Noise-enhanced dynamic single leg balance in subjects with functional ankle instability

2012-12-06ScottRossBrentArnold

Journal of Sport and Health Science 2012年2期

Scott Ross*,Brent Arnold

Virginia Commonwealth University,Richmond,VA 23284,USA

Scott Ross*,a,Brent Arnold

Virginia Commonwealth University,Richmond,VA 23284,USA

Background:Stochastic resonance stimulation(SRS)transmits subsensory electricalGaussian white noise into the body to enhance sensorimotor function.This therapy has improved static single leg balance in subjects with functionalankle instability.However,the effectof this stimulation on dynamic single leg balance is not known.Improvements in dynamic single leg balance with SRS may have implications for enhancing functional rehabilitation for ankle instability.Thus,the purpose of this study was to determine the effects of SRS on dynamic single leg balance in subjects with functional ankle instability.

Methods:This study was an experimental research design and data were collected in a sports medicine research laboratory.Twelve subjects with functionalankle instability(69±15 kg;173±10 cm;21±2 years)reported a history of ankle sprains and instability atthe ankle with physical activity.A single leg jump-landing test was used to assess dynamic balance.Subjects were required to jump between 50%and 55%of the maximal vertical jump height,land on a single leg atop a force plate,and stabilize as quickly as possible.Jump-landing tests were performed with and without SRS.Three trials were performed for each treatment condition(SRS and control).A randomized block design was used to determine testorder.Anterior/posterior and medial/lateraltime-to-stabilization were computed to assess dynamic balance.Lessertime indicated better stability.One-tailed paired samples t tests were used for analysis(α≤0.05).

Results:SRS improved anterior/posterior time-to-stabilization(stochastic resonance=1.32±0.31 s,control=1.74±0.80 s,p=0.03),butdid not enhance medial/lateral time-to-stabilization(stochastic resonance=1.95±0.40 s,control=1.92±0.48 s,p=0.07).

Conclusion:Clinicians mightuse SRS to facilitate balance improvements with sagittalplane dynamic single leg balance exercises that patients may not be able to perform otherwise.

Chronic;Posture;Sprain;Stochastic resonance

1.Introduction

Ankle sprains are common injuries thatoccur during physical activity,and this pathology has been linked to health impairments.1—3Functionalankle instability(FAI)often occurs following sprains,causing sensations of instability at the ankle and recurrentankle sprains.4The causalfactors of FAI are not fully understood,but researchers indicate that deficits in sensorimotor function,eversion strength,and balance are associated with this injury.5—7These factors are not mutually exclusive and may be linked in a way that allows one impairment to exacerbate another.5For example,researchers have identified sensorimotor impairments associated with FAI as being one source of poor balance.5Interestingly,balance deficits are importantto identify because these impairments have been indicative ofankle sprains.8As a resultofbalance deficits association with FAI,clinicians include both sensorimotor and balance exercises in rehabilitation protocols to prevent recurrentsprains and to improve ankle stability.

Therapeutic exercises or devices that facilitate balance improvements may have implications for enhancing rehabilitation by allowing patients to perform exercises earlier in the healing process.A complimentary therapy known as stochastic resonance stimulation(SRS)can facilitate balance improvements immediately9or more quickly than rehabilitation alone.10,11SRS introduces subsensory Gaussian white noise (either electrical or mechanical)through the skin to enhance the ability of mechanoreceptors to detect and transmit weak sensory signals.12,13This noise can add constructively to subthreshold signals to make detectable signals and can change ion permeability to bring membrane potentials closer to threshold.14,15Evidence indicates that muscle spindles can be affected by SRS,allowing these mechanoreceptors to detect afferent signals and,in turn,increase efferent output.13As a result,researchers have investigated the treatmenteffects of SRS on balance because muscle spindles are crucial for initiating reflexive muscle contractions that positively impact postural stability.9—11,16—18

SRS has immediately improved static balance in healthy individuals,patients with sensorimotor deficits,and individuals with FAI.9—11,16—18These immediate enhancements occur while a person receives SRS during a balance task.Interestingly,SRS may be better for improving balance in individuals with sensorimotor dysfunction than those without impairments.17A recentresearch reportsupports the effectiveness of SRS for enhancing balance in individuals with FAI who have sensorimotor deficits.9Static single leg balance was improved by 8%when subjects with FAI who were administered SRS during a balance task.9These immediate improvements may serve to permit individuals with FAI to perform balance activities during therapy thatthey mightnotbe able to perform otherwise.

