Michael A. Busa

Gait Impairments in Persons With Multiple Sclerosis Across Preferred and Fixed Walking Speeds

OBJECTIVES To investigate (1) whether previously observed changes in gait parameters in individuals with multiple sclerosis (MS) are the result of slower preferred walking speeds or reflect adaptations independent of gait speed; and (2) the changes in spatiotemporal features of the unstable swing phase of gait in people with MS. DESIGN Cross-sectional study assessing changes in gait parameters during preferred, slow (0.6m/s), medium (1.0m/s), and fast (1.4m/s) walking speeds. SETTING Gait laboratory with instrumented walkway and motion capture system. PARTICIPANTS MS group with mild to moderate impairment (n=19, 16 women) with a median Expanded Disability Status Scale score of 3.75 (range, 2.5-6), and a sex- and age-matched control group (n=19). INTERVENTIONS Not applicable. MAIN OUTCOME MEASURES Gait speed, stride length, stride width, cadence, dual support time, swing time, and timing of swing foot and body/head center of mass during swing phase. RESULTS Individuals with MS walked at slower preferred speeds with longer dual support times compared with controls. In fixed-speed conditions, dual support times were longer and swing times were shorter in MS compared with controls. Stride width was wider for all speed conditions in the MS group. In fixed-speed conditions, the MS group positioned their head and body centers of mass closer to the anterior base of support boundary when entering the unstable equilibrium of the swing phase. CONCLUSIONS Longer dual support time is part of a gait strategy in MS that is apparent even when controlling for the confounding effect of slower preferred speed. However, a gait strategy featuring longer dual support times may have limitations if potentially destabilizing swing dynamics exist, which especially occur at walking speeds other than preferred for people with MS.

Time-to-contact and multiscale entropy identify differences in postural control in adolescent idiopathic scoliosis

Previous reports on changes in postural control in adolescent idiopathic scoliosis (AIS) compared to healthy controls have been inconsistent. This may suggest center of pressure (COP) sway parameters are not sufficient for determining the ability to maintain quiet upright stance indicating more complex measures may be needed to examine postural control in AIS. The purpose of this investigation was to compare postural control between AIS of different severity levels and healthy controls using time-to-contact (TtC), the complexity index of multiscale entropy (C(r)), and COP sway parameters. Thirty-six AIS patients were classified as pre-bracing or pre-operative and compared to 10 healthy control subjects. Overall, the AIS patients showed significantly greater COP sway in mediolateral direction, but deficits with respect to the anteroposterior direction were only systematically identified with the time-to-contact and entropy measures. The multiscale entropy (C(r)) results indicate that those with AIS utilize a different control strategy from healthy controls in the mediolateral direction that is more constrained, less complex and less adaptable. AIS severity further reduced this adaptability in the anteroposterior direction. These results indicate it is necessary to examine both planes of motion when investigating postural control in AIS. Additionally, the application of the measures used to assess the nature of the postural control changes in AIS should also be considered.

Multiscale entropy：A tool for understanding the complexity of postural control

Clinical disorders often are characterized by a breakdown in dynamical processes that contribute to the control of upright standing. Disruption to a large number of physiological processes operating at different time scales can lead to alterations in postural center of pressure (CoP) fluctuations. Multiscale entropy (MSE) has been used to identify differences in fluctuations of postural CoP time series between groups with and without known physiological impairments at multiple time scales. The purpose of this paper is to: 1) review basic elements and current developments in entropy techniques used to assess physiological complexity; and 2) identify how MSE can provide insights into the complexity of physiological systems operating at multiple time scales that underlie the control of posture. We review and synthesize evidence from the literature providing support for MSE as a valuable tool to evaluate the breakdown in the physiological processes that accompany changes due to aging and disease in postural control. This evidence emerges from observed lower MSE values in individuals with multiple sclerosis, idiopathic scoliosis, and in older individuals with sensory impairments. Finally, we suggest some future applications of MSE that will allow for further insight into how physiological deficits impact the complexity of postural fluctuations; this information may improve the development and evaluation of new therapeutic interventions.

Multiscale entropy identifies differences in complexity in postural control in women with multiple sclerosis

Loss of postural center-of-pressure complexity (COP complexity) has been associated with reduced adaptability that accompanies disease and aging. The aim of this study was to identify if COP complexity is reduced: (1) in those with Multiple Sclerosis (MS) compared to controls; (2) when vision is limited compared to remaining intact; and (3) during more demanding postural conditions compared to quiet standing. Additionally, we explored the relationship between the COP complexity and disease severity, fatigue, cutaneous sensation and central motor drive. Twelve women with MS and 12 age-matched controls were tested under quiet standing and postural maximal lean conditions with normal and limited vision. The key dependent variable was the complexity index (CI) of the center of pressure. We observed a lower CI in the MS group compared to controls in both anterior-posterior (AP) and medio-lateral (ML) directions (ps<0.002), during the performance of maximal self-regulated leans (AP: p<0.001; ML: p=0.018), and under limited vision (AP: p=0.001; ML: p=0.006). No group-by-vision interaction (p>0.05) was observed, indicating that limiting vision did not impact COP complexity differently in the two groups. Decreased cutaneous sensitivity was associated with lower CI values in the AP direction among those with MS (r(2)=0.57); all other measures did not exhibit significant relationships. The findings reported here suggest that (1) MS is associated with diminished COP complexity under both normal and challenging postures, and (2) complexity is strongly correlated with cutaneous sensitivity, suggesting the unique contribution of impaired somatosensation on postural control deficits in persons with MS.

