Postural Control Strategies are Dependent on Reach Direction in the ...

RESEARCH REPORT © 2016 Human Kinetics - IJATT 21(6), pp. 33-39 http://dx.doi.org/10.1123/ijatt.2016-0004

Postural Control Strategies are Dependent on Reach Direction in the Star Excursion Balance Test Tyler R. Keith, ATC, Tara A. Condon, ATC, Ayana Phillips, Patrick O. McKeon, PhD, ATC, CSCS, and Deborah L. King, PhD • Ithaca College The Star Excursion Balance Test (SEBT) is a valid and reliable measure of dynamic postural control. Center of pressure (COP) behavior during the SEBT could provide additional information about direction-dependent SEBT balance strategies. The purpose of this study was to quantify spatiotemporal COP differences using COP area and velocity among three different SEBT reach directions (anterior, posteromedial, posterolateral). The anterior direction COP velocity was significantly lower than both posterior directions. However, the anterior COP area was significantly greater than posterior. Based on COP behavior, the anterior and posterior reach directions appear to use different postural control strategies on the SEBT. Key Words: center of pressure, dynamic balance, postural stability

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alance is an essential ability that is necessary for all human movement. Dynamic balance, or dynamic postural control (DPC), is Key Points defined as maintaining Center of pressure behavior is different for a balanced base of supthe anterior and posterior reach directions port while completing in the Star Excursion Balance Test (SEBT) a movement action.1 In for healthy adults. clinical settings, the Star Excursion Balance Test Center of pressure area was greater for (SEBT) is often used to anterior versus posterior reaches, however assess DPC. It has excelcenter of pressure velocity was lower for lent reliability and has the anterior compared with the posterior become a practical tool reaches. for evaluating dynamic balance deficits.1–4 The Based on the link between center of SEBT has been used pressure and reach direction, it is apparent to identify individuals that the anterior and posterior directions with pathological musuniquely challenge the maintenance of culoskeletal conditions dynamic balance. of the lower extremity, INTERNATIONAL JOURNAL OF ATHLETIC THERAPY & TRAINING

such as chronic ankle instability (CAI),5–8 patellofemoral pain,9 and anterior cruciate ligament (ACL) deficiency.10 It has also been used to identify those at an increased risk of injury and to measure improvements related to rehabilitation or training outcomes.11–19 Overall, DPC is an important sensorimotor control factor related to lower extremity injury and rehabilitation and the SEBT is a valid and reliable DPC assessment tool. Although the SEBT has been an advantageous clinical tool, understanding the contributing factors to SEBT performance can increase our understanding of factors that regulate DPC. It is widely accepted that the SEBT involves aspects of strength, balance, and range of motion.1 Evidence supports a connection between the anterior reach direction and weight bearing ankle dorsiflexion1,20 and that posteromedial and posterolateral reaches may be dependent on proximal NOVEMBER 2016  33

factors such as hip strength.1,21 However, despite the SEBT being a clinical test of DPC, it is unclear how postural control strategies are used during the SEBT. The most common method for assessing balance is capturing center of pressure (COP) excursions during a static or dynamic balance task on a force plate.21 COP excursions are calculated from the three-dimensional forces and moments derived from the interaction between the ground and the base of support as a person attempts to maintain standing balance. By examining the velocity and area of COP excursions, it is possible to gain insight into the temporal and spatial elements of postural control. These variables have been shown to be sensitive to detecting deficits in postural control related to lower extremity injury and rehabilitation.22–24 The assessment of the spatial and temporal characteristics of COP excursions during SEBT performance would enhance our current understanding of how the directions of the SEBT uniquely challenge DPC. From this information, further insight of the postural control strategies used may improve understanding of causative mechanisms of direction-dependent reach performance on the SEBT. Understanding the relationship between SEBT performance and COP behavior in healthy individuals specifically establishes a baseline for comparison in future research involving unhealthy individuals. Therefore, the purpose of this study was to quantify spatial and temporal differences in the COP profiles across the three different reach directions in the SEBT. We hypothesized that each direction would uniquely challenge DPC and there would be detectable direction-dependent differences in COP excursion temporal and spatial characteristics.

Instrumentation Ground reaction force (GRF) data were sampled at 200 Hz from an AMTI portable force plate (ACS-+, Watertown, MA) during the SEBT using Balance Clinic software (v2.02.02 AMTI Watertown, MA). GRFs were recorded from the start of the task, with the subject balancing on one foot, to the end of the task, with the subject back in the static one-footed balancing position. Each trial included two static single leg balance portions (one at the beginning and one at the end of each reach), and a dynamic balance portion during the reaching task itself. Average COP excursion velocity and COP excursion area, defined as a 95% COP confidence ellipse, were calculated from the COP positional data (Figure 1) using Balance Clinic software. COP range and COP length of travel, total distance of path traveled, in the anterior-posterior (AP) and medial-lateral (ML) directions, and mean AP and ML COP positions were also calculated (LabVIEW 2013, National Instruments, Austin, TX). Measuring tapes (1.5 m in length) aligned with directions of reach and secured to the platform were used to record maximal reach distance for each trial of the SEBT (Figure 2).