However,a dynamic balance test may be more useful than a static assessment for determining the effects of SRS on function.Dynamic balance is important to examine because individuals report symptoms of ankle instability while performing physical activity and dynamic stability is necessary for completing functional therapeutic exercises in rehabilitation.Single leg jump-landing tests have been used to assess the effects of FAI on dynamic balance.19—21A common measure used to assess dynamic balance is time-to-stabilization(TTS),which has been reported as an accurate test for identifying anterior/posterior(A/P)and medial/lateral(M/L) posturalstability deficits associated with FAI.19—21In addition, TTS has been used to assess treatment effects of coordination training with and without SRS on single leg dynamic balance.11Thus,TTS is an appropriate measure for assessing the immediate treatment effects of SRS on dynamic balance and it has potential for providing an indication of how individuals might perform functional balance activities in rehabilitation.

The usefulness of SRS forimmediately improving dynamic single leg balance may enhance rehabilitation for FAI.While in theory this therapy may be clinically effective,no evidence has been published on the immediate effects of SRS on dynamic single leg balance in subjects with FAI.We believe thatthis significantgap in literature needs addressed to clarify potentialbenefits of SRS on dynamic single leg balance.Thus, the purpose of this study was to determine immediate benefits of SRS on A/P and M/L TTS in subjects with FAI.We hypothesized that A/P and M/L TTS would improve with SRS over a control condition.

2.Materials and methods

2.1.Subjects

Subjects read and signed a consent form approved by the Committee forthe Protection of the Rights of Human Subjects prior to their participation in this study.Five males and seven females with unilateral FAI(69±15 kg;173±10 cm;21±2 years)participated in this study.Seven subjects had FAI on their dominant leg(leg used to kick a ball),while the remaining five subjects had FAI on their non-dominant leg. The inclusion criteria for FAI were a minimum of one ankle sprain that required immobilization,report at least two“giving-way”sensations at the ankle within the past year,and participate in physical activity for more than 3 h per week. Subjects reported an average of3±1 ankle sprains and 5±4“giving-way”sensations within the 12 months prior to their participation in this study.Additionally,subjects had an average score of 31±5 on the Ankle Joint Functional Assessment Tool(AJFAT)(values equalto or greater than 26 are indicative of FAI).21Potential subjects were excluded if they sustained an ankle sprain within 6 weeks of inquiring about participating in this study.Additional exclusion criteria were a history of lower extremity injuries(other than sprains of the ankle)and impairments that affected balance(e.g., vestibular or visual impairments).Mechanical ankle joint instability was neither an inclusion or exclusion criteria.

2.2.Single leg jump-landing protocol

First,we assessed subjects maximum verticaljump height. The starting position for this maximum verticaljump testwas 70 cm from a Vertec(Sports Imports,Columbus,OH,USA).19The Vertec has adjustable plastic rods thatcan be setto specific heights to assess maximum jump height.Subjects were permitted to use a jump technique that allowed them to jump maximally;however,they were required to perform a twofooted takeoffand jump from a standing position.Subjectswere not allowed to take steps prior to jumping.The maximum vertical jump height was assessed three times,and the highest jump was recorded as the subject’s maximum jump height.

The single leg jump-landing testwasthen performed.Plastic rodson the Vertec were setat50%—55%ofsubjects’maximum jump heights.19Subjects began this teststanding 70 cm away from the Vertec,which was aligned with the center of a force plate(Bertec force plate model#4060;Bertec Corp., Columbus,OH,USA).19They were then instructed to use a jumping technique thatallowed them to generate enough force to reach between 50%and 55%of their maximum jump height with their fingertips.19Subjects were required to reach atleast the 50%percentmark,butcould notjump higher than 55%of theirmaximum jump height.19They were allowed to swing their arms during the jump,butwere required to hold their reaching arm at 180 degrees of shoulder flexion after taking off.19This reaching arm was ipsilateralto the leg with FAI.Aftertouching within the 50%—55%range,subjects landed on their leg with FAI atop the force plate,stabilized quickly,and remained as motionless as possible in a single leg stance for 20 s.

