Sensitivity of discrete symmetry metrics: Implications for metric choice.

Gait asymmetry is present in several pathological populations, including those with Parkinson's disease, Huntington's disease, and stroke survivors. Previous studies suggest that commonly used discrete symmetry metrics, which compare single bilateral variables, may not be equally sensitive to underlying effects of asymmetry, and the use of a metric with low sensitivity could result in unnecessarily low statistical power. The purpose of this study was to provide a comprehensive assessment of the sensitivity of commonly used discrete symmetry metrics to better inform design of future studies. Monte Carlo simulations were used to estimate the statistical power of each symmetry metric at a range of asymmetry magnitudes, group/condition variabilities, and sample sizes. Power was estimated by repeated comparison of simulated symmetric and asymmetric data with a paired t-test, where the proportion of significant results is equivalent to the power. Simulation results confirmed that not all common discrete symmetry metrics are equally sensitive to reference effects of asymmetry. Multiple symmetry metrics exhibit equivalent sensitivities, but the most sensitive discrete symmetry metric in all cases is a bilateral difference (e.g. left-right). A ratio (e.g. left/right) has poor sensitivity when group/condition variability is not small, but a log-transformation produces increased sensitivity. Additionally, two metrics which included an absolute value in their definitions showed increased sensitivity when the absolute value was removed. Future studies should consider metric sensitivity when designing analyses to reduce the possibility of underpowered research.

Effects of arm swing amplitude and lower limb asymmetry on motor variability patterns during treadmill gait.

Motor variability is a fundamental feature of gait. Altered arm swing and lower limb asymmetry (LLA) may be contributing factors having been shown to affect the magnitude and dynamics of variability in spatiotemporal and trunk motion. However, the effects on lower limb joints remain unclear. Full-body kinematics of 15 healthy young adults were recorded during treadmill walking using the Computer-Assisted Rehabilitation Environment system. Participants completed six trials, combining three arm swing (AS) amplitude (normal, active, held) and two LLA (symmetrical, asymmetrical) conditions. The mean standard deviation (meanSD), maximum Lyapunov exponent (λmax), detrended fluctuation analysis scaling exponent of range of motion (DFAα), and sample entropy (SaEn) were computed for tridimensional trunk, pelvis, and lower limb joint angles, and compared using repeated-measures ANOVAs. Relative to normal AS, active AS increased meanSD of all joint angles, λmax of frontal plane hip and ankle angles, and SaEn of sagittal plane ankle angles. Active AS, however, did not affect λmax or SaEn of trunk or pelvis angles. LLA increased meanSD of sagittal plane joint angles, λmax of Euclidean norm trunk angle and of lower limb joint angles, and SaEn of ankle dorsiflexion/ plantarflexion, but decreased SaEn of tridimensional trunk angles and hip rotation in the slower moving leg. Alterations in lower limb variability with active AS and LLA suggest that young adults actively exploit their lower limb redundancies to maintain gait. This appears to preserve trunk stability and regularity during active AS but not during LLA.

Recovery of dynamic stability during slips unaffected by arm swing in people with Parkinson's Disease.

The arm elevation strategy assists in recovering stability during slips in healthy young and elderly individuals. However, in people with Parkinson's Disease, one of the main motor symptoms affecting the upper limbs is reduced arm swing which intensifies throughout the course of the disease before becoming absent. This holds direct implications for these individuals when encountering slips as the arm elevation strategy is an integral component in the interlimb slip response to restore stability. Arm swing's effect in recovering from slips in people with Parkinson's Disease though remains unexamined. Twenty people with Parkinson's Disease (63.78 ± 8.97 years) walked with restricted and unrestricted arm swing conditions on a dual-belt treadmill where slips were induced on the least and most affected sides. Data were collected on the CAREN Extended System (Motek Medical, Amsterdam, NL). The Margin of Stability, linear and angular trunk velocities, as well as step length, time, and width were calculated. Data were examined during the slipped step and recovery step. The restricted arm swing condition, compared to unrestricted, caused a faster step time during the slipped step. Compared to the most affected leg, the least affected had a wider step width during the slipped step. During the recovery step, the least affected leg had a larger anteroposterior Margin of Stability and longer step time than the most affected. No differences between our arm swing conditions suggests that the normal arm swing in our participants was not more effective at restoring stability after an induced slip compared to when their arm motion was restricted. This may be due to the arm elevation strategy being ineffective in counteracting the slip's backward destabilization in these individuals. Differences between the legs revealed that our participants were asymmetrically impaired in their slip recovery response.

Walking on Mild Slopes and Altering Arm Swing Each Induce Specific Strategies in Healthy Young Adults.

