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Assessment of dynamic balance via measurement of lower extremities tortuosity a
a
a
Moataz Eltoukhy , Christopher Kuenze , Hyung-Pil Jun , Shihab b
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Asfour & Francesco Travascio a
Department of Kinesiology and Sport Sciences, University of Miami, Coral Gables, FL, USA b
Department of Industrial Engineering, University of Miami, Coral Gables, FL, USA Published online: 21 Apr 2015.
Click for updates To cite this article: Moataz Eltoukhy, Christopher Kuenze, Hyung-Pil Jun, Shihab Asfour & Francesco Travascio (2015) Assessment of dynamic balance via measurement of lower extremities tortuosity, Sports Biomechanics, 14:1, 18-27, DOI: 10.1080/14763141.2015.1025238 To link to this article: http://dx.doi.org/10.1080/14763141.2015.1025238
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Sports Biomechanics, 2015 Vol. 14, No. 1, 18–27, http://dx.doi.org/10.1080/14763141.2015.1025238
Assessment of dynamic balance via measurement of lower extremities tortuosity MOATAZ ELTOUKHY1, CHRISTOPHER KUENZE1, HYUNG-PIL JUN1, SHIHAB ASFOUR2, & FRANCESCO TRAVASCIO2 1
Department of Kinesiology and Sport Sciences, University of Miami, Coral Gables, FL, USA, and Department of Industrial Engineering, University of Miami, Coral Gables, FL, USA
2
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(Received 19 September 2014; accepted 13 February 2015)
Abstract Tortuosity describes how twisted or how much curvature is present in an observed movement or path. The purpose of this study was to investigate the differences in segmental tortuosity between Star Excursion Balance Test (SEBT) reach directions. Fifteen healthy participants completed this study. Participants completed the modified three direction (anterior, posteromedial, posterolateral) SEBT with three-dimensional motion analysis using an 8 camera BTS Smart 7000DX motion analysis system. The tortuosity of stance limb retro-reflective markers was then calculated and compared between reach directions using a 1 £ 3 ANOVA with repeated measures, while the relationship between SEBT performance and tortuosity was established using Pearson product moment correlations. Anterior superior iliac spine tortuosity was significantly greater ( p , 0.001) and lateral knee tortuosity was lesser ( p ¼ 0.018) in the anterior direction compared to the posteromedial and posterolateral directions. In addition, second metatarsal tortuosity was greater in the anterior reach direction when compared to posteromedial direction ( p ¼ 0.024). Tortuosity is a novel biomechanical measurement technique that provides an assessment of segmental movement during common dynamic tasks such as the SEBT. This enhanced level of detail compared to more global measures of joint kinematic may provide insight into compensatory movement strategies adopted following lower extremity joint injury.
Keywords: Dynamic postural control, segmental motion, Star Excursion Balance Test
Introduction The Star Excursion Balance Test (SEBT) is a commonly used clinical assessment of dynamic balance (Gribble, Hertel, & Plisky, 2012). This assessment has been utilised as a measure of performance and injury risk among healthy active populations (Plisky, Rauh, Kaminski, & Underwood, 2006), as well as an indicator of altered dynamic stability in populations with a history of lower extremity injuries such as lateral ankle sprain (Doherty et al., 2014; Olmsted, Carcia, Hertel, & Shultz, 2002) and anterior cruciate ligament (ACL) rupture (Herrington, Hatcher, Hatcher, & McNicholas, 2009). The integration of hip, knee, and ankle mobility with comprehensive lower extremity neuromuscular control during movement sets the Correspondence: Francesco Travascio, Department of Industrial Engineering, University of Miami, 1251 Memorial Drive MEB 268, Coral Gables, FL, USA, E-mail: [email protected] q 2015 Taylor & Francis
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SEBT apart from many of the more commonly utilised static measures of postural control. The relative simplicity, strong reliability, and sensitivity to intervention in healthy and injured populations make the SEBT an attractive option for clinicians attempting to track prospective changes in a patient’s dynamic balance (Gribble, Kelly, Refshauge, & Hiller, 2013; Robinson & Gribble, 2008a). The primary goal of the SEBT is to assess global postural stability during a dynamic task using basic equipment in short amount of time (Gribble et al., 2012). There are many studies that have investigated the reliability and effectiveness of the SEBT as a measure of injury risk and dynamic postural control following injury. However, there is limited available evidence about the stability of coordinated segmental motion throughout the test (Gribble, Hertel, & Denegar, 2007; Robinson & Gribble, 2008a). A more thorough description of lower extremity segmental motion patterns and relative stability of motion during completion of the SEBT may provide clinicians important information regarding the potential source of strong versus poor performance on the SEBT. In addition, the ability to understand the relationship between these measures of movement stability and the more commonly utilised composite score may help to strengthen the diagnostic and predictive qualities of the SEBT as a measure of dynamic balance. Tortuosity is a common measure used in many areas of biomedical research and clinical medicine (Grisan, Foracchia, & Ruggeri, 2008; Hart, Goldbaum, Cote, Kube, & Nelson, 1997). In the case of biomechanics, tortuosity describes how twisted or how much curvature is present in an observed segmental movement. For example, a very linear and deterministic joint or segmental movement would be considered to have low tortuosity, while a more hesitant or curvilinear movement utilised to achieve the same task would be considered to have high tortuosity. Based on this definition, a segment displaying greater tortuosity throughout a reach distance would be characterised as having continuous rapid changes of direction resulting in a twisted pattern of movement. It is possible that, despite achieving an equal reach distance on the SEBT, differences in the strategy utilised to complete the test coupled with the relative smoothness or stability of segmental movement may provide detail related to the potential source of movement dysfunction that is not available based on composite performance on non-instrumented dynamic balance tasks. Additional detail related to the source of dysfunction may enable clinicians to better understand the symptoms related to orthopaedic injury as well as target intervention towards improvements in specific segmental stability. Currently, it is unclear if increased or decreased tortuosity of a movement is predictive of overall performance or potential injury risk. Therefore, the purpose of this study was to investigate the differences in segmental tortuosity between reach conditions in the SEBT. Our secondary purpose was to evaluate the relationship between threedimensional (3D) tortuosity of foot, shank, and thigh segments with SEBT reach distance. We hypothesised that participants with lower segmental tortuosity would perform better on the SEBT. Methods Fifteen participants completed this descriptive laboratory study and all measures were completed during a single session. This was a pilot study to establish methodology to assess segmental tortuosity during a dynamic task and therefore we utilised a sample of convenience. All participants provided informed consent prior to enrolling in this study. Procedures were approved by our University of Miami Institutional Review Board for human subject’s research. Participants were included if they were physically active (moderate exercise at least three times a week for 30 minutes), had no history of significant lower
