Attentional Distractions Do Not Influence Lumbar Spine Local Dynamic Stability during Repetitive Flexion-Extension Movements.

The association between low back pain and lumbar spine local dynamic stability (LDS) appears to be modulated by if and how someone catastrophizes about pain, suggesting that the cognitive perceptions of pain may influence an individual's ability to control lumbar spine motion. Previous work also demonstrates that directing cognitive resources and attentional focus can influence movement performance. Therefore, we aimed to examine whether distracting attentional focus would influence lumbar spine LDS during repetitive flexion-extension movements. Sixteen participants performed repetitive spine flexion-extension movements under two baseline conditions (pre- and post-), and while attentional focus was distracted by either an external sensory stimulus or a cognitive-motor dual-task, both targeted at the hands. Lumbar spine LDS was examined over 30 continuous movement repetitions using maximum Lyapunov exponents. In comparison to both Baseline and Post-Baseline trials, the perceived mental workload was significantly elevated during the cognitive-motor dual-task trial but not the external sensory stimulus trial. The only statistically significant effect on LDS occurred in the Post-Baseline trial, where LDS was higher than in the cognitive-motor dual-task. In combination with previous work, these findings suggest that distracting attentional focus during repetitive lumbar spine flexion-extension movements does not have a negative influence on lumbar spine LDS.

Effects of Different Wearable Resistance Placements on Running Stability.

Stability during running has been recognized as a crucial factor contributing to running performance. This study aimed to investigate the effects of wearable equipment containing external loads on different body parts on running stability. Fifteen recreational male runners (20.27 ± 1.23 years, age range 19-22 years) participated in five treadmill running conditions, including running without loads and running with loads equivalent to 10% of individual body weight placed on four different body positions: forearms, lower legs, trunk, and a combination of all three (forearms, lower legs, and trunk). A tri-axial accelerometer-based smartphone sensor was attached to the participants' lumbar spine (L5) to record body accelerations. The largest Lyapunov exponent (LyE) was applied to individual acceleration data as a measure of local dynamic stability, where higher LyE values suggest lower stability. The effects of load distribution appear in the mediolateral (ML) direction. Specifically, running with loads on the lower legs resulted in a lower LyE_ML value compared to running without loads (p = 0.001) and running with loads on the forearms (p < 0.001), trunk (p = 0.001), and combined segments (p = 0.005). These findings suggest that running with loads on the lower legs enhances side-to-side local dynamic stability, providing valuable insights for training.

Exploring how metronome pacing at varying movement speeds influences local dynamic stability and coordination variability of lumbar spine motion during repetitive lifting.

Auditory metronomes have been used to preserve movement consistency when examining local dynamic stability (LDS) and coordination variability (CV) of lumbar spine motion during repetitive movements. However, the potential influence of the metronome itself on these outcome measures has rarely been considered. Therefore, this study investigated the influence of different metronome paces (i.e., lifting speeds) on measures of lumbar spine LDS and thorax-pelvis CV during a repetitive lifting/lowering task in comparison to self-paced movements. Ten participants completed 5 repetitive lift/lower trials, where participants completed 35 consecutive repetitions (analysis on last 30 repetitions) at a self-selected pace for the first and last trial, and were paced by a 10 lift/min, 15 lift/min, and 20 lift/min metronome, in randomized order, for the remaining three trials. The average self-paced lift/lower speed before and after experiencing the three different metronome paced speeds was 16.2 (±1.02) and 17.2 (±0.73) lifts/min, respectively, and the most-preferred metronome pace trial was 15 lifts/min. Thorax-pelvis CV during the self-paced trials were similar (p > 0.05) to the 15 lift/min metronome paced trials, while greater thorax-pelvis CV was observed for the 10 lift/min compared to the 15 lift/min and 20 lift/min and second self-paced trial (all p < 0.026). This movement speed effect was not observed for lumbar spine LDS; however, more-dynamically stable movements were observed during all metronome paced trials in comparison to the self-paced trials. This study highlights that careful consideration is required when employing a metronome to control/manipulate movement characteristics while examining neuromuscular control using non-linear dynamical systems measures.

The relationship between the local dynamic stability of gait to cognitive and physical performance in older adults: A scoping review.

