R.E.A. Van Emmerik

Resonant frequencies of arms and legs identify different walking patterns

The present study is aimed at investigating changes in the coordination of arm and leg movements in young healthy subjects. It was hypothesized that with changes in walking velocity there is a change in frequency and phase coupling between the arms and the legs. In addition, it was hypothesized that the preferred frequencies of the different coordination patterns can be predicted on the basis of the resonant frequencies of arms and legs with a simple pendulum model. The kinematics of arms and legs during treadmill walking in seven healthy subjects were recorded with accelerometers in the sagittal plane at a wide range of different velocities (i.e., 0.3-1. 3m/s). Power spectral analyses revealed a statistically significant change in the frequency relation between arms and legs, i.e., within the velocity range 0.3-0.7m/s arm movement frequencies were dominantly synchronized with the step frequency, whereas from 0.8m/s onwards arm frequencies were locked onto stride frequency. Significant effects of walking speed on mean relative phase between leg and arm movements were found. All limb pairs showed a significantly more stable coordination pattern from 0.8 to 1.0m/s onwards. Results from the pendulum modelling demonstrated that for most subjects at low-velocity preferred movement frequencies of the arms are predicted by the resonant frequencies of individual arms (about 0.98Hz), whereas at higher velocities these are predicted on the basis of the resonant frequencies of the individual legs (about 0.85Hz). The results support the above-mentioned hypotheses, and suggest that different patterns of coordination, as shown by changes in frequency coupling and phase relations, can exist within the human walking mode.

Postural orientation: Age-related changes in variability and time-to-boundary

The relation between age-specific postural instability and the detection of stability boundaries was examined. Balance control was investigated under different visual conditions (eyes open/closed) and postural orientations (forward/backward lean) while standing on a force platform. Dependent variables included center of pressure variability and the time-to-contact of the center of pressure with the stability boundaries around the feet (i.e., time-to-boundary). While leaning maximally, older individuals (ages 55-69) showed increased center of pressure variability compared to no lean, while younger subjects (ages 24-38) showed a decrease. These significant differences were found only in anterior-posterior direction. No significant age-specific differences were found between eyes open and eyes closed conditions. Time-to-boundary analysis revealed reduced spatio-temporal stability margins in older individuals in both anterior-posterior and medio-lateral directions. Time-to-boundary variability, however, was not significantly different between the groups in both medio-lateral and anterior-posterior direction. These results show the importance of boundary relevant center of pressure measures in the study of postural control, especially concerning the lateral instability often observed in older adults.

Postural control in women with multiple sclerosis: effects of task, vision and symptomatic fatigue.

People with multiple sclerosis (MS) often report problems with balance, which may be most apparent during challenging postural tasks such as leaning or reaching, and when relying on non-visual sensory systems. An additional obstacle facing people with MS is a high incidence of symptomatic fatigue (>70%). The purpose of this study was to investigate the changes in balance during upright stance in individuals with mild-to-moderate disability due to MS under normal and restricted vision and different levels of self-reported fatigue. Limb loading asymmetry, sway and magnitude of postural shift in center of pressure, and time-to-contact the stability boundary of the center of mass and center of pressure were assessed during quiet standing and maximal lean and reach tasks. Compared to controls, people with MS displayed greater postural sway, greater loading asymmetry, and shorter time-to-contact during quiet standing. In the postural perturbation tasks the MS group had smaller postural shifts and reduced stability compared to controls in the direction perpendicular to the lean and reach. Limiting vision increased loading asymmetry during quiet standing and postural instability during backward lean in the MS group. Inducing additional fatigue in the MS group did affect postural control in the more challenging balance conditions but had no impact during quiet upright standing. The results of this study indicate subtle changes in postural control during standing in people with mild-to-moderate impairments due to MS.

Changing coordinative structures in complex skill acquisition

Abstract The present paper puts forward the concept of coordinative structures as a way to move from the dynamics of the behavioral outcome towards a characterization of what the subject is actually doing to achieve that outcome. In a discovery learning experiment, five subjects learned to perform slalom-like movements on a ski apparatus over a 7-day period. Kinematics of body segments and apparatus were recorded in three dimensions using an opto-electronic Selspot camera system. A stepwise analysis of the learning process was performed in terms of the platform dynamics, the apparatus-subject interaction (i.e., dynamics of the relative phase between platform and centre of mass movements), and the subject (i.e., body configuration and the dynamics of the position of centre of mass). Results showed that stability, as indicated by phase plane measures, was established early in practice in movements of the platform, relative phase, and centre of mass. Later in practice, additional changes were observed in the height and coefficient of variation of height of the centre of mass. These differences in the timing of changes suggest that learning can be interpreted as proceeding globally in three different ‘stages’. The coordinative structure in each of the stages is interpreted as a different instance of pendulum systems. At the end of the paper, we contemplate the potential of low-dimensional, subject-related descriptions of complex movement behavior to advance neurological models of movement control.