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extremity injury which resulted in surgical intervention, and had no reported lower extremity injury that resulted in reduced physical activity, loss of time from work or sports or seeking out formal medical care within the last six weeks. Participants were instructed to wear spandex exercise clothing throughout the testing session. Prior to testing, bilateral anthropometric measurements (mm) were taken including: leg length (anterior superior iliac spine [ASIS] to medial malleolus), knee width, and ankle width. Demographic information for both groups are provided in Table I. Following enrolment, participants completed patient reported outcome measures including the Lower Extremity Functional Scale (LEFS) (Binkley, Stratford, Lott, & Riddle, 1999), Tegner Activity Level Scale (Briggs et al., 2009), as well as a Visual Analog Scale (VAS) for current lower extremity pain. The SEBT is a valid and reliable assessment of dynamic balance (Gribble et al., 2012). Based on recommendations by Hertel and co-workers (Hertel, Braham, Hale, & OlmstedKramer, 2006), the testing procedure was limited to three reach directions: anterior, posteromedial, posterolateral (Figure 1). All subjects were instructed on the proper completion of the SEBT and were asked to complete at least three practice trials in all three testing directions prior to initiation of the data collection session. Each subject was asked to stand on one leg with shoes off in the centre of the testing area. Stance leg was the left leg for all participants due to experimental set-up for a supplemental portion of this investigation. The foot was positioned with the most distal aspect of the great toe at the starting line when performing the anterior reach trials and with the heel at the starting line when performing both the posteromedial and posterolateral trials. Subjects were instructed to keep both hands on their hips and the foot of the balance limb flat on the floor throughout each reach. Subjects were asked to reach with his/her free limb in the desired direction and tap their foot at the terminal reach position. Any deviation from these instructions was considered a failed trial and the trial was repeated. The point of contact between the reaching limb and the floor was recorded and measured (cm) for each trial. The mean was calculated for the three successful trials in each reach direction and was then normalised by the subject’s leg length. In addition, the means for each direction were also taken and normalised to the subject’s height in order to report a composite SEBT score. We performed a 3D motion analysis of the SEBT using an 8 camera BTS Smart 7000DX motion analysis system (BTS Bioengineering, Milano, Italy). Retro-reflective markers were affixed bilaterally over the left and right posterior superior iliac spine (PSIS), ASIS, lateral mid-thigh, lateral femoral condyle, lateral mid-calf, and lateral malleolus using two-sided tape in accordance with the plug-in-gait model (Kadaba, Ramakrishnan, & Wootten, 1990). Static trials were collected to calibrate the marker set-up and provide reference for SEBT trial analysis. During the test, frames were collected at a frequency of 250 Hz. Trajectories of Table I. Participant demographics.
Participants (#) Age (years) Height (cm) Weight (kg) Body mass index Tegner activity score VAS of pain (cm) LEFS score
Males
Females
Combined
4 20.5 ^ 1.9 182.5 ^ 4.2 81.9 ^ 7.4 24.6 ^ 2.6 7.0 ^ 2.0 0.7 ^ 1.1 75.7 ^ 5.1
11 21.7 ^ 1.9 164.7 ^ 6.4 62.6 ^ 11.6 23.1 ^ 4.2 5.2 ^ 1.4 0.1 ^ 0.2 78.1 ^ 3.1
15 21.4 ^ 1.9 169.4 ^ 10.0 67.7 ^ 13.7 23.5 ^ 3.8 5.6 ^ 1.7 0.2 ^ 0.5 77.6 ^ 3.6
Note: VAS, Visual Analog Scale; LEFS, Lower Extremity Functional Scale.
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Figure 1. Star Excursion Balance Test and motion analysis set up with the reflective markers placed on the subject’s lower extremity landmarks.
the balance limb segments and centre of mass location in the anterior – posterior, medial – lateral, or vertical directions were calculated as the weighted sum of the centre of mass of every segment of the body in the same direction. Tortuosity estimates the twisted nature of a pattern of motion. In this investigation, we defined tortuosity as the measurement of 3D twistedness of movement through space of each independent marker throughout the course of the SEBT test in each of the three directions. Based on this definition, a segment displaying greater tortuosity throughout a reach distance would be characterised as having continuous rapid changes of direction resulting in a twisted pattern of movement. Currently, there is no unanimous mathematical definition for tortuosity in 3D, and several metrics have been proposed. Hereby, an approach referred to as ‘sum of angles metric’ (SOAM) is adopted. Such method consists in integrating the total curvature of the motion pattern, and normalising it by the path length (Smedby et al., 1993). More specifically, the following expression can be used for evaluating SOAM: P Ck k SOAM ¼ ; ð1Þ L where Ck is the pattern curvature at the point k, and L is the path length. Details on the computation of Ck are reported in the work of Bullit and co-workers (Bullitt, Gerig, Pizer, Lin, & Aylward, 2003). In the following, SOAM is adopted to evaluate the tortuosity of body segments motion during SEBT, and its values are reported in degrees per millimetre (deg/mm). Means and standard deviations were calculated for all measures related to the SEBT and segmental tortuosity. Between task tortuosity was compared using a 1 (group) £ 3 (task: anteromedial reach, posteromedial reach, and posterolatereal reach) repeated measures ANOVA and significant group main effects were further investigated using simple comparisons between each pair of tasks. In addition, Pearson product moment correlations (r) were calculated between SEBT performance in each reach direction and the associated segmental tortuosity in measured in the same reach direction. Lastly, simple forward entry linear regression was utilised to evaluate the ability of tortuosity of individual segments to predict performance on the SEBT in the associated reach direction. We selected forward entry due to the limited sample size in an attempt to limit the number of significant
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predictors present in our regression models. A priori alpha-level was established as p # 0.05 and all statistical analysis was completed using the SPSS version 18.0 (IBM, Chicago, Illinois, USA).