BACKGROUND: Local dynamic stability (LDS) has become accepted as a gait stability indicator. The deterioration of gait stability is magnified in older adults. RESEARCH QUESTION: What is the current state in the field regarding rthe relationship between LDS and cognitive and/or physical function in older adults? METHODS: A scoping review design was used to search for peer-reviewed literature or conference proceedings published through May 2023 for an association between LDS and cognitive (e.g., Montreal Cognitive Assessment) or physical performance (e.g., Timed Up & Go Test) in older adults. Only studies investigating gait stability via LDS during controlled walking, when dealing with a subject group consisting of healthy older adults, and quantifying LDS relationship to cognitive and/or physical measure were included. We analysed data from the studies in a descriptive manner. RESULTS: In total, 814 potentially relevant articles were selected, of which 15 met the inclusion criteria. We identified 37 LDS quantifiers employed in LDS-cognition and/or LDS-physical performance relationship assessment. Nine measures of cognitive and 20 measures of physical performance were analysed. Most studies estimated LDS quantities using triaxial acceleration data. However, there was a variance in sensor placement and signal direction. Out of the 56 studied relationships of LDS to physical performance measures, sixteen were found to be relevant. Out of 22 studied relationships between LDS and cognitive measures, only two were worthwhile. SIGNIFICANCE: Considering the heterogeneity of the utilized LDS (caused by different sensors locations, signals, and signal directions as well as variety of computational approaches to estimate LDS) and cognitive/physical measures, the results of this scoping review does not indicate a current need for a systematic review with meta-analysis. To assess the overall utility of LDS to reveal a relationship between LDS to cognitive and physical performance measures, an analysis of other subject groups would be appropriate.

Relationship between gait stability indices and gait parameters comprising joint angles based on walking data from 288 people.

Stability during walking is essential because falling accidents may lead to severe injuries. In this study, we calculated the margin of stability (MoS) and the maximum Lyapunov exponent (λs), which are two major stability indices for walking, using a gait database representing 300 healthy people. Previously, the relationships between these indices and other gait parameters, including joint angles, have not been investigated in such a large subject pool. Therefore, we determined the relationships between these stability indices and the gait parameters by calculating correlation coefficients and performing multiple regression analysis. The results indicated that MoS is dominated by walking speed in the forward direction and associated with various joint angles in the lateral direction. Conversely, no relationships were identified between λs and the gait parameters. Although both MoS and λs are considered as measures of gait stability, they are independent. The results of this study suggest that MoS and λs represent different aspects of gait motion.

Ankle strategy assistance to improve gait stability using controllers based on in-shoe center of pressure in 2 degree-of-freedom powered ankle-foot orthoses: a clinical study.

BACKGROUND: Although the ankle strategy is important for achieving frontal plane stability during one-leg stance, previously developed powered ankle-foot orthoses (PAFOs) did not involve ankle strategies because of hardware limitations. Weakness of movement in frontal plane is a factor that deteriorates gait stability and increases fall risk so it should not be overlooked in rehabilitation. Therefore, we used PAFO with subtalar joint for frontal plane movement and tried to confirm that the existence of it is important in balancing through clinical experiments. METHODS: We developed a proportional CoP controller to assist ankle strategy or stabilizing moment and enhance eversion to compensate for the tilting moment with 2 dof PAFO. It was true experimental study, and we recruited seven healthy subjects (30 ± 4 years) who did not experience any gait abnormality participated in walking experiments for evaluating the immediate effect of subtalar joint of PAFO on their gait stability. They walked on the treadmill with several cases of controllers for data acquisitions. Indices of gait stability and electromyography for muscle activity were measured and Wilcoxon signed-rank tests were used to identify meaningful changes. RESULTS: We found that subjects were most stable during walking (in terms of largest Lyapunov exponents, p < 0.008) with the assistance of the PAFO when their electromyographic activity was the most reduced (p < 0.008), although postural sway increased when a proportional CoP controller was used to assist the ankle strategy (p < 0.008). Other indices of gait stability, kinematic variability, showed no difference between the powered and unpowered conditions (p > 0.008). The results of the correlation analysis indicate that the actuator of the PAFO enhanced eversion and preserved the location of the CoP in the medial direction so that gait stability was not negatively affected or improved. CONCLUSIONS: We verified that the developed 2 dof PAFO assists the ankle strategy by compensating for the tilting moment with proportional CoP controller and that wearer can walk in a stable state when the orthosis provides power for reducing muscle activity. This result is meaningful because an ankle strategy should be considered in the development of PAFOs for enhancing or even rehabilitating proprioception. Trial registration 7001988-202003-HR-833-03.