Tasks Three reach directions of the SEBT, anterior (ANT), posterolateral (PL), and posteromedial (PM), were selected

Methods Participants Twenty-eight healthy, college-aged students (13 females, 15 males, age = 19.75 ± 1.04 y, height = 171.43 ± 12.31 cm, mass = 78.72 ± 22.60 kg) volunteered to participate in the study. Subjects were excluded if they had an ankle injury in the last 6 weeks, had a concussion in the last 6 months, or had known musculoskeletal lower extremity injuries or disorders that affect balance. Before participating, all subjects gave written informed consent in accordance to Ithaca College Human Subjects Review guidelines.

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Figure 1 Demonstration of a 95% center of pressure (COP) confidence ellipse, calculated from the COP positional data using Balance Clinic software. (Recording from subject 17 reaching in the posterolateral direction of the Star Excursion Balance Test).

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for analysis based on recommendations of Gribble et al.1 Each subject completed four practice trials and three successful measurement trials for each direction of the SEBT.25 All trials were performed on the right then left foot before starting another reach direction. ANT reaches were performed first, followed by PL and then PM. Subjects were allowed to rest between trials until ready for the next trial.

with the y axis of the force plate and the toe and heel aligned with the tape markings. Upon command, the subject stood on one foot with hands on hips for 3 s and then (1) reached maximally, keeping hands on hips and the balancing foot flat, (2) firmly but gently touched their great toe to the measuring tape, (3) returned to the starting position, and (4) maintained static balance for three seconds. Reaches were repeated until three successful trials were obtained in each direction on each foot. All testing was performed with subjects standing barefoot.

Procedures After providing informed consent, age, weight, height, and standing left and right foot length and width were measured. Subjects identified their preferred kicking leg (defined as the foot the subjects used to kick a ball), which was recorded as their dominant leg. Foot length measures were transferred to the force plate and marked with tape. Subjects then performed the four practice trials in the first reach direction and, once ready, the subject’s foot was carefully centered on the force plate with the midline of the foot aligned

Statistical Analysis Descriptive statistics were calculated for the demographic, reach distance, and COP excursion variables. Each subject’s three trial mean was used for all analyses. The effect of leg dominance on SEBT performance was determined with a 2 × 3 repeated-measures ANOVA using reach distance as the dependent variable. Based on results, dominant and nondominant leg data were pooled for subsequent analyses such that a total of 56 limbs from 28 participants were assessed. Separate one-way (1 × 3) repeated-measures ANOVAs were used to compare COP excursion velocity, area, range, length of travel, and mean COP position between the three reach directions for the pooled data. Significant effects were explored using Bonferroni post hoc comparisons. Alpha was set a priori to .05.

Results There were no significant differences between legs for average reach distance (Table 1). Using pooled data, ANT reach distances were smaller than PM (p < .001) and PL (p = .001). PL reach distance was smaller than

Figure 2

Set-up of Star Excursion Balance Test on force plate showing three reach directions (PL = posterolateral; PM = posteromedial; ANT = anterior).

Table 1 Reach Distance (cm) for Dominant and Nondominant Legs and Pooled Data

Anterior Posteromedial Posterolateral

Nondominant Leg ( N = 28)

Dominant Leg ( N = 28)

Pooled Data ( N = 56)

76.9 ± 8.0 92.4 ± 9.5 80.2 ± 11.1

76.0 ± 8.2 87.7 ± 8.3 84.2 ± 10.1

69.5 ± 7.1* 81.9 ± 9.1# 74.8 ± 10.5

Note. 2 (leg) × 3 (direction) repeated-measures ANOVA: no significant leg effect (F 1,27 = 1.74, p = .198). 2 (leg) × 3 (direction) ANOVA: significant direction effect (F 1,27 = 40.302, p < .001). One-way repeated-measures ANOVA with the pooled data: significant effect (F2 = 63.733, p ≤ .001). *Anterior significantly different than posteromedial and posterolateral. ^Anterior significantly different from posterolateral. #Posteromedial significantly different than posterolateral.


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PM (p < .001). ANT COP velocity was significantly lower than PM (p = .001) and PL (p = .036). ANT COP area was significantly greater than PM (p = .008) and PL (p < .001). AP COP range was significantly greater for ANT reaches as compared with PM and PL (p < .05). The AP length of travel of the COP was significantly shorter for ANT as compared with PL (p < .001) but not PM (p = .08). ANT, PM, and PL ML length of travel were significantly different from each other. Mean AP COP position was significantly more forward for the ANT reaches as compared with PM (p < .001) and PL (p < .001). There were no other significant differences in COP measures between either PM or PL. Reach distances and COP areas, velocities, ranges, lengths of travel, and mean positions are provided in Table 2.