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Results Means and standard deviations were calculated for SEBT performance (Table II) and segmental tortuosity (Table III). ASIS tortuosity was significantly greater ( p ¼ 0.001 and 0.001) and lateral knee tortuosity was significantly lesser ( p ¼ 0.018 and 0.034) in the anterior direction when compared with both the posteromedial and posterolateral directions, respectively (Table III). In addition, second metatarsal tortuosity was greater in the anterior reach direction when compared to posteromedial direction ( p ¼ 0.048). Greater SEBT anterior reach distance was significantly correlated with lesser second metatarsal head marker tortuosity (r ¼ -0.562, p ¼ 0.037), greater posteromedial reach distance was correlated with lesser ASIS marker tortuosity (r ¼ -0.576, p ¼ 0.039), and greater posterolateral reach distance was correlated with greater calcaneal marker tortuosity (r ¼ -0.560, p ¼ 0.040; Table IV). In addition, anterior (R 2 ¼ 0.315, p ¼ 0.037), posteromedial (R2 ¼ 0.573, p ¼ 0.014), and posterolateral reach distance were all significantly predicted by marker tortuosity (R 2 ¼ 0.313, p ¼ 0.037; Table V). Discussion The SEBT is a commonly used clinical assessment of dynamic balance that has been shown to be sensitive to postural alterations caused by previous history of lower extremity injury (Gribble et al., 2012). The primary purpose of this study was to assess 3D lower extremity tortuosity during the SEBT in order to assess differences in movement strategy between each of the three reach distance most commonly utilised as part of the test. Tortuosity has been chosen as a metric since it can quantify the twistedness of patterns of motion by averaging the total curvature of the trajectory by the length of the movement excursion. Frequent changes in the direction of motion increase the overall score of tortuosity. We found that, in a healthy population, tortuosity scores increased up to three orders of magnitude when comparing markers proximal to the centre of mass (e.g. ASIS, PSIS, etc.) to the most distal ones (second metatarsal), see Table III. This increase in tortuosity is expected: all the markers are connected by a kinematic chain and the twistedness of movements of distal markers is the result of their intrinsic twisted motions and the changes of direction of more proximal markers. We also observed that, across the lower extremity (ASIS, lateral knee, and second metatarsal), increased curvature over the course of motion is most common in the anterior reach distance as represented by greater tortuosity when compared to the posteromedial and posterolateral directions (Table III). Our findings are consistent with previous studies that have shown between reach direction differences when assessing lower extremity joint motion
Table II. Performance on the Star Excursion Balance Test. Percentage leg length (%) Anterior reach Posteromedial reach Posterolateral reach Composite score
70.72 ^ 8.00 90.36 ^ 8.16 92.67 ^ 8.16 253.76 ^ 20.18
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Assessment of dynamic balance Table III. Descriptive information for sum of angle metric calculation of marker tortuosity (deg/mm).
ASIS PSIS Lateral thigh Lateral knee joint Fibula Lateral malleolus Calcaneus Second metatarsal head Estimated centre of mass
Anterior reach
Posteromedial reach
Posterolateral reach
p
70.58 ^ 17.28a,b 61.49 ^ 14.70 86.08 ^ 22.34 84.12 ^ 23.29a,b 289.29 ^ 647.02 1187.83 ^ 1536.20 3855.36 ^ 5667.15 10139.88 ^ 15030.30a 71.08 ^ 23.24
44.44 ^ 14.81 59.08 ^ 16.27 77.32 ^ 18.25 97.32 ^ 26.00 146.01 ^ 54.61 673.09 ^ 252.48 1917.18 ^ 1028.94 2661.03 ^ 1786.03 70.31 ^ 26.25
51.52 ^ 13.01 57.10 ^ 12.64 89.14 ^ 19.62 104.14 ^ 32.72 139.71 ^ 48.19 583.09 ^ 211.69 2187.36 ^ 2902.39 1890.10 ^ 1995.70 56.50 ^ 12.35
,0.001c 0.708 0.086 0.018c 0.496 0.153 0.323 0.024c 0.210
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Note: PSIS, posterior superior iliac spine; ASIS, anterior superior iliac spine. a Anterior reach is significantly different from posteromedial reach. b Anterior reach is significantly different from posterolateral reach. c Significant main effect for test.
strategies utilised during the SEBT in both healthy (Fullam, Caulfield, Coughlan, & Delahunt, 2014; Overmoyer & Reiser, 2013; Robinson & Gribble, 2008b) and previously injured populations (Bastien et al., 2014; Herrington et al., 2009; Olmsted et al., 2002). It has been previously reported that the anterior reach direction results in the poorest performance as represented as percentage of limb length (Gribble et al., 2012). While thought to be due to the high demand this position puts on the quadriceps muscle group (Gribble et al., 2007; Herrington et al., 2009; Robinson & Gribble, 2008b) and the impact that limited dorsiflexion range of motion may have (Hoch, Staton, & McKeon, 2011), it is notable that this task also resulted in the greatest degree of tortuosity at markers representing the pelvis (ASIS), knee (lateral knee), and foot (second metatarsal). The Table IV. Pearson product moment correlations (r) for SEBT performance and individual marker tortuosity.
ASIS PSIS Lateral thigh Lateral knee joint Fibula Lateral malleolus Calcaneus Second metatarsal head Estimated centre of mass
r p r p r p r p r p r p r p r p r p
Anterior
Posteromedial
Posterolateral
0.024 0.934 -0.070 0.813 -0.193 0.509 -0.165 0.574 0.033 0.911 -0.080 0.787 -0.407 0.149 -0.562a 0.037 -0.049 0.869
-0.576a 0.039 -0.352 0.238 -0.117 0.704 0.068 0.825 0.001 0.997 -0.367 0.217 -0.472 0.104 -0.253 0.404 -0.052 0.866
-0.210 0.470 -0.458 0.100 -0.113 0.700 0.053 0.857 -0.018 0.951 0.379 0.181 0.560a 0.037 0.204 0.484 -0.207 0.477
Note: PSIS, posterior superior iliac spine; ASIS, anterior superior iliac spine. a Indicates a significant correlation coefficient.
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Table V. Statistically significant regression models for predicting Star Excursion Balance Test reach distance based on marker tortuosity.
Anterior reach distance Posteromedial reach distance Posterolateral reach distance
Predictors
Standardised regression coefficient
R2
p
Second metatarsal ASIS Lateral knee Calcaneus
-0.562 -0.923 0.601 0.560
0.315 0.573
0.037 0.014
0.313
0.037
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Note: ASIS, anterior superior iliac spine.