Static and local dynamic stability of subjects with knee joint osteoarthritis.

Stability is defined as the ability to control the amplitude and velocity of center of gravity (CoG) changes during quiet standing, and to decrease the risk of falling during walking. Few studies exist which examine the local dynamic stability of those with knee joint osteoarthritis (OA). Nevertheless, it is debatable whether the stability of OA subjects differs from that of normal subjects or not. The aim of this study was to evaluate the static and local dynamic stability of the subjects with knee OA in comparison to normal subjects. Fifteen OA subjects and 15 healthy subjects with matching age, weight, and gender participated in this study. A Qualysis motion analysis system incorporating a Kistler force plate was used to record data during quiet standing and when walking on a treadmill. Static stability was evaluated based on center of pressure (COP) sways excursion, path length and velocity in mediolateral and anteroposterior axes. Local dynamic stability was evaluated by the use of short Lyapunov exponent when subjects walk on treadmill. A two-sample test-test was used for the final analysis (α = 0.05). Mean values of the COP excursion in mediolateral and anteroposterior directions, and the velocity of COP in both mediolateral and anteroposterior directions, differed significantly between healthy and OA subjects (p < 0.05). Local dynamic stability of OA subjects appeared to be reduced compared to healthy subjects, especially when ambulating at higher walking speeds (p-value < 0.05). Study results demonstrated that both static and local dynamic stability decreased in OA subjects. This may be in part due to altered control mechanisms which are required for response to perturbations during standing and walking to ensure stability. As a decrease in local dynamic stability is correlated to an increased risk of falling, it is important that this group should receive appropriate treatment interventions to improve standing and walking stability.

Cognitive dual-task alters Local Dynamic Stability of lower extremity during common movements.

A dual-task paradigm is most commonly used in the field of biomechanics to understand the effect of multi-tasking or cognitive load on motor performance. The Local Dynamic Stability (LDS) is most commonly used to quantify motor performance, but there are still several unknown effects of this metric with varied task conditions and cognitive demands. Therefore, this study used motion capture to collect biomechanical data from 28 healthy collegiate participants during a walk and jog task both with and without a semantic fluency task to investigate the effects of task speed, limb dominance, and semantic fluency on LDS. This study showed that the change in ankle plantarflexion LDS during a jog was dependent on self-selected speed. Participants with slower jogging speeds increased ankle plantarflexion LDS during a dual-task, while individuals with faster jogging speeds decreased ankle plantarflexion LDS during a dual-task. This study also found that subjects compensate during gait by both increasing and decreasing LDS in different degrees of freedom of the lower extremity. This study did not find evidence of limb dominance effecting the change in LDS during a dual-task while walking or jogging. These findings reveal where healthy adults compensate for simple movement patterns while multitasking. Future work should further explore the role and relationship between trunk movement and lower extremity compensation, and could help give further context to how the LDS can be interpreted by researchers and clinicians alike.

Nonlinear analysis of dynamic stability in walking with toe-only rocker sole shoes in elderly.

BACKGROUND: Elderly subjects are at the risk of falling. One type of shoe intervention used for this group of the subjects is the shoe with rocker. The aim of this study was to investigate the effects of shoes with various degrees of rockers on dynamic stability of elderly subjects while walking. METHOD: 15 elderly subjects were recruited in this study. A motion analysis system was used to record the motions of body while walking on a treadmill. The local dynamic stability (LDS) was evaluated based on use of Lyapunov exponent of center of mass (COM) movement. The subjects were asked to walk barefoot, with shoe with no rocker and with shoe with various rockers (10, 20, 30 and 40°). RESULTS: The mean values of LDS (λmax-S) in anteroposterior direction were 0.95 ± 0.46, 0.78 ± 0.51 and 0.74 ± 0.54 in bare foot, shoes with no rocker and shoe with 10° rocker, respectively. The mean value of LDS (λmax-S) in vertical direction varied between 1.21 and 1.23. There was no significant difference between LDS of elderly subjects while walking with shoes with various rocker angles. DISCUSSION: Use of shoes with various rocker angles dose not influence on dynamic stability of elderly subjects while walking. Therefore, it is recommended to use this kind of shoe intervention for other trapeutic purposes.