Slopes are present in everyday environments and require specific postural strategies for successful navigation; different arm strategies may be used to manage external perturbations while walking. It has yet to be determined what impact arm swing has on postural strategies and gait stability during sloped walking. We investigated the potentially interacting effects of surface slope and arm motion on gait stability and postural strategies in healthy young adults. We tested 15 healthy adults, using the CAREN-Extended system to simulate a rolling-hills environment which imparted both incline (uphill) and decline (downhill) slopes (± 3°). This protocol was completed under three imposed arm swing conditions: held, normal, active. Spatiotemporal gait parameters, mediolateral margin of stability, and postural kinematics in anteroposterior (AP), mediolateral (ML), and vertical (VT) directions were assessed. Main effects of conditions and interactions were evaluated by 2-way repeated measures analysis of variance. Our results showed no interactions between arm swing and slope; however, we found main effects of arm swing and main effects of slope. As expected, uphill and downhill sections of the rolling-hills yielded opposite stepping and postural strategies compared to level walking, and active and held arm swings led to opposite postural strategies compared to normal arm swing. Arm swing effects were consistent across slope conditions. Walking with arms held decreased gait speed, indicating a level of caution, but maintained stability comparable to that of walking with normal arm swing. Active arm swing increased both step width variability and ML-MoS during downhill sections. Alternately, ML-MoS was larger with increased step width and double support time during uphill sections compared to level, which demonstrates that distinct base of support strategies are used to manage arm swing compared to slope. The variability of the rolling-hills also required proactive base of support changes despite the mild slopes to maintain balance.

The effect of various arm and walking conditions on postural dynamic stability when recovering from a trip perturbation.

BACKGROUND: Appropriately responding to unexpected perturbations, such as a trip, is critical to sustain balance and avoid falling during walking. RESEARCH QUESTION: How do arm motion and walking asymmetry affect postural stability when recovering from a trip perturbation? METHODS: Fifteen healthy young individuals, who had no experience with treadmill induced perturbations, participated in this study. The Computer-Assisted Rehabilitation Environment system (CAREN-Extended) was used to simulate unexpected perturbations while walking symmetrically and asymmetrically with various arm swings (normal, bound, released). Whole-body angular momentum (WBAM), peak trunk angular velocities, Center of Mass (COM), step width and stance time were analyzed before and when recovering from trip perturbations. RESULTS: Participants were able to recover their postural stability within three strides following the sudden anterior-posterior trip perturbation. The perturbation increased peak trunk angular velocity, the COM excursion and WBAM but did not affect stance time and step width. The arm conditions had significant effects on peak trunk angular velocity, WBAM and step width during pre-perturbation. Walking conditions had a significant effect on all variables during pre-perturbation; however, post-perturbation showed significant effects only for peak trunk angular velocity, WBAM, and COM. SIGNIFICANCE: Unexpected perturbation had effects on most of gait variables; nevertheless, participants fully recovered and adapted their gait pattern to sudden perturbations even without using their arms while walking symmetrically and asymmetrically. Arm movements could help young individuals recover after a perturbation but are not essential for perturbations of moderate magnitude. The effect of medial-lateral perturbations on gait still need to be investigated.

Active arm swing and asymmetric walking leads to increased variability in trunk kinematics in young adults.

Fall induced injuries are a leading cause for occupational injuries with the majority originating from challenging same-level walking surfaces. Despite current perturbation and fall prevention paradigms, occupational fall prevalence remains stable. Typically, these paradigms do not account for arm swing which has been demonstrated to affect the center of mass' movement during walking. This study examined the effect of different arm swing on postural control during symmetric and asymmetric walking. Fifteen healthy young adults (age = 23.4 ± 2.8) walked symmetrically and asymmetrically with three arm motions (normal, held, and active) on a split-belt treadmill CAREN Extended-System (Motek Medical, Amsterdam, NL). Mean, standard deviation and maximal values of trunk linear and angular velocity, and whole-body angular momentum were calculated in all three axes; additionally, step length, time and width mean and Coefficient of Variation, Margin of Stability and Harmonic Ratios were calculated. Compared to normal and held conditions, active arm increased trunk linear and angular velocity standard deviation, max velocity values, mean step length and time, as well as the Coefficient of Variation for step length, time, and width. Furthermore, whole-body angular momentum increased as a function of arm swing amplitude. Active arm swing further reduced Harmonic Ratios in the mediolateral and anteroposterior directions. Asymmetric walking increased average step time, and width as well as increased the Coefficient of Variation for step length and time but reduced left average step length and step width Coefficient of Variation. Further, asymmetric walking increased mediolateral Margin of Stability and reduced anteroposterior and mediolateral Harmonic Ratios. Finally, results demonstrated that actively increasing arm swing increases trunk linear and angular velocity variability in healthy young adults during symmetric and asymmetric treadmill walking. Findings may be due to active arm swing and asymmetric walking causing a disproportional contribution to trunk and center of mass movement causing participants to modify their base of support to maintain stability.