Discussion The most important finding from this investigation was that COP behavior was different for reaching in the anterior direction compared with the posterior directions. These findings support our hypothesis that different reach directions in the SEBT, specifically anterior versus posterior reaches, present unique DPC challenges. Our findings are supported by the literature that suggests different factors, such as range of motion and muscular strength, are related to SEBT performance in the anterior versus posterior directions. The anterior direction of the SEBT is highly correlated with weight-bearing ankle dorsiflexion and is influenced primarily by the quadriceps and triceps surae, which control ankle and knee flexion torques during the SEBT.1,21,23–28 Thus, the anterior reach is often associated with a distal control strategy. In comparison, the

posterior reaches have been shown to have a higher level of hamstring activity and flexion of the torso, which provides a counterbalance for the weight and momentum of the posteriorly reaching limb.26 This suggests that posterior reaches rely predominantly on a proximal control strategy.1,26 The descriptive nature of this study does not enable a causative explanation for lower COP velocity for anterior direction as compared with posterior directions to be formulated. However, there are insights that can be gained from the information provided by the COP variables. COP velocity is a spatiotemporal variable used to describe control of balance,27,28 which may suggest that, during anterior reaches, balance is more carefully controlled. However, further research is needed to confirm this speculation. The slower velocity in the anterior direction could also be due to the mechanical limit associated with ankle dorsiflexion, which is correlated with anterior reach.1,21 The end range of dorsiflexion provides a physical constraint that creates a more closed pack position for the ankle in the sagittal plane, as compared with posterior reaches, and may require less control from the ankle musculature. Another possible explanation for the difference in COP velocity is that the smaller musculature surrounding the more distal joints may provide finer control of COP excursions as compared with the larger more proximal muscles of the hip situated farther up from the base of support. Smaller muscles use smaller motor units, allowing for a finer control of movement.29 Given the reliance of anterior reaches on distal degrees of freedom,1,20,30,31 this postulation warrants further investigation. COP excursion area was greater for the anterior reaches as compared with the posterior directions. It is well established that COP excursion area is an indi-

Table 2 Means (± SD) for COP Dependent Variables Velocity (cm/s) Area (cm2) ML range (cm) AP range (cm) ML length of travel (cm) AP length of travel (cm) ML mean position (cm) AP mean position (cm)

ANT

PM

PL

8.5 ± 1.9* 27.2 ± 9.2* 3.36 ± 1.38 8.6 ± 1.9* 24.9 ± 7.2* 36.1 ± 8.3^ 0.1 ± 0.8 1.1 ± 1.2*

9.3 ± 1.6 23.2 ± 6.9 3.8 ± 0.7 6.5 ± 1.23 30.0 ± 10.0# 37.8 ± 10.0 0.0 ± 0.9 –0.5 ± 1.0

9.2 ± 1.8 22.5 ± 6.6 3.7 ± 1.8 6.3 ± 1.5 27.8 ± 9.7 39.0 ± 11.1 0.1 ± 0.6 –0.5 ± 1.2

Abbreviations: COP = center of pressure; ML = medial-lateral; AP = anterior-posterior; ANT = anterior; PM = posteromedial; PL = posterolateral. *ANT significantly different than PM and PL. ^ANT significantly different from PL. #PM significantly different than PL.

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cator of balance performance in quiet standing, with larger areas corresponding to poorer postural control or a more exploratory postural control behavior.28,32 The larger COP area for the anterior reaches could be indicative of inferior balance as compared with the posterior directions. Increased COP displacement has been observed in elderly,33 elderly fallers,34 and individuals with ACL injuries35–37 and low back pain38,39 during quiet standing tasks. The SEBT, however, is a dynamic balancing task. Anterior reaches may require greater movement of the center of mass (COM) to complete the movement task to ‘reach as far as possible’ while maintaining a stable position. Greater movement of the COM is accompanied by greater displacement of the COP and may not necessarily indicate less stability as compared with the posterior reaches. Further investigation is needed to determine if there are differences in COM displacement between anterior and posterior reaches in the SEBT. COP range and length of travel, while providing greater detail of the movement of the COP, did not further distinguish the three reach directions. AP length of travel was shorter and AP range was longer for anterior reach direction as compare with the posterior lateral direction. The smaller total AP COP length of travel for the anterior direction supports the postulation of a tighter control of balance while the larger COP AP range indicates that the larger COP excursion area was due to increased AP displacement, as opposed to ML displacement. The mean anterior reach AP COP position was in front of the middle of the foot, possibly due to the maximally required dorsiflexion during anterior reaches.1,20 However, the COP mean position, as calculated in this study, is the mean position for the whole task including the initial one leg standing balance, the reaching task, and the final one leg balance recovery periods. Further study is needed to determine if the increased anterior range is used during the voluntary movement, or if the increased anterior range occurs in the preparatory or recovery single leg stance positions. Based on the differences in COP behavior among the anterior and posterior directions, it may be that balance assessments such as limits of stability40,41 and variables such as end point excursion may provide greater insight into the relationship between COP behavior and SEBT performance. Fractal dimension (FD) of COP has been measured during the SEBT with CAI and lateral ankle sprain individuals demonstrating less complex COP excursion paths as compared with copers or controls.8,42,43 FD provides an estimate of INTERNATIONAL JOURNAL OF ATHLETIC THERAPY & TRAINING