Enhancing Postural Stability and Adaptability in Multiple Sclerosis

People living with multiple sclerosis (MS) consistently rate balance and gait impairments as having the greatest negative impacts on their quality of life. Our research aims to understand the sensorimotor contributions to balance dysfunction and difficulty with walking in people with MS, with specific attention paid to how fatigue, muscle weakness, and sensory loss interact to limit physical function and mobility. Here, we relate aspects of somatosensory loss and symptomatic fatigue to balance function, and provide new insights in our understanding of the mechanisms of balance and gait dysfunction in MS through the use of novel analytical methods and experimental paradigms. We first review the existing methods and paradigms to assess postural and gait stability in research on MS. Next, we introduce novel measures to assess the stability and adaptability of posture and gait in people with MS that are based on nonlinear and complex systems methods. These novel methods include (1) boundary-relevant measures of postural stability and control (postural “time to contact”), and (2) entropy measures for assessing postural and gait adaptability. These novel methods allow us to differentiate between postural and gait variability caused by dysfunction that may interfere with movement control, and variability that is functional and provides stable and adaptable movement patterns. Finally, we discuss how these methods and paradigms could help to develop innovative treatments for balance and gait dysfunction in people with MS.

Head and Tibial Acceleration as a Function of Stride Frequency and Visual Feedback during Running.

Individuals regulate the transmission of shock to the head during running at different stride frequencies although the consequences of this on head-gaze stability remain unclear. The purpose of this study was to examine if providing individuals with visual feedback of their head-gaze orientation impacts tibial and head accelerations, shock attenuation and head-gaze motion during preferred speed running at different stride frequencies. Fifteen strides from twelve recreational runners running on a treadmill at their preferred speed were collected during five stride frequencies (preferred, ±10% and ±20% of preferred) in two visual task conditions (with and without real-time visual feedback of head-gaze orientation). The main outcome measures were tibial and head peak accelerations assessed in the time and frequency domains, shock attenuation from tibia to head, and the magnitude and velocity of head-gaze motion. Decreasing stride frequency resulted in greater vertical accelerations of the tibia (p<0.01) during early stance and at the head (p<0.01) during early and late stance; however, for the impact portion the increase in head acceleration was only observed for the slowest stride frequency condition. Visual feedback resulted in reduced head acceleration magnitude (p<0.01) and integrated power spectral density in the frequency domain (p<0.01) in late stance, as well as overall of head-gaze motion (p<0.01). When running at preferred speed individuals were able to stabilize head acceleration within a wide range of stride frequencies; only at a stride frequency 20% below preferred did head acceleration increase. Furthermore, impact accelerations of the head and tibia appear to be solely a function of stride frequency as no differences were observed between feedback conditions. Increased visual task demands through head gaze feedback resulted in reductions in head accelerations in the active portion of stance and increased head-gaze stability.

Allometrically Scaled Children's Clinical and Free-Living Ambulatory Behavior.

PURPOSE This study aimed to compare clinical and free-living walking cadence in school-age children and to examine how the allometric scaling of leg length variability affects objective ambulatory activity assessment. METHODS A total of 375 children (154 boys and 221 girls, 9-11 yr old) completed GAITRite-determined slow, normal, and fast walks and wore accelerometers for 1 wk. Dependent variables from clinical assessment included gait speed, cadence, and step length, whereas steps per day, peak 1-min cadence, and peak 60-min cadence were assessed during free living. Analogous allometrically scaled variables were used to account for leg length differences. Free-living times above clinically determined individualized slow, normal, and fast cadence values were calculated. Differences in dependent variables between sex and sex-specific leg length tertiles were assessed. RESULTS Clinically assessed cadence (mean ± SD) was 90.9 ± 15.2 (slow), 113.8 ± 12.9 (normal), and 148.9 ± 20.9 (fast) steps per minute, respectively. During free living, participants accumulated 8651 ± 2259 steps per day. Peak 1-min cadence was 113.4 ± 12.4 steps per minute and peak 60-min cadence was 60.1 ± 11.4 steps per minute. Allometrically scaling gait variables to leg length eliminated the previously significant leg length effect observed in both clinical and free-living gait variables but did not affect the observation that girls exhibited lower levels of free-living ambulatory behavior measured by mean steps per day. On average, all groups spent <15 min·d above clinically determined slow cadence; this was unaffected by leg length. CONCLUSION Allometrically scaling gait variables to leg length significantly affected the assessment of ambulatory behavior, such that different leg length groups appear to walk in a dynamically similar manner. Leg length effects on free-living ambulatory behavior were also eliminated by implementing estimates of time spent above individualized cadence cut points derived from clinical gait assessment.

