THE RELATIONSHIP BETWEEN VARIOUS MODES OF SINGLE LEG POSTURAL CONTROL ASSESSMENT

Abstract

Purpose/Background:

While various techniques have been developed to assess the postural control system, little is known about the relationship between single leg static and functional balance. The purpose of the current study was to determine the relationship between the performance measures of several single leg postural stability tests.

Methods:

Forty six recreationally active college students (17 males, 29 females, 21±3 yrs, 173±10 cm) performed six single leg tests in a counterbalanced order: 1) Firm Surface-Eyes Open, 2) Firm Surface-Eyes Closed, 3) Multiaxial Surface-Eyes Open, 4) Multiaxial Surface-Eyes Closed, 5) Star Excursion Balance Test (posterior medial reach), 6) Single leg Hop-Stabilization Test. Bivariate correlations were conducted between the six outcome variables.

Results:

Mild to moderate correlations existed between the static tests. No significant correlations existed involving either of the functional tests.

Conclusions:

The results indicate that while performance of static balance tasks are mildly to moderately related, they appear to be unrelated to functional reaching or hopping movements, supporting the utilization of a battery of tests to determine overall postural control performance.

Level of Evidence:

3b

INTRODUCTION

Subconscious maintenance of postural stability unconsciously underlies all movements, from planned skill executions to reflexive responses following unexpected perturbations. In works that have considered postural stability from an orthopedic perspective, a wide variety of tests, conditions, and variables have been utilized to determine and describe the construct of postural stability. Although several authors have proposed different approaches to categorizing many of the tests,^1,2 no single classification system has been universally adopted. While double leg stance analysis dominates the postural control literature, measuring single leg postural stability is more applicable in clinical and research sports medicine settings for a variety of reasons.

Sports and physical activities require periods of time where an individual must rely on a single leg base of support, therefore, the use of single leg tests to measure postural stability in a clinical or research sports medicine settings is both logical and warranted. Relying on a single leg base of support requires the postural control system to reorganize the total body center of mass over a narrow base of support.³ In addition to the functional applicability, assessing single leg postural stability reduces the number of peripheral sensory sources and muscle strategies that may serve to compensate for peripheral deficiencies.³ In a clinical setting, the convenience of making bilateral comparisons or examining bilateral differences in cases of unilateral orthopedic injury further contributes to the applicability of single leg tests to determine postural control ability.

Current examination systems and rehabilitation programs used with patients who have experienced a lower extremity injury often stress static (fixed based of support) stance proficiency as a prerequisite for more dynamic and functional exercises. While this suggestion makes intuitive sense, the degree to which static stance proficiency relates to dynamic and functional postural control performance remains largely undocumented. Research examining both static and/or dynamic balance ability independently is widely available, yet little research has directly examined the relationship between static, dynamic and functional balance particularly for single leg balance.^4–8 Drawing consensus across the studies considering the relationship between various modes of single leg postural control assessment is difficult because of the varying population characteristics, the different tests used and the varied results. The relationship between static single leg balance with the star excursion balance test (SEBT) and stepping/stabilization on foam has been studied the most. While Clark and colleagues⁴ reported a significant relationship between a static test and the SEBT, Nakagawa and Hoffman⁷ failed to reveal any relationship. Likewise, Hrysomallis and colleagues⁸ demonstrated a significant relationship between a static test and stepping/stabilization test while Nakagawa and Hoffman⁷ failed to reveal any relationship. With regards to the relationship between static test performance and functional activities, one study⁶ examining single leg hop for distance demonstrated that weak to moderate relationships (r = .37 to .63) with several single leg stance tests, while an earlier study⁵ failed to reveal any relationship between single leg stance stability measures measured using a forceplate and a battery of walking and coordination tasks.

Thus, based on the above results, there is clearly a need for further research examining the relationships between various modes of single leg postural control assessment so clinicians have an objective basis for selecting a particular test or training task in the evaluative and rehabilitation stages of orthopedic injury. Establishing whether relationships exist between static stance and functional postural control is the first step in providing evidence regarding whether the different testing modes may be assessing unique aspects of the construct of postural stability. Therefore, the purpose of this investigation was to determine the relationship between performances of several single leg postural stability tests in a sample of recreationally active college students.