COP excursion complexity representing the shape of the COP signal.44 Our findings provide novel spatiotemporal information on postural control strategies during the SEBT. Combined with studies of kinematics45–48 and electromyography,26,49–52 a more comprehensive insight related to SEBT performance can be gleaned and begins to provide an understanding of mechanisms contributing to SEBT performance.

Limitations This study is not without limitations, and further study is required to determine casual relationships between COP behavior and SEBT performance. Our results apply to only healthy young adults. Subjects for this study did not have any ankle musculoskeletal injuries. Immediate clinical meaningfulness is limited; though establishing baseline characteristics in a healthy population is essential to future development of criteria to identify individuals at risk for injury or prone to poor recovery outcomes post injury. We did not exclude individuals with knee or hip injuries incurred before 6 months of testing, which may raise concerns about the qualification of our subjects as healthy. However, to be included, subjects self-reported no known balance impairments. In addition, we did not measure range of motion or strength of the lower extremity joints. Lower extremity strength and flexibility are two factors that are related to SEBT performance and could provide additional insight of COP behavior during the SEBT. The cross-sectional design of our study does not enable us to determine the stability of the direction-dependent COP behavior during the SEBT over time. Determining the reliability of COP behavior is an important step to being able to discriminate injured or diseased populations.

Clinical Implications This investigative study was conducted on healthy individuals and does not have immediate clinical meaningfulness. The main purpose of this study was to understand balance control strategies as measured by COP movement during the SEBT, and to establish normative data that can later be explored to examine variations in clinical populations, such as individuals with CAI or ACL deficiency. Because the SEBT is a widely-used outcome measure, it is important to build a strong understanding of the contributing factors to SEBT performance. According to current research, the discriminatory ability of the SEBT as a clinician-oriented outcome can be enhanced with NOVEMBER 2016  37

laboratory measures.42,53 By understanding mechanical and sensorimotor constructs that contribute to SEBT performance, such as COP excursion area and velocity, clinicians will better be able to address specific limitations of their patients and identify classification schemes for clinical predictive tools. As clinical norms are established in future research, clinicians may want to supplement the SEBT with laboratory measures of COP behavior to better identify individuals at risk for injury or a poor recovery outcome.

Future Research To build on the findings from this study, it is important to examine additional factors that could contribute to COP behavior. Particularly interesting would be to examine COP behavior separately in the different phases of the SEBT: the initial one-legged balance, the reaching movement itself, and the final recovery one-legged balance. Since the SEBT requires volitional control of an induvial COM and exploration of an individual’s limits of stability, additional COP measures such as time to boundary and FD may prove fruitful. Combining kinematics with kinetics of these three phases could allow researchers to tease out anticipatory, reactive, and voluntary components of postural control in the SEBT. However, this is the first study to compare COP during different reach directions of the SEBT, and our results suggest that COP can successfully distinguish anterior reaches from posterior reaches and enable us to make distinctions between the control strategies on the SEBT. Our findings can be used as a baseline for the normal movement of the COP during the SEBT and provide an initial understanding for COP behavior during the SEBT.

Conclusion COP behavior can be defined by using COP excursion area and velocity alone without further need for examining COP range and length of travel, allowing the use of only two common COP parameters to assess COP behavior during the SEBT. The three reach directions provide unique challenges to the sensorimotor system as evidenced by differing COP behavior during reach performance. The anterior direction uniquely limits COP behavior compared with the posterior directions in healthy individuals. Further research is needed to identify whether this relationship is consistent over time and whether it is influenced by injury and/or rehabilitation.  38  NOVEMBER 2016

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Tyler R. Keith, Tara A. Condon, Ayana Phillips, Patrick O. McKeon, and Deborah L. King are with the Department of Exercise and Sports Science, Ithaca College, Ithaca, NY. Melanie L. McGrath, PhD, ATC, University of Nebraska at Omaha, is the report editor for this article.

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