Influence of back muscle fatigue on dynamic lumbar spine stability and coordination variability of the thorax-pelvis during repetitive flexion-extension movements.

Previous work has identified that individuals adopt different dynamic lumbar spine stability responses when experiencing back muscle fatigue, and that the neuromuscular system adjusts multi-joint coordination in response to fatigue. Therefore, this study was designed to determine whether distinct differences in coordination and coordination variability would be observed for those who stabilize, destabilize, or demonstrate no change in dynamic stability when their back muscles are fatigued. Thirty participants completed two repetitive trunk flexion-extension trials (Rested, Fatigued) during which lumbar flexion-extension dynamic stability, thorax-pelvis movement coordination, and coupling angle variability (CAV) were assessed. Dynamic stability was evaluated using maximum Lyapunov exponents (λmax) with participants being allotted to stabilizer, destabilizer, or no change groups based on their stability response to fatigue. Each flexion-extension repetition was further segregated into two phases (flexion, extension) and vector coding analyses were implemented to determine thorax-pelvis coordination and CAV during each movement phase. Results demonstrated that when fatigued, ∼30% of individuals adopted more stable (lower λmax) flexion-extension movements and greater CAV during the extension phase, ∼17% of individuals became less stable (higher λmax) and exhibited decreased CAV during the extension phase, and the remaining âˆ¼53% of individuals expressed no change in dynamic stability or CAV. Additionally, more in-phase coordination patterns were generally observed across all individuals when fatigued. Altogether, this study highlights the heterogeneous nature of lumbar spine movement behaviours within a healthy population in response to fatigue.

Associations of low-back pain and pain-related cognitions with lumbar movement patterns during repetitive seated reaching.

BACKGROUND: Development of more effective interventions for nonspecific chronic low back pain (LBP), requires a robust theoretical framework regarding mechanisms underlying the persistence of LBP. Altered movement patterns, possibly driven by pain-related cognitions, are assumed to drive pain persistence, but cogent evidence is missing. AIM: To assess variability and stability of lumbar movement patterns, during repetitive seated reaching, in people with and without LBP, and to investigate whether these movement characteristics are associated with pain-related cognitions. METHODS: 60 participants were recruited, matched by age and sex (30 back-healthy and 30 with LBP). Mean age was 32.1 years (SD13.4). Mean Oswestry Disability Index-score in LBP-group was 15.7 (SD12.7). Pain-related cognitions were assessed by the 'Pain Catastrophizing Scale' (PCS), 'Pain Anxiety Symptoms Scale' (PASS) and the task-specific 'Expected Back Strain' scale(EBS). Participants performed a seated repetitive reaching movement (45 times), at self-selected speed. Lumbar movement patterns were assessed by an optical motion capture system recording positions of cluster markers, located on the spinous processes of S1 and T8. Movement patterns were characterized by the spatial variability (meanSD) of the lumbar Euler angles: flexion-extension, lateral-bending, axial-rotation, temporal variability (CyclSD) and local dynamic stability (LDE). Differences in movement patterns, between people with and without LBP and with high and low levels of pain-related cognitions, were assessed with factorial MANOVA. RESULTS: We found no main effect of LBP on variability and stability, but there was a significant interaction effect of group and EBS. In the LBP-group, participants with high levels of EBS, showed increased MeanSDlateral-bending (p = 0.004, η2 = 0.14), indicating a large effect. MeanSDaxial-rotation approached significance (p = 0.06). SIGNIFICANCE: In people with LBP, spatial variability was predicted by the task-specific EBS, but not by the general measures of pain-related cognitions. These results suggest that a high level of EBS is a driver of increased spatial variability, in participants with LBP.

Effects of barefoot vs. shod walking during indoor and outdoor conditions in younger and older adults.

