Sensitivity of the Wolf’s and Rosenstein’s Algorithms to Evaluate Local Dynamic Stability from Small Gait Data Sets: Response to Commentaries by Bruijn et al.

Assessing gait stability using the Largest Lyapunov Exponent (λ1) has become popular, especially because it may be a key measure in evaluating gait abnormalities in patient populations. However, clinical settings usually involve having small gait data sets and accurate determination of λ1 estimates from such sets is difficult. In an effort to address this issue, Cignetti et al.2 recently identified that λ1 estimates using the algorithm of Wolf et al.9 (W-algorithm) were more sensitive than those using the algorithm of Rosenstein et al.7 (R-algorithm) in order to capture age-related decline in gait stability from small data sets. Thus, they advocated the use of the former algorithm. Some concerns about the study were expressed afterwards by Bruijn et al.1 and we welcome the opportunity to discuss them in the present letter

Complexity and Human Gait

Recently, the complexity of the human gait has become a topic of major interest within the field of human movement sciences. Indeed, while the complex fluctuations of the gait patterns were, for a long time, considered as resulting from random processes, the development of new techniques of analysis, so-called nonlinear techniques, has open new vistas for the understanding of such fluctuations. In particular, by connecting the notion of complexity to the one of chaos, new insights about gait adaptability, unhealthy states in gait and neural control of locomotion were provided. Through methods of evaluation of the complexity, experimental results obtained both with healthy and unhealthy subjects and theoretical models of gait complexity, this review discusses the tremendous progresses made about the understanding of the complexity in the human gait variability. Recientemente, la complejidad de la marcha humana se está convirtiendo en un tema de gran interés en el campo de la ciencia del movimiento humano. De hecho, mientras las fluctuaciones complejas de los patrones de la marcha fueron, durante mucho tiempo, consideradas como resultado de procesos al azar, el desarrollo de nuevas técnicas de análisis, las llamadas técnicas no lineales, ha abierto nuevas vías para el entendimiento de tales fluctuaciones. En particular, mediante la conexión de la noción de complejidad con la de caos, se están obteniendo nuevos conocimientos sobre la adaptabilidad de la marcha, las condiciones patológicas en la marcha y el control neural de la locomoción. Mediante métodos de evaluación de la complejidad, los resultados experimentales obtenidos tanto con individuos sanos como no sanos y con modelos teóricos de la complejidad de la marcha, esta revisión habla de los enormes progresos efectuados sobre el entendimiento de la complejidad en la variabilidad de la marcha humana

Wearing a safety harness during treadmill walking influences lower extremity kinematics mainly through changes in ankle regularity and local stability

Background: Wearing a harness during treadmill walking ensures the subject’s safety and is common practice in biomedical engineering research. However, the extent to which such practice influences gait is unknown. This study investigated harness-related changes in gait patterns, as evaluated from lower extremity kinematics during treadmill walking. Findings: Healthy subjects (n = 10) walked on a treadmill at their preferred speed for 3 minutes with and without wearing a harness (LiteGait®, Mobility Research, Inc.). In the former condition, no weight support was provided to the subjects. Lower extremity kinematics was assessed in the sagittal plane from the mean (meanRoM), standard deviation (SDRoM) and coefficient of variation (CoVRoM) of the hip, knee, and ankle ranges of motion (RoM), as well as from the sample entropy (SampEn) and the largest Lyapunov exponent (LyE) of the joints’ angles. Wearing the harness increased the meanRoM of the hip, the SDRoM and the CoVRoM of the knee, and the SampEn and the LyE of the ankle. In particular, the harness effect sizes for both the SampEn and the LyE of the ankle were large, likely reflecting a meaningful decline in the neuromuscular stabilizing control of this joint. Conclusions: Wearing a harness during treadmill walking marginally influences lower extremity kinematics, resulting in more or less subtle changes in certain kinematic variables. However, in cases where differences in gait patterns would be expressed through modifications in these variables, having subjects walk with a harness may mask or reinforce such differences

Executive function orchestrates regulation of task-relevant gait fluctuations

Humans apply a minimum intervention principle to regulate treadmill walking, rapidly correcting fluctuations in the task-relevant variable (step speed: SS) while ignoring fluctuations in the task-irrelevant variables (step time: ST; step length: SL). We examined whether the regulation of fluctuations in SS and not in ST and SL depends on high-level, executive function, processes. Young adults walked on a treadmill without a cognitive requirement and while performing the cognitive task of dichotic listening. SS fluctuations became less anti-persistent when performing dichotic listening, meaning that taxing executive function impaired the ability to rapidly correct speed deviations on subsequent steps. Conversely, performing dichotic listening had no effect on SL and ST persistent fluctuations. Findings suggest that high-level brain processes are involved only in regulating gait task-relevant variables