Single leg jump-landing tests were performed under SRS and control(no SRS)conditions.Stochastic resonance stimulator units(Afferent Corp.,Providence,RI,USA)with surface electrodes(2×2 cm)self-adhesive gel pads(Model Platinum 896,230,Axelgaard Mfg.Co.,Ltd.,Fallbrook,CA, USA)were placed on the skin over the muscle bellies of the lateral soleus,peroneus longus,and tibialis anterior.9Additionally,electrodes were placed on the anterior talofibular ligament and deltoid ligament.Stimulators delivered SRS via subsensory electrical noise(Gaussian white noise,zero mean, SD=0.05 mA)to ankle muscles and ligaments.The noise amplitude of 0.05 mA has been used in previous SRS studies to improve balance.9

Three practice trials were performed prior to data collection.Then,subjects performed three trials for each treatment condition.A randomized block design was used to determine test order for SRS and control conditions.Subjects were blinded to treatment conditions because SRS was subsensory. During SRS trials,the device was turned on and subjects were then instructed to jump immediately.The SRS was then shut off after subjects stepped off of the force plate.Lastly,subjects were retested if they failed to jump within the 50%—55% range,hopped on their test leg after landing,or touched the ground with their non-weight bearing leg after landing.

2.3.Data collection and reduction

Ground reaction force data were collected from the force plate ata sampling rate of 180 Hz.19Signals were then passed through a BNC adapter chassis that was interfaced with an analog-to-digital board within a personal computer.These signals were then converted to ground reaction force vectors and moments.Data were filtered using a second order recursive low-pass Butterworth digital filter with an estimated optimum cutoff frequency of 12.53 Hz.19

A customized LabVIEW(National Instruments Corp., Austin,TX,USA)software program computed A/P and M/L TTS.A/P and M/L components of the ground reaction force data were analyzed separately for each subject,but the same procedure was used for both components.First,the last10 s of the ground reaction forces were analyzed to find the smallest absolute ground reaction force range for each component.19These ranges were accepted as the optimal range of variation values.19A/P and M/L components of the ground reaction force data were then rectified.19An unbounded third order polynomial was fit from the peak force to the last data point for each component.19TTS for each component was the point where the unbounded third order polynomial was equal to or less than the respective optimal range of variation value.19

2.4.Statistical analysis

Average A/P and M/L TTS values for each treatment condition were computed in PASW version 18.0(SPSS,Inc., Chicago,IL,USA).Alpha levelwas set a priori at p≤0.05 to indicate statistical significance.One-tailed paired samples t tests compared SRS to control conditions for A/P and M/L TTS.Effect size d values were calculated for each t test.22Average percent improvements for each TTS measure were also computed for all subjects and average improvement of eight subjects who improved with SRS(subjects who did not improve were removed).No improvements were defined as increased TTS with SRS over a control condition.Lastly,to provide insight on why some subjects did not improve with SRS,we computed effect size d values for comparing responders and non-responders on frequency of sprains, frequency of“giving-way”,and score on the AJFAT.

3.Results

SRS significantly improved A/P TTS over the control condition(SRS=1.32±0.31 s,Control=1.74±0.80 s; t(11)=-2.04,p=0.03;d=0.76).The average percent improvement for A/P TTS with SRS was 24%(n=12)and increased to 34%(n=8;SRS=1.32±0.35 s, Control=2.01±0.86 s)when four subjects who did not improve were removed.SRS did not affect M/L TTS (SRS=1.95±0.40 s,Control=1.92±0.48 s;t(11)=-0.20, p=0.42;d=-0.07).The average percent improvement for M/L TTS with SRS was 2%(n=12)and increased to 15% (n=8;SRS=1.75±0.30 s,Control=2.06±0.50 s)when four subjects who did notimprove were removed.

