference means sin-gle-leg recoveries had lower R values thanbipedal stance.

Discussion

The role of balancing tests is becoming more promin-ent in medicine and sports sciences as well. Trainingof coordination and proprioception are used not onlyfor increasing athletic performance. These are now anintegral part of skills development for children andrehabilitation programmes for various orthopaedic(e.g. crucial ligament tear, arthrosis, joint replace-ment) and neurological alterations (e.g. stroke,dementia, Alzheimer’s disease). There is also the pos-sibility of applying these tests as diagnostic tools andresearch efforts are made to this end.

Besides static posturography, the sudden perturb-ation test is becoming prevalent (M€uller et al. 2004;Petr�o et al. 2017). Evaluation measures include per-formance measures such as the balance index (M€ulleret al. 2004), the time of balancing (Giboin et al. 2015)and Lehr’s damping ratio (Kiss 2011). The directionalratio characterises platform motion trajectory (Petr�oand Kiss 2017) and thus the recovery action itself.Clinicians opt to perform these tests with participantsadopting either a single-leg or a bipedal stance.However, to our knowledge, no study had been car-ried out to compare the performance and recoveryaction execution simultaneously between the twostances. Thus, the objective of the present study wasto determine whether balance recovery performancelevels, as measured by Lehr’s damping ratio, and plat-form motion trajectory, as characterised by the direc-tional ratio, differ between dominant single-leg andbipedal balance recoveries for the young, healthy

population. In essence, our study found that platformtrajectories differ while reaching similar perform-ance levels.

The finding that multiple single-leg (D1z and R1z)recoveries scored statistically similarly means thatrepeating the perturbation test for 3–5 times had noeffect on the measured parameters; the same can bestated for bipedal recoveries (Table 1, upper left andlower right quadrants). The Friedman test showed nosignificant difference in damping between (dominant)single-leg and bipedal recoveries (v2(5)¼ 3.987,p¼ 0.551). Earlier, the same was observed for a groupof elderly participants (Kiss 2012). This proves ourfirst hypothesis. At the same time, significant differen-ces (p� 0.002) were found in the directional ratio forall pairwise comparisons between single-leg andbipedal recoveries (Table 1, lower left quadrant),which proves our second hypothesis. This suggeststhat the two different balancing tasks can be per-formed with similar effectiveness but reaching thiseffectiveness requires a different recovery action.

The R-D plot represents the interaction betweenbalancing effectiveness and platform trajectory, whichallows for the analysis of balance recovery action andeffectiveness at the same time; in this study, by com-paring single-leg and bipedal stances (Figure 5). TheR-D plot (Figure 5) shows that without the use of R,the two distributions could not be distinguished. Theboxplot (Figure 3) shows that for D, the interquartilerange of bipedal recoveries completely overlap withsingle-leg recoveries (D¼ 4.3–7.5%), while the totalminimum-maximum range for single-leg stance is farlarger (D¼ 1.7–13.5%). The interesting finding here isthat when looking at an individual single-leg recoveryaction, it can either: succeed at completing the task,but fail to reach the same effectiveness, succeed atcompleting the task and reach the same effectiveness,or even supercede the effectiveness of bipedalstance recoveries.

Considering single-leg recoveries that fail to reachthe effectiveness of the bipedal stance (D< 4%), theseattempts tend to score lower R values (R< 5.5)(Figure 5). Inspecting the platform trajectory showsthat they are circular in nature; an example is shownin Figure 1(b). This suggests that in such cases, themore elliptical trajectory was adopted to makethe balance recovery possible rather than to increasethe effectiveness of it. Considering recoveries withsimilar D scores to bidepal recoveries (4–8%), single-leg recoveries have notably lower R values (R< 4)(Figure 5). This means that in order to reach similar

Table 1. Results of post-hoc Dunn test for directional ratiovariables; adjusted significance levels are shown.

R11 R12 R13 R21 R22 R23R11 –R12 1 –R13 1 1 –R21 <0.001 0.002 0.002 –R22 <0.001 <0.001 <0.001 1 –R23 <0.001 <0.001 <0.001 1 1 –

Bold font indicates significant values.

598 B. PETR�O ET AL.

effectiveness, single-leg recoveries need to be per-formed with a more circular trajectory.

It is noteworthy that single-leg recoveries can out-perform bipedal recoveries in terms of damping(D> 8%), consequently resulting in faster balanceregain while scoring lower than bipedal R values(R< 8). Here, the lower R-value indicates not only amore elliptical trajectory. A visual inspection of thecorresponding platform trajectories reveals that thesebalancing actions quickly turn the initial ML perturb-ation into dominantly AP directed oscillation (anexample is presented in Figure 1(d)). This ML-APchanging pattern was overall a rare observation in theparticipant group and the ability to perform thisrecovery action could possibly indicate superior bal-ancing capacity. Thus, this motion pattern is a candi-date for being selected as a positivediagnostic marker.

Let us address how these findings may be of helpto clinicians. Our study verified that recovering bal-ance after a sudden perturbation test is different (sig-nificant difference in R) in the two stances even if theachieved performance is similar (no significant differ-ence in D). Based on this, rehabilitation protocolsshould include balance training for both single-legand bipedal stances. It follows that evaluations of testresults should utilise performance and motion charac-teristic parameters simultaneously for both stances,easily visualised by an R-D plot. Our results showthat this is a meaningful measure of balance recoveryactions. This methodology might reveal whether atraining protocol improves balancing performance,changes the recovery action or achieves both.

A limitation of this study is that balance recoveryactions were analysed only in a young, healthy group; thegroup was also mixed with regards to gender. Futureinvestigations should address the effects of age, gender,lateral dominance, and different sports backgrounds onthe directional ratio. Further research is proposed to iden-tify different recovery actions and strategies, such as theML-AP changing pattern described here. Our study uti-lised only platform motion tracking since this is the mostwidely available data acquisition method in clinical prac-tice. For the identification of recovery strategies, surfaceelectromyography could also be used to better characterisethe associated neuromuscular regulatory mechanisms.

Conclusion

Recovering balance on a free oscillating platform afterthe sudden perturbation is inherently differentwhen performed in a single-leg or a bipedal stance.

However, a previous study found that elderly groupsachieve similar effectiveness in both stances if thedominant leg is used. The current study examinedand proved that this is true for young adults as welland analysed recovery actions by the resulting plat-form trajectory. It was demonstrated that in the sin-gle-leg stance, more elliptical platform trajectoriesenable slow, but successful balance recovery orimprove its performance to score similarly to bipedalrecovery. A motion pattern of single-leg recoveryaction that outperformed bipedal stance was alsoidentified. A diagnostic methodology was proposedthat utilises performance and motion characteristicparameters simultaneously. Further research of usingthis test as a diagnostic tool is suggested to incorpor-ate participants of different age, gender, lateral dom-inance, athletic background, and health with a focuson identifying further motion patterns that can indi-cate superior, normal or impaired balancing abilities.

Acknowledgements

The authors thank Gergely Nagym�at�e and Krist�of R�acz fortheir assistance with conducting the measurements.

Disclosure statement

No potential conflict of interest was reported bythe authors.

Funding

This project was supported by the Hungarian ScientificFund [grant number K115894.]

ORCID

B�alint Petr�o http://orcid.org/0000-0003-3920-5161Rita M Kiss http://orcid.org/0000-0003-3607-8435

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