METHODS

Subjects

Forty-six recreationally active, healthy, college students (17 males, 29 females, age=21±3 years, height =173±10 cm, mass =68±12 kg) participated in this study. All subjects were free of any lower extremity orthopedic related injuries or neurologic disorders and previous history of head injury or balance disorders as indicated by a medical history questionnaire. Prior to participation, informed consent from each participant was acquired using a local institutional review board approved consent document.

Design

Each subject attended one testing session that lasted approximately 25 minutes in duration. Within the testing session, each subject performed a battery of six single leg postural stability tests: 1) Firm Surface-Eyes Open (FIEO), 2) Firm Surface-Eyes Closed (FIEC), 3) Multiaxial Surface-Eyes Open (MAEO), 4) Multiaxial Surface-Eyes Closed (MAEC), 5) Posterior-medial reach of the Star Excursion Balance Test (SEBT), 6) Single Leg Hop-Stabilization test (SLHS). All testing procedures were conducted using the dominant limb of each participant. Dominant limb was defined as the preferred leg to use to kick a ball. Additionally, the order in which tests were administered was unique for each participant by randomly assigning a particular order from all possible testing orders.

Firm and Multiaxial Surface Testing

The eyes open and eyes closed firm surface testing was conducted using a Bertec 4060-NC Force Plate and AM-6700 amplifier (Bertec Co., Columbus, OH) sampled at 100Hz using a LabVIEW (National Instruments, Austin, TX) based acquisition program. A similar single leg testing protocol previously developed and demonstrated to be reliable by Goldie et al⁹ was used. Subjects were instructed to stand as motionless as possible, maintain their hands on the iliac crests and their non-dominant limb in 30° of hip and knee flexion (Figure 1). During each trial, subjects were encouraged, if it became necessary, to touchdown on the force platform with their non-dominant limb.⁹ For each testing condition, subjects performed one seven second practice trial followed by three identical seven second test trials. The authors own pilot work has supported the reliability for this trial duration as being compatible with 15s trials, but with the advantage of a reduced number of touch downs.¹⁰ Thirty-second rest periods were allotted between trials by allowing the subject to touch down their contralateral limb.

Figure 1.

Standardized testing position for the firm surface testing.

Similar stance and trial procedures as the firm surface testing were used for the multiaxial surface testing (Figure 2). A commercially available multiaxial surface (Gordons Balance Board, Gordons Research and Development, Inc, Pinckneyville, IL) was placed directly on the force plate. The stability of the multiaxial surface is adjustable by interchanging the column support, and/or column length. For the purposes of this investigation, a moderate stability level (green column, first hole length) was used based on pilot work with 10 additional subjects. Pilot work established the cross correlational coefficients (zero phase lag) between multiaxial platform tilt and center of pressure excursion to be .91±.06 and .80±.17 for anterior-posterior and medial-lateral, respectively, supporting the usage of a center of pressure derived measure to reflect multiaxial surface performance.

Figure 2.

Standardized testing position for the multiaxial surface testing.

In the event that three or more compensatory events occurred during the firm and multiaxial surface testing, subjects were given one re-test trial. Compensatory events were defined according to the Balance Error Scoring System (BESS).¹¹ In short, the BESS was developed as a portable and objective method of assessing static postural stability without requiring complex or expensive instrumentation. The tester scores balance performance by counting the number of errors committed during a trial. The BESS errors considered compensatory events for the current study included lifting the hands off the iliac crests, opening the eyes during eyes closed trials, stepping or stumbling, moving hip into more than 30° of flexion or abduction, and lifting forefoot or heel.

Custom software was written to conduct data reduction procedures. This included smoothing the force and moment data obtained from the force plate (10 Hz cutoff) and calculating the average center of pressure velocity during the middle 5 seconds of data.