Dynamics of movement coordination and tremor during gait in Parkinson's disease

Abstract Movement coordination, transition and stability aspects of gait in individuals with Parkinsons disease were investigated. Changes in movement coordination were evaluated by means of relative phase changes between upper and lower extremities and stability of patterns by means of the standard deviation of relative phase. Spectroscopic analysis of individual limb oscillators and of the relative phase between these oscillators was used to assess the contribution of lower and higher (e.g., tremor) frequency components on the coordination dynamics. Coordination changes in five patients and five age-matched controls were evaluated by means of accelerometry while movement velocity was manipulated as a control parameter on a motor-driven treadmill. Relative phase changes and stability features showed gradual changes in upper and lower extremity oscillations with walking velocity, a finding consistent with model predictions by Schoner et al. (1990) regarding gait changes in the quadrupedal walking mode. Parkinsons disease patients showed overall less adaptations in relative phase than the control group, especially those involving the upper extremities. Frequency analyses of relative phase also demonstrated that 1. (a) systematic scaling of walking velocity can affect the dominant frequency of Parkinsonian tremor, 2. (b) higher order frequency components in relative phase can play a functional, stabilizing role, and 3. (c) 1/ƒ scaling properties of the relative phase power spectrum can identify the higher degree of constraint on the coordination dynamics in Parkinsons disease.

LYAPUNOV EXPONENT ESTIMATION FOR HUMAN GAIT ACCELERATION SIGNALS

While it is not possible to analytically define the Lyapunov exponents for biological systems, estimating the spectrum of exponents or simply the largest exponent is possible. Two algorithms are often used for experimental data: the Wolf algorithm [1] and the Rosenstein algorithm [2]. Rosenstein’s algorithm was developed to address limitations inherent in the Wolf algorithm, including: reliability issues with small data sets, computational intensity, and sensitivity to noise. Wolf and Rosenstein both suggest a minimum of 10 data points for Lyapunov estimation, where d is the dimension of the attractor; however, Rosenstein et al. claim their algorithm performs well with smaller data sets. How well these algorithms perform on small biological data sets (<10) is of interest.

Prospective dynamic balance control during the swing phase of walking: Stability boundaries and time-to-contact analysis

This study examined the prospective control of the swing phase in young healthy adults while walking at preferred speed over unobstructed ground and during obstacle clearance. Three aspects of swing were examined: (1) the relation of the body Center of Mass (CoM) to the stability boundaries at the base of support; (2) a dynamic time-to-contact analysis of the CoM and swing foot to these boundaries; and (3) the role of head movements in the prospective control of gait and field of view assessment. The time-to-contact analysis of CoM and swing foot showed less stable swing dynamics in the trail foot compared to the lead foot in the approach to the unstable equilibrium, with the CoM leading the swing foot and crossing the anterior stability boundary before the swing foot. Compensations in temporal coupling occurred in the trail limb during the late swing phase. Time-to-contact analysis of head movement showed stronger prospective control of the lead foot, while fixation of the field of view occurred earlier in swing and was closer to the body in the obstacle condition compared to unobstructed walking. The dynamic time-to-contact analysis offers a new approach to assessing the unstable swing phase of walking in different populations.

Ecological gait dynamics: stability, variability and optimal design

Variability–stability relationships are the very foundation of flexible biological movements. For footwear design, stability is best defined as pattern stability at the level of segmental relationships with regard to goal-oriented performance. Footwear design should seek to optimize the variation in segmental relationships while maintaining the overall locomotion pattern, providing a system that is both adaptable to upcoming events and stable to unanticipated perturbations. As footwear is designed for a variety of uses other than forward, continuous running on a flat terrain, an examination of the fundamental assumptions and applicability of traditional gait dynamics is necessary. Expanding evaluation of gait dynamics to non-forward, non-continuous motion across different terrains would seem appropriate, given the ubiquity of these movements in everyday life. Optimizing footwear design for different tasks requires trade-offs between design and material selection and a principled scientific approach to understanding these trade-offs is required. Evaluation of these non-traditional gait dynamics and incorporation of goal-oriented performance requires consideration of the behavioural consequences of design and perception–action coupling. Understanding variability–stability relationships remains at the center of any attempt to understand footwear performance and design implications in these expanded gait dynamics. The basis for changing historical perspectives on variability–stability relationships is discussed with regard to maintaining and changing coordinative patterns, information–movement relationships, injury prevention, and optimizing performance through footwear design. A principled approach to building a footwear performance space from empirical data is provided, and it is suggested that such an approach offers objective metrics for making appropriate trades in footwear design to optimize performance across categories of intended use.

Coupling of postural and manual tasks in expert performers

The purpose of this study was to investigate the integration of bimanual rhythmic movements and posture in expert marching percussionists. Participants (N=11) performed three rhythmic manual tasks [1:1, 2:3, and 2:3-F (2:3 rhythm played faster at a self-selected tempo)] in one of three postures: sitting, standing on one foot, and standing on two feet. Discrete relative phase, postural time-to-contact, and coherence analysis were used to analyze the performance of the manual task, postural control, and the integration between postural and manual performance. Across all three rhythms, discrete relative phase mean and variability results showed no effects of posture on rhythmic performance. The complexity of the manual task (1:1 vs. 2:3) had no effect on postural time-to-contact. However, increasing the tempo of the manual task (2:3 vs. 2:3-F) did result in a decreased postural time-to-contact in the two-footed posture. Coherence analysis revealed that the coupling between the postural and manual task significantly decreased as a function of postural difficulty (going from a two-footed to a one-footed posture) and rhythmic complexity (1:1 vs. 2:3). Taken together, these results demonstrate that expert marching percussionists systematically decouple postural and manual fluctuations in order to preserve the performance of the rhythmic movement task.