Anterior-posterior and medial-lateral control of sway in infants during sitting acquisition does not become adult-like

We examined (1) how sitting postural control in infants develops in the anterior–posterior (A/P) and medial–lateral (M/L) directions of sway, and (2) whether this control is already adult-like during the late phase of infant\u27s sitting acquisition. COP data were acquired from 14 healthy infants (from the onset of sitting until independent sitting) and 21 healthy adults while sitting on a force platform. Attractor dimensionality (CoD: correlation dimension), attractor predictability (LyE: largest Lyapunov exponent), and sway variability (RMS: root-mean square) were calculated from the COP data to evaluate postural control. In the A/P direction, sitting was mastered by the infants by decreasing the active degrees of freedom of the postural system (decreased CoD), using a more predictable and (locally) stable sway (decreased LyE), and increasing sway variability (increased RMS). Control of sitting became practically simple, stable and exploratory with infant development. This may support the hypothesis that the sitting posture serves as the foundation for the development of other motor skills, as reaching. In the M/L direction, only sway variability decreased with development, possibly due to changes in the infant\u27s body dimensions. Taken together, these findings indicate that early in development the focus is more in the A/P than the M/L direction. Adults’ postural control was found more adaptable than the infants in both directions, involving more active degrees of freedom and less predictable sway patterns. Identifying the factors that make the dynamics of the postural system adult-like requires further research

Effects of aging on the relationship between cognitive demand and step variability during dual-task walking

A U-shaped relationship between cognitive demand and gait control may exist in dual-task situations, reflecting opposing effects of external focus of attention and attentional resource competition. The purpose of the study was twofold: to examine whether gait control, as evaluated from step-to-step variability, is related to cognitive task difficulty in a U-shaped manner and to determine whether age modifies this relationship. Young and older adults walked on a treadmill without attentional requirement and while performing a dichotic listening task under three attention conditions: non-forced (NF), forced-right (FR), and forced-left (FL). The conditions increased in their attentional demand and requirement for inhibitory control. Gait control was evaluated by the variability of step parameters related to balance control (step width) and rhythmic stepping pattern (step length and step time). A U-shaped relationship was found for step width variability in both young and older adults and for step time variability in older adults only. Cognitive performance during dual tasking was maintained in both young and older adults. The U-shaped relationship, which presumably results from a trade-off between an external focus of attention and competition for attentional resources, implies that higher-level cognitive processes are involved in walking in young and older adults. Specifically, while these processes are initially involved only in the control of (lateral) balance during gait, they become necessary for the control of (fore-aft) rhythmic stepping pattern in older adults, suggesting that attentional resources turn out to be needed in all facets of walking with aging. Finally, despite the cognitive resources required by walking, both young and older adults spontaneously adopted a “posture second” strategy, prioritizing the cognitive task over the gait task

Use of Motor Abundance in Young and Older Adults during Dual-Task Treadmill Walking

Contains fulltext : 110120.pdf (publisher's version ) (Open Access)Motor abundance allows individuals to perform any task reliably while being variable in movement's particulars. The study investigated age-related differences in this feature when young adults (YA) and older adults (OA) performed challenging tasks, namely treadmill walking alone and while performing a cognitive task. A goal function for treadmill walking was first defined, i.e., maintain constant speed at each step, which led to a goal equivalent manifold (GEM) containing all combinations of step time and step length that equally satisfied the function. Given the GEM, amounts of goal-equivalent and non-goal-equivalent variability were afterwards determined and used to define an index providing information about the set of effective motor solutions relative to the GEM. The set was limited in OA compared to YA in treadmill walking alone, indicating that OA made less flexible use of motor abundance than YA. However, this differentiation between YA and OA disappeared when concurrently performing the cognitive task. It is proposed that OA might have benefited from cognitive compensation