Star Excursion Balance Test (SEBT)

The original SEBT consists of a series of 8 postural stability tests that incorporates single- leg stance of one leg with a maximum targeted reach with the contralateral leg.¹² The subject stands at the center of a grid pattern laid on the floor consisting of 8 lines extending at 45° increments from the center of the grid in a star pattern. For each respective reach, the distance the subject is able to extend the free leg and tap down on the respective grid line while maintaining a single leg stance is marked and measured from the center of the grid. The results of factor analysis revealed the posterior-medial reach to be most representative of performance in all eight directions.¹³ Thus, in the current investigation, the average of the three posterior-medial reach trials was normalized to body height and was used as the measure of SEBT performance (Figure 3). Because subjects were required to maintain their hands on their iliac crests during all other tests comprising the investigation, a modification to the original SEBT description was made to include the requirement of the hands remaining on the iliac crests during each reach. A previous study found that following a practice session (both participants and testers), the intratester reliability of the SEBT has been reported to range between .82 and .96, while intertester reliability was reported to range between .81 and .93.¹⁴ Additionally, based on the documentation concerning multiple exposure effects in the posterior medial reach direction by Hertel et al,¹⁴ four practice trials were given to the subjects prior to the test trials.

Figure 3.

Completion of the posterior-medial reach of the Star Excursion Balance Test.

Single leg Hop-Stabilization Test (SLHS)

The Single Leg Hop-Stabilization Test^15,16 requires the subject to sequentially single leg hop through a ten point floor pattern using one leg while maintaining the hands on the iliac crests (Figure 4). The dimensions of the floor pattern are modified according to the height of each subject. Subjects are instructed to begin on the start mark with their dominant foot covering the tape mark, facing forward, with the hands on the iliac crests. They are required to hop to the first tape mark, and upon landing, maintain single leg balance with their hands on their iliac crests for five seconds. At the conclusion of five seconds, the subject is then instructed to hop to the next successive tape mark, repeating the procedure through all ten points of the floor pattern. Performance during the test is determined using separate criteria for the two phases of the test (landing and balance).¹⁵ The landing phase includes landing on the tape mark and establishing body control. The balance phase encompasses the subsequent 5 seconds after body control is established during which the subject maintains single leg balance before the next jump is executed. Criteria for error in landing and balance are included in Table 1. Intertester reliability (standard error of measurement) was reported to be .92 (.57) for the landing scores and .70 (.55) for the balancing scores.¹⁵ Prior to the scored trials, in accordance with the multiple exposure effects previously documented,¹⁵ subjects were given ample opportunity to practice the hop-stabilization sequences to become familiar with the hop distances and directions as well as transitioning into static stance.

Figure 4.

Standardized testing position for completion of the landing phase of the Single Leg Hop Stabilization Test.

Table 1.

Single leg hop-stabilization test error scoring system.

Phase	Errors
Landing	Not covering tape mark
	Stumbling on landing
	Foot not facing forward with 10 degrees of rotation
	Hands off hips
Balance	Touching down with nondominant limb
	Nondominant limb touching dominant limb
	Nondominant limb moving into excessive flexion, extension or abduction
	Hands off hips

Data Analysis

Consistent with the purpose of the investigation, Pearson correlational analyses were conducted between the dependent variables describing performance of the single- leg postural stability tests. The level of significance was preset at .05 for all analyses.

RESULTS

Descriptive statistics for the dependent variables are presented in Table 2. Results of the correlational analyses between the six postural stability tests are presented in Table 3. In general, with of the exception of the relationship between the FIEC and MAEO (r=.267, p=.073), significant mild to moderate (r=.371 to .624) relationships were revealed between firm and multiaxial surfaces. No significant relationships (r=.006 to .268) were revealed between the firm and multiaxial surfaces and the SEBT or SLHS.

Table 2.

Means and standard deviations (SD) for the postural tasks.

Test	Mean±SD
Firm surface-eyes open	4.20±1.22 cm/s
Firm surface-eyes closed	10.84±4.06 cm/s
Multiaxial surface-eyes open	13.31±5.42 cm/s
Multiaxial surface-eyes closed	19.82±9.18 cm/s
STAR excursion test (posterior-medial)	38.6±8.8%BH
Single leg hop stabilize-Land errors	3.6±2.0
Single leg hop stabilize-Balance errors	3.7±1.7

BH = Body height

Table 3.

Correlational coefficients between the postural tasks.

