Human Movement Variability, Nonlinear Dynamics, and Pathology: Is There A Connection?

Fields studying movement generation, including robotics, psychology, cognitive science, and neuroscience utilize concepts and tools related to the pervasiveness of variability in biological systems. The concept of variability and the measures for nonlinear dynamics used to evaluate this concept open new vistas for research in movement dysfunction of many types. This review describes innovations in the exploration of variability and their potential importance in understanding human movement. Far from being a source of error, evidence supports the presence of an optimal state of variability for healthy and functional movement. This variability has a particular organization and is characterized by a chaotic structure. Deviations from this state can lead to biological systems that are either overly rigid and robotic or noisy and unstable. Both situations result in systems that are less adaptable to perturbations, such as those associated with unhealthy pathological states or absence of skillfulness

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

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

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

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

Stepping over obstacles of different heights and varied shoe traction alter the kinetic strategies of the leading limb

This study aims to investigate the effects of shoe traction and obstacle height on friction during walking to better understand the mechanisms required to avoid slippage following obstacle clearance. Ten male subjects walked at a self-selected pace during eight different conditions: four obstacle heights (0%, 10%, 20%, and 40% of limb length) while wearing two different pairs of shoes (low and high traction). Frictional forces were calculated from the ground reaction forces following obstacle clearance, which were sampled with a Kistler platform at 960 Hz. All frictional peaks increased with increases in obstacle height. Low traction shoes yielded smaller peaks than high traction shoes. The transition from braking to propulsion occurred sooner due to altered control strategies with increased obstacle height. Collectively, these results provided insights into kinetic strategies of leading limb when confronted with low traction and high obstacle environments

Variability of Lower Extremity Joint Kinematics During Backward Walking in a Virtual Environment

Backward walking (BW) shows significant differences with forward walking (FW) and these differences are potentially useful in rehabilitation. However the lack of visual cues makes BW risky. The purpose of this study was to investigate the effect of visual cues provided by a virtual environment on FW and BW on gait variability. Each subject underwent four conditions of treadmill walking at self-selected pace. The subjects walked backwards in three conditions and forwards in the fourth condition. A virtual corridor was displayed to the subjects in the FW condition (forward optic flow) and two of the backward conditions (forward and backward optic flow). The third BW condition was a control condition (no visual cues). Gait variability measures of the hip, knee and ankle range of motion and the stride interval were analyzed. Magnitude of variability was evaluated with the coefficient of variation and structure of variability with approximate entropy. Significant differences were demonstrated between the FW and the BW gait characteristics as well as in gait variability (for both magnitude and structure of variability). No significant differences were found between the three BW conditions as a result of the direction of visual cues. In order to get optimal benefit of BW in the aged and the diseased, optical flow of visual feedback may need to be manipulated in a different manner than FW. Future studies will explore other parameters of visual cues like the velocity of optic flow and appearance of obstacles to obtain the best visual cue configuration for rehabilitation

The Effect of Virtual Reality on Gait Variability

Optic Flow (OF) plays an important role in human locomotion and manipulation of OF characteristics can cause changes in locomotion patterns. The purpose of the study was to investigate the effect of the velocity of optic flow on the amount and structure of gait variability. Each subject underwent four conditions of treadmill walking at their self-selected pace. In three conditions the subjects walked in an endless virtual corridor, while a fourth control condition was also included. The three virtual conditions differed in the speed of the optic flow displayed as follows – same speed (OFn), faster (OFf), and slower (OFs) than that of the treadmill. Gait kinematics were tracked with an optical motion capture system. Gait variability measures of the hip, knee and ankle range of motion and stride interval were analyzed. Amount of variability was evaluated with linear measures of variability - coefficient of variation, while structure of variability i.e., its organization over time, were measured with nonlinear measures - approximate entropy and detrended fluctuation analysis. The linear measures of variability, CV, did not show significant differences between Non-VR and VR conditions while nonlinear measures of variability identified significant differences at the hip, ankle, and in stride interval. In response to manipulation of the optic flow, significant differences were observed between the three virtual conditions in the following order: OFn \u3e OFf \u3e OFs. Measures of structure of variability are more sensitive to changes in gait due to manipulation of visual cues, whereas measures of the amount of variability may be concealed by adaptive mechanisms. Visual cues increase the complexity of gait variability and may increase the degrees of freedom available to the subject. Further exploration of the effects of optic flow manipulation on locomotion may provide us with an effective tool for rehabilitation of subjects with sensorimotor issues

New insights into anterior cruciate ligament deficiency and reconstruction through the assessment of knee kinematic variability in terms of nonlinear dynamics

Purpose Injuries to the anterior cruciate ligament (ACL) occur frequently, particularly in young adult athletes, and represent the majority of the lesions of knee ligaments. Recent investigations suggest that the assessment of kinematic variability using measures of nonlinear dynamics can provide with important insights with respect to physiological and pathological states. The purpose of the present article was to critically review and synthesize the literature addressing ACL deficiency and reconstruction from a nonlinear dynamics standpoint. Methods A literature search was carried out in the main medical databases for studies published between 1990 and 2010. Results Seven studies investigated knee kinematic variability in ACL patients. Results provided support for the theory of “optimal movement variability”. Practically, loss below optimal variability is associated with a more rigid and very repeatable movement pattern, as observed in the ACL-deficient knee. This is a state of low complexity and high predictability. On the other hand, increase beyond optimal variability is associated with a noisy and irregular movement pattern, as found in the ACL-reconstructed knee, regardless of which type of graft is used. This is a state of low complexity and low predictability. In both cases, the loss of optimal variability and the associated high complexity lead to an incapacity to respond appropriately to the environmental demands, thus providing an explanation for vulnerability to pathological changes following injury. Conclusion Subtle fluctuations that appear in knee kinematic patterns provide invaluable insight into the health of the neuromuscular function after ACL rupture and reconstruction. It is thus critical to explore them in longitudinal studies and utilize nonlinear measures as an important component of post-reconstruction medical assessment. Level of Evidence II

The effects of shoe traction and obstacle height on lower extremity coordination dynamics during walking.

This study aims to investigate the effects of shoe traction and obstacle height on lower extremity relative phase dynamics (analysis of intralimb coordination) during walking to better understand the mechanisms employed to avoid slippage following obstacle clearance. Ten participants walked at a self-selected pace during eight conditions: four obstacle heights (0%, 10%, 20%, and 40% of limb length) while wearing two pairs of shoes (low and high traction). A coordination analysis was used and phasing relationships between lower extremity segments were examined. The results demonstrated that significant behavioral changes were elicited under varied obstacle heights and frictional conditions. Both decreasing shoe traction and increasing obstacle height resulted in a more in-phase relationship between the interacting lower limb segments. The higher the obstacle and the lower the shoe traction, the more unstable the system became. These changes in phasing relationship and variability are indicators of alterations in coordinative behavior, which if pushed further may have lead to falling