Using effectsize d values to detectmean differences,nonresponders had greater mean values than responders on frequency of sprains,frequency of“giving-way”,and score on the AJFAT.Small effect size d values were found for comparing non-responders and responders for frequency of sprains(non-responders=3.00±1.12, responders=2.71±0.94;d=0.28)and frequency of“giving-way”(non-responders=5.50±4.70, responders=4.15±3.76;d=0.32).A high effect size d value was found for comparing non-responders and responders on the AJFAT(non-responders=33.38±4.34, responders=29.79±4.35;d=0.83).

4.Discussion

The most important finding of this study was that SRS delivered to the lower leg muscles and ankle ligaments improved dynamic single leg balance by reducing A/P TTS in subjects with FAI.These findings support the use of subsensory noise as an effective therapy for improving sagittal plane dynamic single leg balance.We did not identify specific neural mechanisms for improving balance with SRS in this study,but we suspect based on the stochastic resonance literature that this complimentary therapy facilitated afferent signal detection and efferentoutput.12,13

Increasing dynamic stability with SRS may have implications on reducing recurrent sprains and allowing individuals with FAIto perform balance exercises in rehabilitation thatthey may notbe able to perform successfully withoutthe use ofSRS. Our current results indicate that A/P dynamic balance was improved by 24%.Previous research has indicated that A/P TTS deficits associated with FAIrange between 22%and 40% when comparing FAIto stable ankles.11,19—21Ourresults ofthis current study indicate that SRS returns A/P TTS to within normal limits of stable ankles.Previous research has also demonstrated thatSRS was effective in improving static single balance in subjects with FAI by 8%over a control condition.9Thus,clinicians may use this complimentary therapy to facilitate static single leg balance and sagittal plane dynamic single leg balance.This therapy may be criticalfor individuals with FAIwho cannotbalance on a single leg or perform single leg hop exercises effectively during rehabilitation.SRS may allow these individuals to perform dynamic single leg balance exercises earlier in therapy,which may facilitate and enhance rehabilitation.Clinically,this SRS treatmenteffectmay translate to reducing recurrent ankle sprains.Researchers have indicated thatbalance training decreasesankle sprain injury and improvements in balance between 4%and 9%have been associated with a reduction in sprains.23Our immediate effect exceeds these improvements,which is one reason we conjecture that this therapy may have implications for decreasing ankle sprains.This theory is purely speculative because we did notstudy the effects of SRS on recurrentankle sprains.Future research should explore the clinical effectiveness of SRS on reducing recurrent ankle sprains in subjects with FAI.

Afferent signal detection is critical for initiating postural reflexive muscle contractions that enhance balance and SRS may facilitate balance improvements because of its ability to increase sensory feedback.Several neural mechanisms exist for SRS to enhance the ability of mechanoreceptors to detect sensory signals.Electrical subsensory noise transmitted transcutaneously can add constructively to subthreshold signals to create suprathreshold ones that can be detected by mechanoreceptors.14In addition,this subsensory noise can stimulate mechanoreceptors to bring membrane potentials closer to threshold by changing ion permeability.15Thus, mechanoreceptors are primed to fire in the presence of real sensory signals,especially subsensory signals that would typically go undetected.15SRS can also contribute to preceding influential activity that converges on gamma motor neurons.13Neurologically,input arising from mechanoreceptors(e.g.,cutaneous,muscle spindle,Golgi tendon organs, articular)increase gamma motor neuron activation.SRS that influences gamma motor neurons can,in turn,activate muscle spindles.13Through these direct and indirect pathways,SRS sensitizes muscle spindles to detect sensory signals that are importantfor maintaining balance and dynamic jointstability.