demands placed upon lower extremity musculature as well as the change in height of the centre of mass associated with completion of the anterior reach distance appears to negatively impact the smoothness or stability of motion when compared to both the posteromedial and posterolateral reach directions (Fullam et al., 2014; Robinson & Gribble, 2008a). Subjective biomechanical evaluation or in some cases the broad evaluation of peak kinematic values have become a more common aspect of clinical care and return to physical activity decision-making; however, the rates of re-injury and chronic instability among those with significant ankle and knee injuries have been largely unaffected. While the SEBT is a simple, resource independent clinical evaluation tool, it may not be sufficiently sensitive to detect subtle changes in lower extremity movement that persist well beyond a return to physical activity following lower extremity joint injury (Arnold, De La Motte, Linens, & Ross, 2009; Howells, Ardern, & Webster, 2011). Based on our findings, the use of tortuosity to assess segmental steadiness during the anterior reach distance may be clinically useful to identify subtle persistent differences in movement patterns within patient populations that have been shown to have difficulty with this task such as those with a history of ACL deficiency, ACL reconstruction, and those with chronic ankle instability. Subsequent investigations must include comparisons to a variety of previously injured populations in order to better understand the relationship between increases or decreases in the tortuosity of specific lower extremity segments in those with known lower extremity movement dysfunction. In addition to assessing the differences in tortuosity between reach directions, we investigated the ability of each segmental motion to predict multi-directional dynamic balance in healthy participants. In each of the three reach distances, 3D segmental tortuosity was found to be related to and predictive of test performance (Tables IV and V). While previous literature has shown total sagittal plane knee and hip displacement to be strongly predictive of posteromedial and posterolateral reach distance performance, this is the first investigation to quantify the relative smoothness and stability of the joint movement in space (Robinson & Gribble, 2008b). Markers associated with the pelvis (ASIS) and knee (lateral knee) were predictive of the posteromedial direction; however, more similar to the anterior reach direction (second metatarsal), the posterolateral reach was best predicted using a marker associated with the foot (calcaneus). This is an interesting departure from previous studies that have used both muscle strength and lower extremity kinematics to explain performance on the SEBT. Most commonly, performance on the posteromedial and posterolateral reach directions has been linked to hip and knee joint range of motion (Robinson & Gribble, 2008a), as well as strength of the lateral hip stabilisers (Leavey, Sandrey, & Dahmer, 2010; McMullen, Cosby, Hertel, Ingersoll, & Hart, 2011). However, these assessments have relied on peak measures of strength or motion instead of measures of steadiness or smoothness. Applying the concept of tortuosity in concert with more
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traditional kinematic and kinetic assessment tools may provide a more complete understanding of the strategies utilised to optimise SEBT performance. In addition, this measurement technique may provide insight regarding alterations in movement strategy and relative smoothness of movement among a group of individuals completing the SEBT. In this study, the movements of body segments were characterised by measuring the motion tortuosity. Such metric has no unique mathematical definition, and several formulations have been proposed. For instance, the most commonly used approach is the ‘distance metric’, which provides the ratio of the path length to the distance between starting and end point of the motion (Bracher, 1982; Hart et al., 1997; Smedby et al., 1993; Zhou, Rzeszotarski, Singerman, & Chokreff, 1994). Although intuitive, this metric is not sensitive to high-frequency wiggling of a motion pattern (Capowski, Kylstra, & Freedman, 1995). Accordingly, large ‘S’ or ‘U’ patterns of motion may provide values of ‘distance metric’ larger than short high-frequency oscillating patterns. An alternative way of representing tortuosity might have been the ‘inflection count metric’, which weights ‘distance metric’ by the number of inflection points characterising the motion pattern (Bullitt et al., 2003). However, such approach presents limitations when evaluating tight helicoidal patterns: helices do not add significant length to the pattern, and do not present inflection points (Koenderink, 1990). Hence, we adopted the SOAM definition in the belief that this could be the better suiting approach for evaluating tortuosity of the specific body motions involved in this analysis, since it overcomes the aforementioned limitations characterising the ‘distance metric’ and the ‘inflection count metric’. There are several limitations to the current study that should be considered when interpreting the results. The SEBT is commonly utilised as an evaluation tool following lower extremity injury in order to identify alterations in dynamic balance. In order to better understand the application and limitations of tortuosity as a predictor of dynamic balance, our sample was limited to young, healthy, physically active individuals. Due to this delimitation, the ability to apply our findings to injured populations is limited. Future research should focus on using this novel biomechanical variable to identify patterns of compensation or altered balance strategies in individuals with a recent or history of lower extremity injury. In addition, our sample of convenience had more females (11) compared to males (4) which may have influenced our results. This was further compounded by the use of the left limb for all participants regardless of limb dominance. Previous investigations related to the influences of gender and limb dominance on dynamic balance strategies have shown a significant impact based on both factors depending on the task selected (Gribble et al., 2012; Gribble, Robinson, Hertel, & Denegar, 2009). Hence, increased variability in our data due to the difference in gender-based balance strategies may have limited the predictive ability of our findings as related to performance on the SEBT. From a methodological prospective, trials were limited to a single limb in this study for logistical reasons. Commonly, between limb asymmetry in SEBT performance is used clinically as an indicator of unilateral alterations in dynamic balance. While not the stated purpose of this study, future investigation of symmetry in lower extremity tortuosity may improve the applicability of this measure in an injured population. Finally, we utilised complex 3D motion analysis to assess tortuosity in this investigation. While this methodology is the most accurate and sensitive approach to assessing tortuosity, it may limit the generalisability and applicability to the clinical setting. Future investigations should focus on developing a more simple 2D assessment to enable the use of more widely available video technology. The SEBT is a common measure utilised for assessing alterations in dynamic balance following lower extremity injury. However, the outcome of the assessment does not provide feedback on the strategies utilised by an individual to complete the test. In this investigation,
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we found that tortuosity, an estimate twisting during a pattern of motion, of the ASIS, lateral knee, and second metatarsal was significantly greater during the anterior reach when compared to the other test components. In addition, it is clear that significant amount of variance in reach distance for each test direction can be predicted using tortuosity. However, the marker with the strongest predictive ability varies based on the task. Tortuosity is a novel biomechanical measurement technique that provides an assessment of segmental movement during common dynamic tasks such as the SEBT. This enhanced level of detail compared to more global measures of joint kinematic may provide insight into compensatory movement strategies adopted following lower extremity joint injury. Disclosure statement
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No potential conflict of interest was reported by the authors.