The effects of arm swing amplitude and lower-limb asymmetry on gait stability.

Changes to arm swing and gait symmetry are symptomatic of several pathological gaits associated with reduced stability. The purpose of this study was to examine the relative contributions of arm swing and gait symmetry towards gait stability. We theorized that actively increasing arm swing would increase gait stability, while asymmetric walking would decrease gait stability. Fifteen healthy, young adults (23.4 ± 2.8 yrs) walked on a split-belt treadmill under symmetric (1.2 m/s) and asymmetric walking (left/right, 5:4 speed ratio) with three different arm swings: held, normal, and active. Trunk local dynamic stability, inter-limb coordination, and spatiotemporal gait variability and symmetry were measured. Active arm swing resulted in improved local trunk stability, increased gait variability, and decreased inter-limb coordination (p < .013). The changes in local trunk stability and gait variability during active arm swing suggests that these metrics quantify fundamentally different aspects of stability and are not always comparable. Split-belt walking caused reduced local trunk stability, increased gait variability, and increased lower limb asymmetry (p < .003). However, the arm swing symmetry was unaffected by gait asymmetry, this suggests that the decreases in gait stability are linked to the increases in gait asymmetry rather than increases in arm swing asymmetry.

Effect of arm motion on postural stability when recovering from a slip perturbation.

The aim of this study was to examine the effects of various arm swing on postural stability and recovery responses to an unexpected slip during treadmill walking. Fifteen healthy young adults (23.4 ±â€¯2.8 years old) participated in this study. The CAREN-Extended system was used to simulate unexpected slip perturbations in a safe environment while walking symmetrically and asymmetrically with various arm swings (normal, bound, released). Whole-body angular momentum (range), peak trunk angular velocities, step width and stance time were extracted before and after perturbations (when recovering from slip). All participants were able to recover their balance after two strides and no falls occurred. There were significant differences (p < 0.05) in most gait parameters between pre- and post-perturbations. Arm conditions had significant effects on all gait parameters during both pre- and post-perturbation except for stance time. Compared to symmetric walking, walking asymmetrically before a perturbation led to larger step width and stance time among the different arm conditions both before and after the perturbations. Despite the presence of significant effects of different arm and walking conditions on most gait parameters during pre- and post-perturbation, participants were able to implement stabilization strategies to prevent fall even when they were prevented from using their normal arm swing, in both symmetric and asymmetric walking. While our results indicate that perturbations were mild to moderate in magnitude, investigations with elderly and faller populations are needed to examine their susceptibility to these arm and walking conditions when trying to regain postural balance.

Medication and trial duration influence postural and pointing parameters during a standing repetitive pointing task in individuals with Parkinson's disease.

We aimed to determine the effects of levodopa medication on the performance of a repetitive pointing task while standing, and to investigate the optimal trial duration in individuals with Parkinson's disease, and older adults. Seventeen individuals with Parkinson's disease (5 freezers) and 9 older adults stood on force platforms for 30 s and 120 s while performing a bilateral repetitive pointing task, tracked by motion capture. Participants with Parkinson's disease were assessed on and off medication and older adults were also assessed on separate days. The main findings were that: 1) on medication, participants with Parkinson's exhibited greater center of pressure root mean square in the medial-lateral direction, greater velocity in the medial-lateral and anterior-posterior directions, and greater range in the medial-lateral direction than off medication; 2) longer trial durations resulted in greater center of pressure range in the medial-lateral and anterior-posterior directions and greater coefficient of variation in finger pointing on the least affected side; 3) Parkinson's participants exhibited larger range in the medial-lateral direction compared to older adults; 4) off medication, freezers presented with less range and root mean square in the anterior-posterior direction than non-freezers; and 5) a correlation emerged between the freezing of gait questionnaire and pointing asymmetry and the coefficient of variation of pointing on the most affected side. Therefore, Parkinson's medication may increase instability during a repetitive pointing task. Longer trials may provide a better depiction of sway by discriminating between those with and without neurological impairment. Individuals with Parkinson's were less stable than older adults, supporting that they are at a greater risk for falls. The greater restrictive postural strategy in freezers compared to non-freezers is likely a factor that augments fall-risk. Lastly, the link between freezing of gait and upper-limb movement indicates that freezing may manifest first in the lower-limbs.