BACKGROUND: Gait stability and variability measures in barefoot and shod locomotion are frequently investigated in younger but rarely in older adults. Moreover, most studies examine gait measures in laboratory settings instead of real-life settings. RESEARCH QUESTIONS: How are gait stability and variability parameters affected by footwear compared to barefoot walking in younger and older adults as well as under indoor vs. outdoor conditions? METHODS: Healthy younger (<35 years) and older adults (>65 years) participated in the randomised within-subject study design. Participants conducted consecutive 25 m walking trials barefoot and with standardised footwear inside and outside. Inertial measurement units were mounted on the participant's foot and used to calculate local dynamic stability (LDS), velocity and minimal toe clearance (MTC), stride length and stride time, including variabilities for these parameters. Linear mixed models were calculated. RESULTS: Data of 32 younger (17 female, 15 male, age: 30 ± 4 years) and 42 older participants (24 female, 18 male, age: 71 ± 4 years) were analysed. MTC variability was higher in shod conditions compared to barefoot (p = 0.048) and in outdoor conditions (p < 0.001). LDS was different between age groups (p < 0.001). Gait velocity and MTC were higher in shod and outdoor conditions (both p < 0.001). Stride length and time were higher in shod conditions (both p < 0.001) and different between outdoor vs. indoor (longer stride length and shorter stride time outdoor, both (p < 0.001) as well as age groups (shorter stride length (p < 0.021) and stride time in older adults (p < 0.001). SIGNIFICANCE: Results suggest that gait stability and variability in older and younger adults are acutely affected by footwear vs. barefoot and indoor vs. outdoor walking conditions, indicating a high adaptiveness of these parameters to different experimental conditions. Consequently, future studies should be careful with generalising results obtained under certain conditions. Findings stress the clinical potential of barefoot walking.

Validity and Sensitivity of an Inertial Measurement Unit-Driven Biomechanical Model of Motor Variability for Gait.

Motor variability in gait is frequently linked to fall risk, yet field-based biomechanical joint evaluations are scarce. We evaluated the validity and sensitivity of an inertial measurement unit (IMU)-driven biomechanical model of joint angle variability for gait. Fourteen healthy young adults completed seven-minute trials of treadmill gait at several speeds and arm swing amplitudes. Trunk, pelvis, and lower-limb joint kinematics were estimated by IMU- and optoelectronic-based models using OpenSim. We calculated range of motion (ROM), magnitude of variability (meanSD), local dynamic stability (λmax), persistence of ROM fluctuations (DFAα), and regularity (SaEn) of each angle over 200 continuous strides, and evaluated model accuracy (RMSD: root mean square difference), consistency (ICC2,1: intraclass correlation), biases, limits of agreement, and sensitivity to within-participant gait responses (effects of speed and swing). RMSDs of joint angles were 1.7-9.2° (pooled mean of 4.8°), excluding ankle inversion. ICCs were mostly good to excellent in the primary plane of motion for ROM and in all planes for meanSD and λmax, but were poor to moderate for DFAα and SaEn. Modelled speed and swing responses for ROM, meanSD, and λmax were similar. Results suggest that the IMU-driven model is valid and sensitive for field-based assessments of joint angle time series, ROM in the primary plane of motion, magnitude of variability, and local dynamic stability.

Measuring Gait Stability in People with Multiple Sclerosis Using Different Sensor Locations and Time Scales.

The evaluation of local divergence exponent (LDE) has been proposed as a common gait stability measure in people with multiple sclerosis (PwMS). However, differences in methods of determining LDE may lead to different results. Therefore, the purpose of the current study was to determine the effect of different sensor locations and LDE measures on the sensitivity to discriminate PwMS. To accomplish this, 86 PwMS and 30 healthy participants were instructed to complete a six-minute walk wearing inertial sensors attached to the foot, trunk and lumbar spine. Due to possible fatigue effects, the LDE short (~50% of stride) and very short (~5% of stride) were calculated for the remaining first, middle and last 30 strides. The effect of group (PwMS vs. healthy participants) and time (begin, mid, end) and the effect of Expanded Disability Status Scale (EDSS) and time were assessed with linear random intercepts models. We found that perturbations seem to be better compensated in healthy participants on a longer time scale based on trunk movements and on a shorter time scale (almost instantaneously) according to the foot kinematics. Therefore, we suggest to consider both sensor location and time scale of LDE when calculating local gait stability in PwMS.

Dynamics of Modular Neuromotor Control of Walking and Running during Single and Dual Task Conditions.

