Preliminary Investigation of Limb Load Symmetry During Quiet Stance & Sit-to-Stand
(1) Department of Mechanical Engineering, Virginia Tech
(2) Department of Biomedical Engineering and Mechanics, Virginia Tech
Kevin P. Granata Biomechanics Lab at Virginia Tech
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Understanding human postural control is critical to assessing musculoskeletal and neurological pathologies.
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Limb symmetry is an important factor in evaluating balance performance [1] in numerous populations such as those with Parkinson's Disease [2], or patients who have undergone knee anterior cruciate ligament reconstruction [3] and ankle arthroplasty [4].​
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Though there is some literature that examined how quiet standing related to other balance functional tasks [5-6], this research area is still not fully understood and lacks data for healthy populations.
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Limb symmetry is also affected by everyday, environmental disturbances, and hence performing asymmetry assessments in non-research settings is important to capture a realistic prespective on human balance.
This website serves as the poster presentation of "Preliminary Investigation of Limb Load Symmetry During Quiet Stance & Sit-to-Stand" for the American Society of Biomechanics 2020 Virtual Conference. The contact information for the authors can be found on the left of this web page. The italicized section headers on the left can be used to navigate to the desired section of the poster. If you have any questions about the project please fill out the webform at the bottom of the page and I can follow up with an answer and/or further discussion!
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The purpose of this study was to examine load asymmetry during quiet stance in a healthy population and determine if that relates to a functional, sit-to-stand, task in a non-laboratory environment.
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A similar level of asymmetry across both tasks is hypothesized.
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The institutional review board at Virginia Tech approved this study and it was conducted at the Annual Meeting of the Biomedical Engineering Society (2019, Philadelphia, PA, Figures 1 - 2).
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Participants were included based on criteria in self-reported questionnaires that excluded those who:
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were under the age of 18
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require assistive devices to walk​ such as crutches or canes
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sustained an injury in the previous 3 months that kept them from being physically active or altered their typical level of physical activity
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A participant's dominant limb was self-reported by answering “Which leg would you kick a soccer ball with?”.​
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Each participant completed:
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a 30 second, eyes open bilateral quiet stance (QS) task
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five eyes open sit-to-stands (STS) as quickly and as safely as possible
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Two PASCO force platforms (Roseville, CA) recorded vertical ground reaction forces (GRF) under each foot throughout both the QS and STS tasks, at a sampling frequency of 20 Hz.
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Load symmetry was quantified using the Normalized Symmetry Index (NSI) [7].
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where t corresponds to a single trial and the denominator represents the maximum and minimum GRF value attained, for that participant, across n trials for a specific task.
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The middle 20-second interval of the QS task for each participant was broken up into three sub trials, and the five STS trials were used to calculate the NSI.
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Paired t-tests were used to assess symmetry differences between tasks (p<0.05).
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Pearson's correlation was used to examine associations between tasks (p<0.05).
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All data, statistical and graphical analysis was done in MATLAB.
Figure 1: Participant attempting a balance task.
Figure 2: Researchers explaining study and providing instructions.
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Data from sixty-three healthy, adults (25.9 ± 6.8 years old, 170.3 ± 9.1 cm, 68.0 ± 14.3 kg, 34 females, 29 males) were collected and analysed.
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Despite a visually large difference in the range of NSI between that for QS (0.692 ± 10.7, mean ± s.d.) and STS (0.824 ± 4.45) as indicated by the standard deviations, the values were not statistically different between tasks, suggesting a similar magnitude of asymmetry between the tasks. (Figure 1, p=0.919).
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The NSI of QS and STS demonstrated a weak significant positive correlation (r = 0.291, p=0.021, Figure 2).
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These results demonstrate that in a healthy population and in non-laboratory settings, asymmetry levels between QS and STS are similar but with weak, significant association.
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In a healthy population, asymmetry during QS is to be expected within a relatively large range in typical, non-research settings, and is hypothesized to be associated with the ease of the balance task.
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QS requires less muscle activation and postural control to maintain balance than a five-time STS.
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Subjects may also displace their weight more asymmetrically in QS due to the reduced risk of falling.
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The large range in asymmetry of QS found here is similar to previous work [8].
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One goal of this study was to collect asymmetry data in real-world settings, which could also be considered a limitation of this work. The testing took place in a noisy, visually stimulating conference environment disturbing subjects’ performance, with limited control over clothing or shoes worn, which restricts capturing the maximum bounds of human balance.
Figure 1: Boxplot comparing NSI values for QS and STS tasks.
Figure 2: Correlation plot for the QS and STS tasks.
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This study demonstrated that limb load asymmetry was consistent and significantly associated between QS and STS, with a larger range of symmetry levels exhibited across participants in QS than in STS.
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Future studies should seek to verify load asymmetry in relation to ease of task, and aim to establish the normal bounds of balance in larger populations.
[1] L. C. Anker et al., Gait Posture, vol. 27, Apr. 2008, doi:10.1016/j.gaitpost.2007.06.002
[2] F. A. Barbieri et al., Hum. Mov. Sci., vol. 63, Feb. 2019, doi:10.1016/j.humov.2018.10.008
[3] A. Gokeler et al., Orthop.Trauma.Surg.Res., vol. 103, Oct. 2017, doi:10.1016/j.otsr.2017.02.015
[4] J. R. Gladish et al., J. Biomech., vol. 83, Jan. 2019, doi:10.1016/j.jbiomech.2018.11.028
[5] V. Talis et al., Clinical Biomechanics, vol. 23, May 2008, doi:10.1016/j.clinbiomech.2007.11.010
[6] Miura et al., Physiother Theory Pract​., vo. 34, July 2018, doi:10.1080/09593985.2017.1422203
[7] R. Queen et al., J. Biomech., vol. 99, Jan. 2020, doi:10.1016/j.jbiomech.2019.109531
[8] J. StodóÅ‚ka et al., Acta Bioeng. Biomech., vol. 19, 2017, doi:10.5277/ABB-00712-2016-0
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