A link between sensorimotor deficits associated with FAI and poor single leg balance has been established,and theoretical framework is developing to explain how individuals with ankle instability cope with impairments to maintain balance.5,24Recently,McKeon etal.24have used the dynamic systems perspective to explain why ankle instability may cause a re-weighting of the sensory system to provide feedback relevant for maintaining balance.Sensory impairments reduce the degrees of freedom(defined as the interaction between the task,organism,and environment)along the lower extremity kinetic chain to decrease the variability in movement execution,making kinetics more predictable.24In the case of ankle instability,movement variability may be decreased because sensory deficits from the organism reduces the degrees of freedom.As a result,the sensorimotor system re-weights sensory input to available functioning mechanoreceptors to allow successful completion of a movement.24During single leg balance,McKeon etal.24speculated that plantar cutaneous receptors and mechanoreceptors in the triceps surae input are re-weighted to provide sensory feedback necessary to make sagittal plane movement less variable and,therefore,more predictable for maintaining stability when mechanoreceptors in ankle ligaments are unavailable.24Although re-weighting sensory input facilitates balance to some degree,sagittalplane instabilities will still be present because maximal input from damaged mechanoreceptors is not available.24Based on the aforementioned information,we speculate that the SRS may have facilitated this re-weighting process to improve dynamic single leg balance.However,SRS could also have allowed ineffective mechanoreceptors to reach threshold and transmit sensory information vitalfor enhancing sagittalplane stability.

We may nothave maximized our treatmenteffects because we did not optimize the noise intensity.Researchers indicate that enhancements with SRS can be optimized at a specific input intensity.9,13Essentially,improvements with SRS will increase to a maximum intensity and decrease thereafter;often worsening compared to a control condition as the intensity approaches threshold.13This phenomenon is often described as stochastic resonance behavior,which can be presented as an inverted“U”shape when plotting percent improvement over a control condition.A limitation to this study is our use of a single subsensory intensity forallsubjects,which could have limited the treatment effect when small percentage improvements for some subjects were combined with high percentage improvements of others.For example,A/P TTS percent improvements with SRS increased 10%when four subjects who did not improve with SRS were removed from analysis. We want to note that this increase was due mainly to the control average A/P TTS value increasing.Furthermore,we did not find improvements in frontalplane dynamic single leg balance.However,M/L TTS percent improvements with SRS increased by 13%when foursubjects who were impaired with SRS were removed from analysis.This increase percentage was due to the SRS M/L TTS value decreasing.Perhaps using an optimized intensity would have produced immediate SRS effects in all subjects.Although the stimulation intensity was not optimized,we want to mention that using the same subsensory intensity for all subjects is the most widely accepted protocol in the SRS literature.

Our analysis comparing responders and non-responders indicates thatthe degree of ankle instability may be a contributing factorto responding(ornotresponding)to SRS.In other words,subjects with greater instability did not improve with SRS.We operationally defined degree of ankle instability by examining the frequency ofsprains,frequency of“giving-way”, and score on the AJFAT.Those with more sprains and“givingway”may have a greaterdegree ofinstability and subjects with greaterscoreson the AJFAT have a decreased ability to perform functionalactivities because of the presence of FAI.Oursample size was smalland we elected to use effectsize d values over t tests to examine potentialdifferences in response.Our d values ranged between 0.28 and 0.83,indicating thatnon-responders had greater means than responders and mean differences between groups should be statistically detectable given adequate power.Future research may explore how these ankle instability factors affectresponse to SRS.

5.Conclusion

We found that SRS is effective forimproving sagittalplane dynamic single leg balance in subjects with FAI.However,this therapy did not improve frontal plane dynamic balance. Clinicians might use this complimentary therapeutic device to facilitate balance improvements with sagittal plane dynamic single leg balance exercises that patients may not be able to perform otherwise.Future research can explore the effects of an optimal SRS intensity on improving dynamic single leg balance in subjects with FAI.

1.McKay G,Goldie P,Payne W,Oakes B.Ankle injuries in basketball: injury rate and risk factors.Br J Sports Med 2001;35:103—8.

2.Verhagen E,van der Beek A,Bouter L,Bahr R,van Mechelen W.A one season prospective cohort study of volleyball injuries.Br J Sports Med 2004;38:477—81.

3.Arnold B,Wright C,Ross SE.Functional ankle instability and healthrelated quality of life.J Athl Train 2011;46:634—41.