The aim of the study was to determine the stability and complexity of muscle synergies to provide insight to the neural control of gait stability in walking and running and when performing a concurrent cognitive dual task. Eighteen healthy young adults performed walking and running at preferred speeds and 120% of preferred speeds in single and dual task conditions. Muscle synergies were determined from the activity of 9 trunk and leg muscles and centre of mass (COM) motion was recorded with an inertial measurement unit. Local dynamic stability, complexity and width of synergies, and stability and complexity of COM motion were determined, in addition to the cross sample entropy to determine the coupling between COM motion and muscle synergies. Increasing locomotion speed increased complexity and decreased stability of COM motion with a concurrent decrease in synergy complexity and stability but with no change in synergy width. The coupling of COM motion and muscle synergies also increased with increasing speed. Vertical COM motion was more complex and less stable but with no change in anterior-posterior or medio-lateral directions in dual task locomotion. Muscle synergies were also more stable in dual task conditions. These findings indicate that changes in neuromotor dynamics may underpin reported changes in COM local stability during gait as the neural commands responsible for generating the movement are altered in response to increasing task demands. Increased cognitive demands lead to more stable neuromotor commands possibly to maintain local stability of COM motion in the anterior-posterior and medio-lateral directions.

Effect of data length on time delay and embedding dimension for calculating the Lyapunov exponent in walking.

The Lyapunov exponent (LyE) is a trending measure for characterizing gait stability. Previous studies have shown that data length has an effect on the resultant LyE, but the origin of why it changes is unknown. This study investigates if data length affects the choice of time delay and embedding dimension when reconstructing the phase space, which is a requirement for calculating the LyE. The effect of three different preprocessing methods on reconstructing the gait attractor was also investigated. Lumbar accelerometer data were collected from 10 healthy subjects walking on a treadmill at their preferred walking speed for 30 min. Our results show that time delay was not sensitive to the amount of data used during calculation. However, the embedding dimension had a minimum data requirement of 200 or 300 gait cycles, depending on the preprocessing method used, to determine the steady-state value of the embedding dimension. This study also found that preprocessing the data using a fixed number of strides or a fixed number of data points had significantly different values for time delay compared to a time series that used a fixed number of normalized gait cycles, which have a fixed number of data points per stride. Thus, comparing LyE values should match the method of calculation using either a fixed number of strides or a fixed number of data points.

Accuracy of image data stream of a markerless motion capture system in determining the local dynamic stability and joint kinematics of human gait.

Assessment of gait parameters is commonly performed through the high-end motion tracking systems, which limits the measurement to sophisticated laboratory settings due to its excessive cost. Recently, Microsoft Kinect (v2) sensor has become popular in clinical gait analysis due to its low-cost. But, determining the accuracy of its RGB-D image data stream in measuring the joint kinematics and local dynamic stability remains an unsolved problem. This study examined the suitability of Kinect(v2) RGB-D image data stream in assessing those gait parameters. Fifteen healthy participants walked on a treadmill during which lower body kinematics were measured by a Kinect(v2) sensor and a optophotogrametric tracking system, simultaneously. Extended Kalman filter was used to extract the lower extremity joint angles from Kinect, while inverse kinematics was used for the gold standard system. For both systems, local dynamic stability was assessed using maximal Lyapunov exponent. Sprague's validation metrics, root mean square error (RMSE) and normalized RMSE were computed to confirm the difference between the joint angles time series of the two systems while relative agreement between them was investigated through Pearson's correlation coefficient (pr). Fisher's Exact Test was performed on maximal Lyapunov exponent to investigate the data independence while reliability was assessed using intraclass correlation coefficients. This study concludes that the RGB-D data stream of Kinect sensor is efficient in estimating joint kinematics, but not suitable for measuring the local dynamic stability.

Local dynamic stability during long-fatiguing walks in people with multiple sclerosis.