Analyse dynamique de la locomotion ski de fond

L'objectif de notre travail était d'identifier les principes qui organisent la motricité du skieur de fond en utilisant le cadre d'analyse offert par la Théorie des Systèmes Dynamiques (TSD). Il apparait que les mouvements segmentaires du skieur peuvent être modélisés par le biais d'un oscillateur auto-entretenu marquant la préférence du système neuromusculaire à s'organiser sous la forme d'un attracteur cycle-limite. Les adaptations du modèle observées avec la contrainte de fatigue indiquent que la nature de l'attracteur demeure inchangée et que seuls ses détails de forme sont modifiés, de telle sorte que le comportement asymptotique cycle-limite des segments devient plus harmonique. L'identification d'une variabilité inter-cycle de nature déterministe dans les mouvements du skieur nécessitera cependant d'envisager la piste d'un attracteur étrange dans les études futures. Au niveau inter-segmentaire, il ressort que seuls certains modes de coordination sont adoptés entre les segments du skieur, caractérisés par des rapports de fréquence d'ordre faible (i.e., 1:1, 2:1) et des phases relatives proches de 0 et 180 (i.e., patrons plus ou moins en phase et antiphase). Par ailleurs, l'augmentation de la contrainte de pente conduit à des changements brutaux entre ces modes préférentiels en raison d'une perte de stabilité de coordination pour des niveaux critiques de pente. Les changements de coordination prennent donc la forme de bifurcations entre les attracteurs qui ponctuent la coordination inter-segmentaire. Ainsi, les propriétés telles qu'attracteur, bifurcation et perte de stabilité identifiées dans notre travail indiquent que la motricité du skieur de fond est gouvernée par des principes d'auto-organisation. De plus, les adaptations comportementales observées avec la fatigue ouvrent de nouvelles perspectives sur l'utilisation qu'il peut être fait de la TSD en vu d'amener une lecture nouvelle de certains processus biologiques.The aim of our study was to identify the principles on which the motor acts of the cross-country skier are organized using the Dynamic Systems Theory (DST). It appears that a self-sustained oscillator reproduce with adequacy the rhythmic movements of the limbs, indicating a preferential limit-cycle organization of the neuromuscular system. The model adaptations observed with the fatigue constraint indicate that the nature of the attractor remains unchanged and that only its shape is modified, so that the asymptotic limit-cycle behaviour of the limbs become more harmonic. However, the deterministic nature of inter-cycle variability revealed in the limb movements of the skier suggests that a strange attractor will have to be considered in future works. Our study also highlights that only few modes of coordination emerged between the limbs of the skier, characterized by low integer frequency ratios (i.e., 1:1 and 2:1) and relative phase values close to 0 and 180 (i.e., patterns more or less in phase and out of phase). Moreover, the increase of the slope constraint conducts to sudden switches between these preferential modes and a loss of coordination stability is observed at their occurrences. Accordingly, changes in the coordination of the skier take the form of bifurcations between the attractors of the coordination dynamics. In sum, the properties as attractor, bifurcation and loss of stability identified in our study favour strongly for a self-organization of the motor acts in cross-country skiing. More, the behavioural adaptations observed in the limb movements of the skier with fatigue prove the relevance of the DST for the interpretation of some biological processes.CHAMBERY -BU Bourget (730512101) / SudocSudocFranceF

Locomotion : physiologie, méthodes d'analyse et classification des principaux troubles

International audienceDepuis son acquisition au cours du développement jusqu'à la sénescence, les troubles de la marche, ainsi que ceux de l'équilibre et de la posture, sont des motifs de consultation fréquents en médecine, et tout spécialement auprès du neurologue, du rhumatologue et du médecin rééducateur. L'approche de ces troubles repose essentiellement sur une analyse clinique systématique associée à un examen neurologique global. La classification proposée dans cet article dans un but très pratique de diagnostic repose sur le déficit cardinal du trouble : équilibre, déficit moteur, trouble hyper- ou hypokinétique, ce qui nécessite de connaître les principaux fondements physiopathologiques de la locomotion. Certains troubles étiquetés complexes intègrent plusieurs dysfonctions dont une dimension cognitive. L'identification du trouble spécifique de la marche peut dans certains cas déboucher sur une conduite thérapeutique adaptée : rééducation fonctionnelle, traitement chimique (lévodopa), chirurgical (dérivation ventriculopéritonéale). Les innovations dans ce domaine sont nombreuses ; il faut en particulier souligner la place que prend la stimulation cérébrale profonde qui donne déjà dans les cibles classiques des résultats dans de nombreux domaines de la pathologie du mouvement (pallidum interne et dystonie, noyau sous-thalamique et maladie de Parkinson), mais qui connaît un développement propre aux troubles posturolocomoteurs avec le ciblage du noyau pédonculopontin en relation directe avec les études les plus récentes en neurosciences chez l'animal et chez l'homme sur les centres locomoteurs