Association between stride time fractality and gait adaptability during unperturbed and asymmetric walking

Human locomotion is an inherently complex activity that requires the coordination and control of neurophysiological and biomechanical degrees of freedom across various spatiotemporal scales. Locomotor patterns must constantly be altered in the face of changing environmental or task demands, such as heterogeneous terrains or obstacles. Variability in stride times occurring at short time scales (e.g., 5-10 strides) is statistically correlated to larger fluctuations occurring over longer time scales (e.g., 50-100 strides). This relationship, known as fractal dynamics, is thought to represent the adaptive capacity of the locomotor system. However, this has not been tested empirically. Thus, the purpose of this study was to determine if stride time fractality during steady state walking associated with the ability of individuals to adapt their gait patterns when locomotor speed and symmetry are altered. Fifteen healthy adults walked on a split-belt treadmill at preferred speed, half of preferred speed, and with one leg at preferred speed and the other at half speed (2:1 ratio asymmetric walking). The asymmetric belt speed condition induced gait asymmetries that required adaptation of locomotor patterns. The slow speed manipulation was chosen in order to determine the impact of gait speed on stride time fractal dynamics. Detrended fluctuation analysis was used to quantify the correlation structure, i.e., fractality, of stride times. Cross-correlation analysis was used to measure the deviation from intended anti-phasing between legs as a measure of gait adaptation. Results revealed no association between unperturbed walking fractal dynamics and gait adaptability performance. However, there was a quadratic relationship between perturbed, asymmetric walking fractal dynamics and adaptive performance during split-belt walking, whereby individuals who exhibited fractal scaling exponents that deviated from 1/f performed the poorest. Compared to steady state preferred walking speed, fractal dynamics increased closer to 1/f when participants were exposed to asymmetric walking. These findings suggest there may not be a relationship between unperturbed preferred or slow speed walking fractal dynamics and gait adaptability. However, the emergent relationship between asymmetric walking fractal dynamics and limb phase adaptation may represent a functional reorganization of the locomotor system (i.e., improved interactivity between degrees of freedom within the system) to be better suited to attenuate externally generated perturbations at various spatiotemporal scales.

Adaptive changes in running kinematics as a function of head stability demands and their effect on shock transmission

This study aimed to identify adaptive changes in running kinematics and impact shock transmission as a function of head stability requirements. Fifteen strides from twelve recreational runners were collected during preferred speed treadmill running. Head stability demands were manipulated through real-time visual feedback that required head-gaze orientation to maintain within boxes of different sizes, ranging from 21° to 3° of visual angle with 3° decrements. The main outcome measures were tibial and head peak accelerations in the time and frequency domains (impact and active phases), shock transmission from tibia to head, stride parameters, and sagittal plane joint kinematics. Increasing head stability requirements resulted in decreases in the amplitude and integrated power of head acceleration during the active phase of stance. During the impact portion of stance tibial and head acceleration and shock transmission remained similar across visual conditions. In response to increased head stability requirements, participants increased stride frequency approximately 8% above preferred, as well as hip flexion angle at impact; stance time and knee and ankle joint angles at impact did not change. Changes in lower limb joint configurations (smaller hip extension and ankle plantar-flexion and greater knee flexion) occurred at toe-off and likely contributed to reducing the vertical displacement of the center of mass with increased head stability demands. These adaptive changes in the lower limb enabled runners to increase the time that voluntary control is allowed without embedding additional impact loadings, and therefore active control of the head orientation was facilitated in response to different visual task constraints.

Additional helmet and pack loading reduce situational awareness during the establishment of marksmanship posture

Abstract The pickup of visual information is critical for controlling movement and maintaining situational awareness in dangerous situations. Altered coordination while wearing protective equipment may impact the likelihood of injury or death. This investigation examined the consequences of load magnitude and distribution on situational awareness, segmental coordination and head gaze in several protective equipment ensembles. Twelve soldiers stepped down onto force plates and were instructed to quickly and accurately identify visual information while establishing marksmanship posture in protective equipment. Time to discriminate visual information was extended when additional pack and helmet loads were added, with the small increase in helmet load having the largest effect. Greater head-leading and in-phase trunk–head coordination were found with lighter pack loads, while trunk-leading coordination increased and head gaze dynamics were more disrupted in heavier pack loads. Additional armour load in the vest had no consequences for Time to discriminate, coordination or head dynamics. This suggests that the addition of head borne load be carefully considered when integrating new technology and that up-armouring does not necessarily have negative consequences for marksmanship performance. Practitioner Summary: Understanding the trade-space between protection and reductions in task performance continue to challenge those developing personal protective equipment. These methods provide an approach that can help optimise equipment design and loading techniques by quantifying changes in task performance and the emergent coordination dynamics that underlie that performance.