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Hart, W. E., Goldbaum, M., Cote, B., Kube, P., & Nelson, M. R. (1997). Automated measurement of retinal vascular tortuosity. Proceedings: A Conference of the American Medical Informatics Association, 459–463. Herrington, L., Hatcher, J., Hatcher, A., & McNicholas, M. (2009). A comparison of Star Excursion Balance Test reach distances between ACL deficient patients and asymptomatic controls. The Knee, 16, 149 –152. doi:10. 1016/j.knee.2008.10.004 Hertel, J., Braham, R. A., Hale, S. A., & Olmsted-Kramer, L. C. (2006). Simplifying the Star Excursion Balance Test: Analyses of subjects with and without chronic ankle instability. The Journal of Orthopaedic Sports Physical Therapy, 36, 131 –137. doi:10.2519/jospt.2006.36.3.131 Hoch, M. C., Staton, G. S., & McKeon, P. O. (2011). Dorsiflexion range of motion significantly influences dynamic balance. Journal of Science and Medicine in Sport, 14, 90–92. doi:10.1016/j.jsams.2010.08.001 Howells, B. E., Ardern, C. L., & Webster, K. E. (2011). Is postural control restored following anterior cruciate ligament reconstruction? A systematic review. Knee Surgery, Sports Traumatology, Arthroscopy, 19, 1168–1177. doi:10.1007/s00167-011-1444-x Kadaba, M. P., Ramakrishnan, H. K., & Wootten, M. E. (1990). Measurement of lower extremity kinematics during level walking. Journal of Orthopaedic Research, 8, 383–392. doi:10.1002/jor.1100080310 Koenderink, J. J. (1990). Solid shape (Vol. 2). Cambridge: Cambridge University Press. Leavey, V. J., Sandrey, M. A., & Dahmer, G. (2010). Comparative effects of 6-week balance, gluteus medius strength, and combined programs on dynamic postural control. Journal of Sports Rehabilitation, 19, 268 –287. McMullen, K. L., Cosby, N. L., Hertel, J., Ingersoll, C. D., & Hart, J. M. (2011). Lower extremity neuromuscular control immediately after fatiguing hip-abduction exercise. Journal of Athletic Training, 46, 607–614. Olmsted, L. C., Carcia, C. L., Hertel, J., & Shultz, J. S. (2002). Efficacy of the Star Excursion Balance Tests in detecting reach deficits in subjects with chronic ankle instability. Journal of Athletic Training, 37, 501–506. Overmoyer, G. V., & Reiser, R. F. (2013). Relationships between asymmetries in functional movements and the Star Excursion Balance Test. Journal of Strength and Conditioning Research, 27, 2013–2024. doi:10.1519/JSC. 0b013e3182779962 Plisky, P. J., Rauh, M. J., Kaminski, T. W., & Underwood, F. B. (2006). Star Excursion Balance Test as a predictor of lower extremity injury in high school basketball players. The Journal of Orthopaedic Sports Physical Therapy, 36, 911– 919. doi:10.2519/jospt.2006.2244 Robinson, R., & Gribble, P. (2008a). Kinematic predictors of performance on the Star Excursion Balance Test. Journal of Sports Rehabilitation, 17, 347 –357. Robinson, R., & Gribble, P. (2008b). Support for a reduction in the number of trials needed for the Star Excursion Balance Test. Archives of Physical Medicine and Rehabilitation, 89, 364 –370. doi:10.1016/j.apmr.2007.08.139 ¨ ., Ho¨gman, N., Nilsson, S., Erikson, U., Olssori, A. G., & Walldius, G. (1993). Two-dimensional Smedby, O tortuosity of the superficial femoral artery in early atherosclerosis. Journal of Vascular Research, 30, 181–191. doi:10.1159/000158993 Zhou, L., Rzeszotarski, M. S., Singerman, L. J., & Chokreff, J. M. (1994). The detection and quantification of retinopathy using digital angiograms. IEEE Transactions on Medical Imaging, 13, 619 –626. doi:10.1109/42. 363106
Sports Biomechanics Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/rspb20
Assessment of dynamic balance via measurement of lower extremities tortuosity a
a
a
Moataz Eltoukhy , Christopher Kuenze , Hyung-Pil Jun , Shihab b
b
Asfour & Francesco Travascio a
Department of Kinesiology and Sport Sciences, University of Miami, Coral Gables, FL, USA b
Department of Industrial Engineering, University of Miami, Coral Gables, FL, USA Published online: 21 Apr 2015.
Click for updates To cite this article: Moataz Eltoukhy, Christopher Kuenze, Hyung-Pil Jun, Shihab Asfour & Francesco Travascio (2015) Assessment of dynamic balance via measurement of lower extremities tortuosity, Sports Biomechanics, 14:1, 18-27, DOI: 10.1080/14763141.2015.1025238 To link to this article: http://dx.doi.org/10.1080/14763141.2015.1025238
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Sports Biomechanics, 2015 Vol. 14, No. 1, 18–27, http://dx.doi.org/10.1080/14763141.2015.1025238
Assessment of dynamic balance via measurement of lower extremities tortuosity MOATAZ ELTOUKHY1, CHRISTOPHER KUENZE1, HYUNG-PIL JUN1, SHIHAB ASFOUR2, & FRANCESCO TRAVASCIO2 1
Department of Kinesiology and Sport Sciences, University of Miami, Coral Gables, FL, USA, and Department of Industrial Engineering, University of Miami, Coral Gables, FL, USA
2
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(Received 19 September 2014; accepted 13 February 2015)
Abstract Tortuosity describes how twisted or how much curvature is present in an observed movement or path. The purpose of this study was to investigate the differences in segmental tortuosity between Star Excursion Balance Test (SEBT) reach directions. Fifteen healthy participants completed this study. Participants completed the modified three direction (anterior, posteromedial, posterolateral) SEBT with three-dimensional motion analysis using an 8 camera BTS Smart 7000DX motion analysis system. The tortuosity of stance limb retro-reflective markers was then calculated and compared between reach directions using a 1 £ 3 ANOVA with repeated measures, while the relationship between SEBT performance and tortuosity was established using Pearson product moment correlations. Anterior superior iliac spine tortuosity was significantly greater ( p , 0.001) and lateral knee tortuosity was lesser ( p ¼ 0.018) in the anterior direction compared to the posteromedial and posterolateral directions. In addition, second metatarsal tortuosity was greater in the anterior reach direction when compared to posteromedial direction ( p ¼ 0.024). Tortuosity is a novel biomechanical measurement technique that provides an assessment of segmental movement during common dynamic tasks such as the SEBT. This enhanced level of detail compared to more global measures of joint kinematic may provide insight into compensatory movement strategies adopted following lower extremity joint injury.
Keywords: Dynamic postural control, segmental motion, Star Excursion Balance Test
Introduction The Star Excursion Balance Test (SEBT) is a commonly used clinical assessment of dynamic balance (Gribble, Hertel, & Plisky, 2012). This assessment has been utilised as a measure of performance and injury risk among healthy active populations (Plisky, Rauh, Kaminski, & Underwood, 2006), as well as an indicator of altered dynamic stability in populations with a history of lower extremity injuries such as lateral ankle sprain (Doherty et al., 2014; Olmsted, Carcia, Hertel, & Shultz, 2002) and anterior cruciate ligament (ACL) rupture (Herrington, Hatcher, Hatcher, & McNicholas, 2009). The integration of hip, knee, and ankle mobility with comprehensive lower extremity neuromuscular control during movement sets the Correspondence: Francesco Travascio, Department of Industrial Engineering, University of Miami, 1251 Memorial Drive MEB 268, Coral Gables, FL, USA, E-mail: [email protected] q 2015 Taylor & Francis
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SEBT apart from many of the more commonly utilised static measures of postural control. The relative simplicity, strong reliability, and sensitivity to intervention in healthy and injured populations make the SEBT an attractive option for clinicians attempting to track prospective changes in a patient’s dynamic balance (Gribble, Kelly, Refshauge, & Hiller, 2013; Robinson & Gribble, 2008a). The primary goal of the SEBT is to assess global postural stability during a dynamic task using basic equipment in short amount of time (Gribble et al., 2012). There are many studies that have investigated the reliability and effectiveness of the SEBT as a measure of injury risk and dynamic postural control following injury. However, there is limited available evidence about the stability of coordinated segmental motion throughout the test (Gribble, Hertel, & Denegar, 2007; Robinson & Gribble, 2008a). A more thorough description of lower extremity segmental motion patterns and relative stability of motion during completion of the SEBT may provide clinicians important information regarding the potential source of strong versus poor performance on the SEBT. In addition, the ability to understand the relationship between these measures of movement stability and the more commonly utilised composite score may help to strengthen the diagnostic and predictive qualities of the SEBT as a measure of dynamic balance. Tortuosity is a common measure used in many areas of biomedical research and clinical medicine (Grisan, Foracchia, & Ruggeri, 2008; Hart, Goldbaum, Cote, Kube, & Nelson, 1997). In the case of biomechanics, tortuosity describes how twisted or how much curvature is present in an observed segmental movement. For example, a very linear and deterministic joint or segmental movement would be considered to have low tortuosity, while a more hesitant or curvilinear movement utilised to achieve the same task would be considered to have high tortuosity. Based on this definition, a segment displaying greater tortuosity throughout a reach distance would be characterised as having continuous rapid changes of direction resulting in a twisted pattern of movement. It is possible that, despite achieving an equal reach distance on the SEBT, differences in the strategy utilised to complete the test coupled with the relative smoothness or stability of segmental movement may provide detail related to the potential source of movement dysfunction that is not available based on composite performance on non-instrumented dynamic balance tasks. Additional detail related to the source of dysfunction may enable clinicians to better understand the symptoms related to orthopaedic injury as well as target intervention towards improvements in specific segmental stability. Currently, it is unclear if increased or decreased tortuosity of a movement is predictive of overall performance or potential injury risk. Therefore, the purpose of this study was to investigate the differences in segmental tortuosity between reach conditions in the SEBT. Our secondary purpose was to evaluate the relationship between threedimensional (3D) tortuosity of foot, shank, and thigh segments with SEBT reach distance. We hypothesised that participants with lower segmental tortuosity would perform better on the SEBT. Methods Fifteen participants completed this descriptive laboratory study and all measures were completed during a single session. This was a pilot study to establish methodology to assess segmental tortuosity during a dynamic task and therefore we utilised a sample of convenience. All participants provided informed consent prior to enrolling in this study. Procedures were approved by our University of Miami Institutional Review Board for human subject’s research. Participants were included if they were physically active (moderate exercise at least three times a week for 30 minutes), had no history of significant lower
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extremity injury which resulted in surgical intervention, and had no reported lower extremity injury that resulted in reduced physical activity, loss of time from work or sports or seeking out formal medical care within the last six weeks. Participants were instructed to wear spandex exercise clothing throughout the testing session. Prior to testing, bilateral anthropometric measurements (mm) were taken including: leg length (anterior superior iliac spine [ASIS] to medial malleolus), knee width, and ankle width. Demographic information for both groups are provided in Table I. Following enrolment, participants completed patient reported outcome measures including the Lower Extremity Functional Scale (LEFS) (Binkley, Stratford, Lott, & Riddle, 1999), Tegner Activity Level Scale (Briggs et al., 2009), as well as a Visual Analog Scale (VAS) for current lower extremity pain. The SEBT is a valid and reliable assessment of dynamic balance (Gribble et al., 2012). Based on recommendations by Hertel and co-workers (Hertel, Braham, Hale, & OlmstedKramer, 2006), the testing procedure was limited to three reach directions: anterior, posteromedial, posterolateral (Figure 1). All subjects were instructed on the proper completion of the SEBT and were asked to complete at least three practice trials in all three testing directions prior to initiation of the data collection session. Each subject was asked to stand on one leg with shoes off in the centre of the testing area. Stance leg was the left leg for all participants due to experimental set-up for a supplemental portion of this investigation. The foot was positioned with the most distal aspect of the great toe at the starting line when performing the anterior reach trials and with the heel at the starting line when performing both the posteromedial and posterolateral trials. Subjects were instructed to keep both hands on their hips and the foot of the balance limb flat on the floor throughout each reach. Subjects were asked to reach with his/her free limb in the desired direction and tap their foot at the terminal reach position. Any deviation from these instructions was considered a failed trial and the trial was repeated. The point of contact between the reaching limb and the floor was recorded and measured (cm) for each trial. The mean was calculated for the three successful trials in each reach direction and was then normalised by the subject’s leg length. In addition, the means for each direction were also taken and normalised to the subject’s height in order to report a composite SEBT score. We performed a 3D motion analysis of the SEBT using an 8 camera BTS Smart 7000DX motion analysis system (BTS Bioengineering, Milano, Italy). Retro-reflective markers were affixed bilaterally over the left and right posterior superior iliac spine (PSIS), ASIS, lateral mid-thigh, lateral femoral condyle, lateral mid-calf, and lateral malleolus using two-sided tape in accordance with the plug-in-gait model (Kadaba, Ramakrishnan, & Wootten, 1990). Static trials were collected to calibrate the marker set-up and provide reference for SEBT trial analysis. During the test, frames were collected at a frequency of 250 Hz. Trajectories of Table I. Participant demographics.
Participants (#) Age (years) Height (cm) Weight (kg) Body mass index Tegner activity score VAS of pain (cm) LEFS score
Males
Females
Combined
4 20.5 ^ 1.9 182.5 ^ 4.2 81.9 ^ 7.4 24.6 ^ 2.6 7.0 ^ 2.0 0.7 ^ 1.1 75.7 ^ 5.1
11 21.7 ^ 1.9 164.7 ^ 6.4 62.6 ^ 11.6 23.1 ^ 4.2 5.2 ^ 1.4 0.1 ^ 0.2 78.1 ^ 3.1
15 21.4 ^ 1.9 169.4 ^ 10.0 67.7 ^ 13.7 23.5 ^ 3.8 5.6 ^ 1.7 0.2 ^ 0.5 77.6 ^ 3.6
Note: VAS, Visual Analog Scale; LEFS, Lower Extremity Functional Scale.
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Figure 1. Star Excursion Balance Test and motion analysis set up with the reflective markers placed on the subject’s lower extremity landmarks.
the balance limb segments and centre of mass location in the anterior – posterior, medial – lateral, or vertical directions were calculated as the weighted sum of the centre of mass of every segment of the body in the same direction. Tortuosity estimates the twisted nature of a pattern of motion. In this investigation, we defined tortuosity as the measurement of 3D twistedness of movement through space of each independent marker throughout the course of the SEBT test in each of the three directions. Based on this definition, a segment displaying greater tortuosity throughout a reach distance would be characterised as having continuous rapid changes of direction resulting in a twisted pattern of movement. Currently, there is no unanimous mathematical definition for tortuosity in 3D, and several metrics have been proposed. Hereby, an approach referred to as ‘sum of angles metric’ (SOAM) is adopted. Such method consists in integrating the total curvature of the motion pattern, and normalising it by the path length (Smedby et al., 1993). More specifically, the following expression can be used for evaluating SOAM: P Ck k SOAM ¼ ; ð1Þ L where Ck is the pattern curvature at the point k, and L is the path length. Details on the computation of Ck are reported in the work of Bullit and co-workers (Bullitt, Gerig, Pizer, Lin, & Aylward, 2003). In the following, SOAM is adopted to evaluate the tortuosity of body segments motion during SEBT, and its values are reported in degrees per millimetre (deg/mm). Means and standard deviations were calculated for all measures related to the SEBT and segmental tortuosity. Between task tortuosity was compared using a 1 (group) £ 3 (task: anteromedial reach, posteromedial reach, and posterolatereal reach) repeated measures ANOVA and significant group main effects were further investigated using simple comparisons between each pair of tasks. In addition, Pearson product moment correlations (r) were calculated between SEBT performance in each reach direction and the associated segmental tortuosity in measured in the same reach direction. Lastly, simple forward entry linear regression was utilised to evaluate the ability of tortuosity of individual segments to predict performance on the SEBT in the associated reach direction. We selected forward entry due to the limited sample size in an attempt to limit the number of significant
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predictors present in our regression models. A priori alpha-level was established as p # 0.05 and all statistical analysis was completed using the SPSS version 18.0 (IBM, Chicago, Illinois, USA).