	F1EO	FIEC	MAEO	MAEC	STAR	SLHS-Land
FIEC	r=.624
	p<001
MAEO	r=.371	r=267
	p=.011	p=.073
MAEC	r=.480	r=.561	r=.540
	p=.001	p<.001	p<.001
STAR	r=−.114	r=−.086	r=.006	r=.179
	p=.449	p=.568	p=.970	p=.234
SLHS-Land	r=.260	r=.188	r=.111	r=−.065	r=−.217
	p=.081	p=.210	p=.462	p=.669	p=.147
SLHS-Balance	r=−.089	r=.029	r=−.268	r=−.173	r=−.174	r=.328
	p=.556	p=.847	p=.071	p=.249	p=.246	p=.026

FIEO = Firm surface-eyes open

FIEC = Firm surface-eyes closed

MAEO = Multiaxial surface-eyes open

MAEC = Multiaxial surface-eyes closed

STAR = Star excursion balance test posterior-medial reach

SLHS-Land = Single leg hop stabilize test-land errors

SLHS-Balance = Single leg hop stablize test-balance errors

DISCUSSION

The major purpose of this investigation was to determine if relationships existed between performances on several single leg postural tests. Significant mild to moderate strength correlations were revealed between performance of the firm and multiaxial surface tests. The most important result was that no significant relationships were revealed between static tests and either of the more dynamic (SEBT and SLHS) tests. These results suggest that while performance on the static stabilization tasks (firm and multiaxial surfaces) appears to be interrelated, proficient performance does not relate to voluntary reaching and hopping movements. Clinically, this raises a question concerning sole use of many traditionally utilized postural control tests (i.e. stationary single leg stance on a force plate) to determining the postural control requirements associated with activities of daily living and physical activity. Rather, the results of this investigation support the notion that a battery of tests, including more functional tests that involved movement, be used to fully evaluate an individual's postural control abilities.

It is important to recognize the characteristics and limitations associated with the research design of this investigation. Correlational investigations, such as the current work, do not offer information regarding cause and effect but rather the presence and magnitude of relationships existing between various dependent variables. Thus, the results of the current investigation must be prudently considered to suggest that static stabilization task (firm and multiaxial) proficiency does not statistically predict either aptitude or inability to perform the voluntary movement tasks (SEBT and SLHS). Further research using an experimental design with static and dynamic postural control training methods is needed to more completely understand the degree to which static stabilization performance may contribute to dynamic and functional balance abilities.

The current study was unique in considering two static stabilization tasks on two different surfaces under both eyes open and closed conditions. With the exception of the relationship between the firm surface-eyes closed and multiaxial surface- eyes open, the only significant relationships revealed in the current study involved these four tests. This was not unexpected as all four tests had a similar objective of directing conscious attention solely upon standing as motionless as possible. Interestingly, in contrast to the moderate strength relationship revealed between performances on the two surfaces under eyes closed, the relationship under eyes open was weak. One possible immediate statistical interpretation of this result may be attributable to the small range of scores for the firm surface- eyes open test. However, because the strongest relationship was demonstrated between the two tests with the smallest range of scores, the eyes open and closed tests on the firm surface, thus, this potential explanation is not the only explanation to be considered.

A second possible explanation for the differences in relationships between the firm and multiaxial surfaces under similar visual conditions may be related to the challenges imposed upon the sensory and motor components of postural control system by the surface and visual conditions. Compared to double leg stance, single leg stance is associated with decreased stability secondary to the narrower base of support, which seems to increase the reliance on visual information, as well as increase the amount of corrective action required at the ankle, knee, hip and trunk.¹⁷ Standing on an unstable surface has been suggested to require faster stabilization mechanisms that originate from proprioceptive input,^18,19 which in turn, increases ankle and trunk corrective action necessary to maintain equilibrium.¹⁷ Thus, in the current study, performance not being correlated between the surfaces with the eyes open may indicate that the challenges imposed may be challenging the postural control system differently or that ability of subjects to adapt to the different challenges is inconsistent. Further research is needed to better clarify how different surfaces, coupled with changes in visual conditions, affects the postural control system.

In addition to the aforementioned studies in the introduction considering the relationships between various single leg balance tests in young adults, Ekdahl et al⁵ considered the relationship between the time required to walk 30m and a coordination task (concomitant arm and contralateral leg flexion occurring within 15s) with traditional forceplate measures of single leg stability (center of pressure path, velocity and area) in 152 healthy participants (78 males, 74 females, aged 20–64 years). Similar to the current investigation, the authors failed to reveal significant relationships between the voluntary movement and stability tasks. Similarly, Lindmark et al²⁰ examined the relationship between performance of a battery of functional tasks and stability during single leg eyes-open stance (center of pressure excursion velocity) in a population of 100 menopausal women. The results of their investigation failed to reveal any significant relationships with the exception of a weak relationship involving the figure eight walk test (r=.25). Coupled with the results of studies described in the introduction to this paper, the current results of static stabilization performances not relating to the two more dynamic test performances adds further support to the notion that the predictive value of static balance performance to dynamic balance is negligible.