4.Tropp H.Commentary:functional ankle instability revisited.J Athl Train 2002;37:512—5.

5.Sefton JM,Hicks-Little CA,Hubbard TJ,Clemens MG,Yengo CM, Koceja DM,et al.Sensorimotor function as a predictor of chronic ankle instability.Clin Biomech 2009;24:451—8.

6.Arnold B,de la Motte S,Linens S,Ross S.Ankle instability is associated with balance impairments:a meta-analysis.Med Sci Sports Exerc 2009;41:1048—62.

7.Arnold B,de la Motte S,Linens S,Ross SE.Concentric evertor strength differences and functional ankle instability:a meta-analysis.J Athl Train 2009;44:653—62.

8.McGuine T,Greene J,BestT,Leverson G.Balance as a predictorof ankle injuries in high school basketball players.Clin J Sport Med 2000;10:239—44.

9.Ross S.Noise-enhanced posturalstability in subjects with functionalankle instability.Br J Sports Med 2007;41:656—9.

10.Ross S,Arnold B,Blackburn J,Brown C,Guskiewicz K.Enhanced balance associated with coordination training with stochastic resonance stimulation in subjects with functional ankle instability:an experimental trial.J Neuroeng Rehabil 2007;4:47.

11.Ross S,Guskiewicz K.Effect of coordination training with and without stochastic resonance stimulation on dynamic posturalstability of subjects with functional ankle instability and subjects with stable ankles.Clin J Sport Med 2006;16:323—8.

12.Collins JJ,Imhoff TT,Grigg P.Noise-enhanced tactile sensation.Nature 1996;383:770.

13.Cordo P,Inglis JT,Verschueren S,Collins JJ,Merfeld DM,Rosenblum S, et al.Noise in human spindles.Nature 1996;383:769—70.

14.Moss F,Ward LM,Sannita WG.Stochastic resonance and sensory information processing:a tutorial and review of application.Clin Neurophysiol2004;115:267—81.

15.Bezrukov SM,Vodyanoy I.Noise-induced enhancement of signal transduction across voltage-dependent ion channels.Nature 1995;378:362—4.

16.Priplata AA,Patritti BL,Niemi JB,Hughes R,Gravelle DC,Lipsitz LA, et al.Noise-enhanced balance control in patients with diabetes and patients with stroke.Ann Neurol 2006;59:4—12.

17.Priplata A,Niemi J,Harry J,Lipsitz L,Collins J.Vibrating insoles and balance control in elderly people.Lancet 2003;362:1123—4.

18.Priplata A,Niemi J,Salen M,Harry J,Lipsitz L,Collins J.Noiseenhanced human balance control.Phy Rev Lett 2002;89:238101.

19.Ross S,Guskiewicz K.Examination of static and dynamic postural stability in individuals with functionally stable and unstable ankles.Clin J Sport Med 2004;14:332—8.

20.Ross S,Guskiewicz K.Single-leg jump-landing stabilization times in subjects with functionally unstable ankles.JAthlTrain 2005;40:298—304.

21.Ross S,Guskiewicz K,Gross M,Yu B.Assessment tools for identifying functional limitations associated with functional ankle instability.J Athl Train 2008;43:44—50.

22.Cohen J.Statistical power analysis for the behavioral sciences.2nd ed. Hillsdale,NJ:Lawrence Erlbaum Associates;1988.

23.Eils E,Rosenbaum D.A multi-station proprioception exercise program in patients with ankle instability.Med Sci Sports Exerc 2001;33:1991—8.

24.McKeon P,Booi M,Branam B,Johnson D,Mattacola C.Lateral ankle ligament anesthesia significantly alters single limb postural control.Gait Posture 2010;32:374—7.

Received 29 March 2012;revised 16 May 2012;accepted 18 May 2012

*Corresponding author.

E-mailaddress:seross@uncg.edu(S.Ross)

aPresent address:The University of North Carolina at Greensboro,Departmentof Kinesiology,250 HHP Building Greensboro,NC,USA.

Peer review under responsibility of Shanghai University of Sport

Production and hosting by Elsevier

Journal of Sport and Health Science

2012年2期