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Results Means and standard deviations were calculated for SEBT performance (Table II) and segmental tortuosity (Table III). ASIS tortuosity was significantly greater ( p ¼ 0.001 and 0.001) and lateral knee tortuosity was significantly lesser ( p ¼ 0.018 and 0.034) in the anterior direction when compared with both the posteromedial and posterolateral directions, respectively (Table III). In addition, second metatarsal tortuosity was greater in the anterior reach direction when compared to posteromedial direction ( p ¼ 0.048). Greater SEBT anterior reach distance was significantly correlated with lesser second metatarsal head marker tortuosity (r ¼ -0.562, p ¼ 0.037), greater posteromedial reach distance was correlated with lesser ASIS marker tortuosity (r ¼ -0.576, p ¼ 0.039), and greater posterolateral reach distance was correlated with greater calcaneal marker tortuosity (r ¼ -0.560, p ¼ 0.040; Table IV). In addition, anterior (R 2 ¼ 0.315, p ¼ 0.037), posteromedial (R2 ¼ 0.573, p ¼ 0.014), and posterolateral reach distance were all significantly predicted by marker tortuosity (R 2 ¼ 0.313, p ¼ 0.037; Table V). Discussion The SEBT is a commonly used clinical assessment of dynamic balance that has been shown to be sensitive to postural alterations caused by previous history of lower extremity injury (Gribble et al., 2012). The primary purpose of this study was to assess 3D lower extremity tortuosity during the SEBT in order to assess differences in movement strategy between each of the three reach distance most commonly utilised as part of the test. Tortuosity has been chosen as a metric since it can quantify the twistedness of patterns of motion by averaging the total curvature of the trajectory by the length of the movement excursion. Frequent changes in the direction of motion increase the overall score of tortuosity. We found that, in a healthy population, tortuosity scores increased up to three orders of magnitude when comparing markers proximal to the centre of mass (e.g. ASIS, PSIS, etc.) to the most distal ones (second metatarsal), see Table III. This increase in tortuosity is expected: all the markers are connected by a kinematic chain and the twistedness of movements of distal markers is the result of their intrinsic twisted motions and the changes of direction of more proximal markers. We also observed that, across the lower extremity (ASIS, lateral knee, and second metatarsal), increased curvature over the course of motion is most common in the anterior reach distance as represented by greater tortuosity when compared to the posteromedial and posterolateral directions (Table III). Our findings are consistent with previous studies that have shown between reach direction differences when assessing lower extremity joint motion
Table II. Performance on the Star Excursion Balance Test. Percentage leg length (%) Anterior reach Posteromedial reach Posterolateral reach Composite score
70.72 ^ 8.00 90.36 ^ 8.16 92.67 ^ 8.16 253.76 ^ 20.18
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Assessment of dynamic balance Table III. Descriptive information for sum of angle metric calculation of marker tortuosity (deg/mm).
ASIS PSIS Lateral thigh Lateral knee joint Fibula Lateral malleolus Calcaneus Second metatarsal head Estimated centre of mass
Anterior reach
Posteromedial reach
Posterolateral reach
p
70.58 ^ 17.28a,b 61.49 ^ 14.70 86.08 ^ 22.34 84.12 ^ 23.29a,b 289.29 ^ 647.02 1187.83 ^ 1536.20 3855.36 ^ 5667.15 10139.88 ^ 15030.30a 71.08 ^ 23.24
44.44 ^ 14.81 59.08 ^ 16.27 77.32 ^ 18.25 97.32 ^ 26.00 146.01 ^ 54.61 673.09 ^ 252.48 1917.18 ^ 1028.94 2661.03 ^ 1786.03 70.31 ^ 26.25
51.52 ^ 13.01 57.10 ^ 12.64 89.14 ^ 19.62 104.14 ^ 32.72 139.71 ^ 48.19 583.09 ^ 211.69 2187.36 ^ 2902.39 1890.10 ^ 1995.70 56.50 ^ 12.35
,0.001c 0.708 0.086 0.018c 0.496 0.153 0.323 0.024c 0.210
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Note: PSIS, posterior superior iliac spine; ASIS, anterior superior iliac spine. a Anterior reach is significantly different from posteromedial reach. b Anterior reach is significantly different from posterolateral reach. c Significant main effect for test.
strategies utilised during the SEBT in both healthy (Fullam, Caulfield, Coughlan, & Delahunt, 2014; Overmoyer & Reiser, 2013; Robinson & Gribble, 2008b) and previously injured populations (Bastien et al., 2014; Herrington et al., 2009; Olmsted et al., 2002). It has been previously reported that the anterior reach direction results in the poorest performance as represented as percentage of limb length (Gribble et al., 2012). While thought to be due to the high demand this position puts on the quadriceps muscle group (Gribble et al., 2007; Herrington et al., 2009; Robinson & Gribble, 2008b) and the impact that limited dorsiflexion range of motion may have (Hoch, Staton, & McKeon, 2011), it is notable that this task also resulted in the greatest degree of tortuosity at markers representing the pelvis (ASIS), knee (lateral knee), and foot (second metatarsal). The Table IV. Pearson product moment correlations (r) for SEBT performance and individual marker tortuosity.
ASIS PSIS Lateral thigh Lateral knee joint Fibula Lateral malleolus Calcaneus Second metatarsal head Estimated centre of mass
r p r p r p r p r p r p r p r p r p
Anterior
Posteromedial
Posterolateral
0.024 0.934 -0.070 0.813 -0.193 0.509 -0.165 0.574 0.033 0.911 -0.080 0.787 -0.407 0.149 -0.562a 0.037 -0.049 0.869
-0.576a 0.039 -0.352 0.238 -0.117 0.704 0.068 0.825 0.001 0.997 -0.367 0.217 -0.472 0.104 -0.253 0.404 -0.052 0.866
-0.210 0.470 -0.458 0.100 -0.113 0.700 0.053 0.857 -0.018 0.951 0.379 0.181 0.560a 0.037 0.204 0.484 -0.207 0.477
Note: PSIS, posterior superior iliac spine; ASIS, anterior superior iliac spine. a Indicates a significant correlation coefficient.