Qualitative analysis of the postural stability tasks used in the current investigation can offer additional insight into the results attained. Close inspection of the demands imposed by the voluntary movement tests suggests that both tests (SEBT and SLHS) required some degree of proficient single leg stabilization for successful completion. The external constraints imposed by all tests in this investigation included the hands remaining on the iliac crests to decrease the amount of compensatory motion arising from upper limb movement. Specifically, during the completion of the entire SEBT participants had to remain in single leg stance equilibrium while the contralateral extremity was extended in a posterior-medial direction. While the quantity of corrective action required to remain in equilibrium was not a direct component of the scoring, it logically follows that being able to control the body's center of mass (COM) over a fixed base of support would have been a prerequisite skill. Likewise, the end of the SLHS landing phase required the participants to control movement of their body's COM following ground contact over a fixed single leg base of support. This was then followed by a brief stance period in which stance was maintained with minimal corrective action. Intuitively, a fixed base of support control would also be a logical prerequisite for optimal SLHS performance. Despite the attractiveness of this theory, no significant relationships between the tasks were revealed. One explanation for the lack of significant relationships revealed could be related to the concentration being focused solely on stabilization during the firm and multiaxial surface tests, whereas during the SEBT concentration was on reaching and during the SLHS concentration was divided between stabilization and anticipating the next hop.

An additional explanation for the lack of significant relationships between the two types of tasks could be that the limiting factor in the voluntary tasks did not reside in the ability to stabilize, but rather in other component(s) of the postural control system. Intuitively, both the SEBT and SLHS appear to require a heightened level of coordination about the ankle, knee, and hip joints. Factoring into the coordination patterns selected is the amount of muscle strength and endurance, as well as available range of motion about each of the joints.³ Again, it is recommended that future research consider using true experimental designs to further explore the factors contributing to proficient performance of the voluntary movement tasks.

It is important to recognize some of the limitations associated with this study. First, this study only used healthy subjects aged 18 to 30 years old. This decision was made to reduce any potentially confounding factors (musculoskeletal, neurologic or vestibular pathologies) on the relationships revealed. The degree to which these results would apply to persons with orthopedic pathology is unknown. In addition, the resulting sample involved a disparity in the number of men and women participants. With largely only firm surface stance being considered, the literature is inconsistent regarding whether there are a sex differences in balance performance. While some studies suggest women have better balance than men,^5,21–23 other studies have revealed better balance in men.^24,25 Complicating the question of whether sex differences exist in balance performance are the differences in anthropometric characteristics (i.e., height, weight, body mass index, etc.) and physical activity history. For the current investigation, even if sex differences in balance performance exist, there does not appear to be any evidence in the literature to suggest an interaction between sex and the relationship between various modes of single leg balance assessment. Nonetheless, this should be regarded as a possible limitation and an area for future research. Finally, we did not formally conduct a reliability study specific to the current investigation. The decision to forego a reliability study was made because of the experience the investigators had with developing several of the tests and methods used in the current investigation,^{10,11,15,17,26} coupled with the extensive protocol rehearsal that was conducted in preparation for the current investigation. Thus, while it is unlikely that the reliability of the testing conducted should deviate from previously published reports, because reliability was not formally document as part of the current investigation it should be recognized as a limitation.

CONCLUSION

Because a strong correlation between the static stabilization tasks and the voluntary movement tasks was not evident, it can be deduced that these types of tasks measure different constructs, each taxing different postural control mechanisms. Until the underlying differences in the tasks are tested and identified, this investigation can be considered to provide a rationale for assessing postural control in young, physically active individuals through use of a battery of tasks. It is suggested that clinicians consider evaluating and exercising the postural control system through both traditional single leg stance activities as well as voluntary, more functional movements. Further research is needed to identify which aspect of the postural control system each task optimally targets and assesses, as well as the limitations to performance of the various tasks.