BACKGROUND: Altered balance/stability during walking is common in people with multiple sclerosis (PwMS). While dynamic gait stability has been related to falling and localized muscle fatigue, it has rarely been studied in MS. Specifically, the effects of walking-related fatigue on dynamic stability are unclear in PwMS. RESEARCH QUESTIONS: 1) Are temporal changes in dynamic stability during long-walks different among PwMS and healthy controls (HC)? 2) Is there a relationship between stability and walking performance changes in PwMS? METHODS: Twenty-five PwMS and ten HC participated in the six-minute walk test (6MWT) wearing six-wireless inertial sensors. Local dynamic stability (LDS) during gait was quantified by maximum-finite-time Lyapunov exponents (λS), where larger λS indicates less stable dynamics. Linear mixed models were fit to compare changes in LDS and walking performance over time among two groups. Additionally, the percent changes in λS and distance from minute 1 to 6 were recorded as Dynamic Stability Index (DSI6-1) and Distance-Walked Index (DWI6-1) respectively. Finally, Pearson correlation compared the association between DSI6-1 and DWI6-1. RESULTS: A significant group*time interaction was found for LDS. PwMS did not have different LDS than HC until minute-4 of walking, and differences persisted at minute-6. Further, PwMS walked significantly shorter distances and demonstrated a greater decline in walking performance (DWI6-1) during the 6MWT. Finally, DSI6-1 and DWI6-1 were significantly correlated in PwMS. Significance The dynamic stability differences among PwMS and HC were only apparent after 3-minutes of walking and ∼60% of PwMS became less stable over time, supporting the use of long walks in MS to capture stability changes during the motor task performance. A significant relationship between the decline in stability and poor walking performance over time during the 6MWT suggested a possible role of walking-related fatigue in the worsening of balance during long walks in PwMS.

Concurrent validity of a wearable IMU for objective assessments of functional movement quality and control of the lumbar spine.

Inertial measurement units (IMUs) are being recognized in clinical and rehabilitation settings for their ability to assess movement-related disorders of the spine for better guidance of treatment-planning and tracking of recovery. This study evaluated the Mbientlab MetaMotionR IMUs, relative to Vicon motion capture equipment in measuring local dynamic stability of the spine (quantified using maximum finite-time Lyapunov exponent; λmax), lumbopelvic coordination (quantified using mean absolute relative phase; MARP), and intersegmental motor variability (quantified using deviation phase; DP) of lumbopelvic segments in 10 participants during 35 cycles of repetitive spine flexion-extension (FE). Intraclass correlations were strong between systems when using both the FE angle time-series and the sum of squares (SS) time-series to measure local dynamic stability (0.807 ≤ICC2,1λmax,FE ≤ 0.919; 0.738 ≤ ICC2,1λmax,SS ≤ 0.868), sagittal-plane lumbopelvic coordination (0.961 ≤ICC2,1MARP ≤ 0.963), and sagittal-plane lumbopelvic variability (0.961 ≤ICC2,1DP ≤ 0.963). It was concluded that the MetaMotionR IMUs can be reliably used for measuring features associated with spine movement quality and motor control during a repetitive FE task. Future work will assess the reliability of sensor placement, performance during multi-directional movements, and ability to discern clinical and healthy populations based on assessment of movement quality and control.

A comparison of methods to quantify control of the spine.

Low back pain (LBP) affects many individuals worldwide. The established association between LBP and spine motor control has led to the development of many control assessment techniques. To understand spine control and LBP, it is essential to know the relationship between assessment techniques. Systems identification (SI) and local dynamic stability (LDS) are two methods of quantifying spine control. SI provides a detailed description of control but uses linearity assumptions, whereas LDS provides a "black box" non-linear assessment during dynamic movements. Therefore, the purpose of this project was to compare control outcomes of SI and LDS. 15 participants completed two tasks (SI and LDS) in a random order. For the SI task, participants were seated and ventrally perturbed at the 10th thoracic vertebrae. They were instructed to resist the perturbations (resist condition) or to relax the trunk (relax condition). Admittance was computed, and a neuromuscular control model quantified lumbar stiffness, damping and muscle spindle feedback gains. For the LDS task, participants completed three repetitive movement blocks consisting of flexion/extension, axial rotation, and complex movements. In each block, the maximum finite-time Lyapunov exponent (λmax) was estimated. A stepwise linear regression determined that λmax during the rotation task was best predicted by SI outcomes in the relax condition (adjusted R2 = 0.83). Many conditions demonstrated no relationship between λmax and SI outcomes. These findings outline the importance of a consistent framework for the assessment of spine control. This could clarify research comparisons and the understanding of the cause/effect role of LBP on spine control.