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Table V. Statistically significant regression models for predicting Star Excursion Balance Test reach distance based on marker tortuosity.
Anterior reach distance Posteromedial reach distance Posterolateral reach distance
Predictors
Standardised regression coefficient
R2
p
Second metatarsal ASIS Lateral knee Calcaneus
-0.562 -0.923 0.601 0.560
0.315 0.573
0.037 0.014
0.313
0.037
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Note: ASIS, anterior superior iliac spine.
demands placed upon lower extremity musculature as well as the change in height of the centre of mass associated with completion of the anterior reach distance appears to negatively impact the smoothness or stability of motion when compared to both the posteromedial and posterolateral reach directions (Fullam et al., 2014; Robinson & Gribble, 2008a). Subjective biomechanical evaluation or in some cases the broad evaluation of peak kinematic values have become a more common aspect of clinical care and return to physical activity decision-making; however, the rates of re-injury and chronic instability among those with significant ankle and knee injuries have been largely unaffected. While the SEBT is a simple, resource independent clinical evaluation tool, it may not be sufficiently sensitive to detect subtle changes in lower extremity movement that persist well beyond a return to physical activity following lower extremity joint injury (Arnold, De La Motte, Linens, & Ross, 2009; Howells, Ardern, & Webster, 2011). Based on our findings, the use of tortuosity to assess segmental steadiness during the anterior reach distance may be clinically useful to identify subtle persistent differences in movement patterns within patient populations that have been shown to have difficulty with this task such as those with a history of ACL deficiency, ACL reconstruction, and those with chronic ankle instability. Subsequent investigations must include comparisons to a variety of previously injured populations in order to better understand the relationship between increases or decreases in the tortuosity of specific lower extremity segments in those with known lower extremity movement dysfunction. In addition to assessing the differences in tortuosity between reach directions, we investigated the ability of each segmental motion to predict multi-directional dynamic balance in healthy participants. In each of the three reach distances, 3D segmental tortuosity was found to be related to and predictive of test performance (Tables IV and V). While previous literature has shown total sagittal plane knee and hip displacement to be strongly predictive of posteromedial and posterolateral reach distance performance, this is the first investigation to quantify the relative smoothness and stability of the joint movement in space (Robinson & Gribble, 2008b). Markers associated with the pelvis (ASIS) and knee (lateral knee) were predictive of the posteromedial direction; however, more similar to the anterior reach direction (second metatarsal), the posterolateral reach was best predicted using a marker associated with the foot (calcaneus). This is an interesting departure from previous studies that have used both muscle strength and lower extremity kinematics to explain performance on the SEBT. Most commonly, performance on the posteromedial and posterolateral reach directions has been linked to hip and knee joint range of motion (Robinson & Gribble, 2008a), as well as strength of the lateral hip stabilisers (Leavey, Sandrey, & Dahmer, 2010; McMullen, Cosby, Hertel, Ingersoll, & Hart, 2011). However, these assessments have relied on peak measures of strength or motion instead of measures of steadiness or smoothness. Applying the concept of tortuosity in concert with more
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traditional kinematic and kinetic assessment tools may provide a more complete understanding of the strategies utilised to optimise SEBT performance. In addition, this measurement technique may provide insight regarding alterations in movement strategy and relative smoothness of movement among a group of individuals completing the SEBT. In this study, the movements of body segments were characterised by measuring the motion tortuosity. Such metric has no unique mathematical definition, and several formulations have been proposed. For instance, the most commonly used approach is the ‘distance metric’, which provides the ratio of the path length to the distance between starting and end point of the motion (Bracher, 1982; Hart et al., 1997; Smedby et al., 1993; Zhou, Rzeszotarski, Singerman, & Chokreff, 1994). Although intuitive, this metric is not sensitive to high-frequency wiggling of a motion pattern (Capowski, Kylstra, & Freedman, 1995). Accordingly, large ‘S’ or ‘U’ patterns of motion may provide values of ‘distance metric’ larger than short high-frequency oscillating patterns. An alternative way of representing tortuosity might have been the ‘inflection count metric’, which weights ‘distance metric’ by the number of inflection points characterising the motion pattern (Bullitt et al., 2003). However, such approach presents limitations when evaluating tight helicoidal patterns: helices do not add significant length to the pattern, and do not present inflection points (Koenderink, 1990). Hence, we adopted the SOAM definition in the belief that this could be the better suiting approach for evaluating tortuosity of the specific body motions involved in this analysis, since it overcomes the aforementioned limitations characterising the ‘distance metric’ and the ‘inflection count metric’. There are several limitations to the current study that should be considered when interpreting the results. The SEBT is commonly utilised as an evaluation tool following lower extremity injury in order to identify alterations in dynamic balance. In order to better understand the application and limitations of tortuosity as a predictor of dynamic balance, our sample was limited to young, healthy, physically active individuals. Due to this delimitation, the ability to apply our findings to injured populations is limited. Future research should focus on using this novel biomechanical variable to identify patterns of compensation or altered balance strategies in individuals with a recent or history of lower extremity injury. In addition, our sample of convenience had more females (11) compared to males (4) which may have influenced our results. This was further compounded by the use of the left limb for all participants regardless of limb dominance. Previous investigations related to the influences of gender and limb dominance on dynamic balance strategies have shown a significant impact based on both factors depending on the task selected (Gribble et al., 2012; Gribble, Robinson, Hertel, & Denegar, 2009). Hence, increased variability in our data due to the difference in gender-based balance strategies may have limited the predictive ability of our findings as related to performance on the SEBT. From a methodological prospective, trials were limited to a single limb in this study for logistical reasons. Commonly, between limb asymmetry in SEBT performance is used clinically as an indicator of unilateral alterations in dynamic balance. While not the stated purpose of this study, future investigation of symmetry in lower extremity tortuosity may improve the applicability of this measure in an injured population. Finally, we utilised complex 3D motion analysis to assess tortuosity in this investigation. While this methodology is the most accurate and sensitive approach to assessing tortuosity, it may limit the generalisability and applicability to the clinical setting. Future investigations should focus on developing a more simple 2D assessment to enable the use of more widely available video technology. The SEBT is a common measure utilised for assessing alterations in dynamic balance following lower extremity injury. However, the outcome of the assessment does not provide feedback on the strategies utilised by an individual to complete the test. In this investigation,
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we found that tortuosity, an estimate twisting during a pattern of motion, of the ASIS, lateral knee, and second metatarsal was significantly greater during the anterior reach when compared to the other test components. In addition, it is clear that significant amount of variance in reach distance for each test direction can be predicted using tortuosity. However, the marker with the strongest predictive ability varies based on the task. Tortuosity is a novel biomechanical measurement technique that provides an assessment of segmental movement during common dynamic tasks such as the SEBT. This enhanced level of detail compared to more global measures of joint kinematic may provide insight into compensatory movement strategies adopted following lower extremity joint injury. Disclosure statement
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No potential conflict of interest was reported by the authors.
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