Dynamic patterns of postural sway in ballet dancers and track athletes

Organization of postural equilibrium in several planes in ballet dancers

This study analyzed the balance strategies of ballet dancers during postural equilibrium in three single leg balance conditions with and without vision and regard to age. Dancers participating formed two groups of 20 dancers each, one aged between 8 and 16 years (young group) and the other aged between 17 and 30 years (adult group). Ground reaction forces (GRFs) (mediolateral (ML), anteroposterior (AP) components, vertical (V)) were recorded. Results analysis enabled us to extract some spatiotemporal data for each component of the GRF (number of GRF oscillations, variability and impulses). Young dancers are characterized, compared to adult dancers, by an instability combined with an increase of oscillations number and a decrease variability mainly visible on the ML component. In the two groups, the absence of vision implies an increase of AP, ML and V impulsions and GRF variability. Balance with the gesturing limb to the rear increases the age and vision effect compared to balances with the limb forward or to the side. Young dancers are less efficient at controlling their balance than adult dancers. This observation may be related to the number of hours practicing dance, which differs between groups. The dancers have a visual dependence to control the postural balance.

To freeze or not to freeze? Affective and cognitive perturbations have markedly different effects on postural control

Similar effects have been reported for diverting attention from postural control and increased anxiety on the characteristics of center-of-pressure (COP) time series (decreased excursions and elevated mean power frequency). These effects have also received similar interpretations in terms of increased postural stiffness, suggesting that cognitive and affective manipulations have similar influences on postural control. The present experiment tested this hypothesis by comparing postural conditions involving manipulations of attention (diverting attention from posture using cognitive and motor dual tasks) and anxiety (standing at a height), and by complementing posturography with electromyographic analyses to directly examine neuromuscular stiffness control. Affective and cognitive manipulations had markedly different effects. Unlike the height condition, diverting attention from balance induced smaller COP amplitudes and higher sway frequencies. In addition, more regular COP trajectories (lower sample entropy) were found in the height condition than the dual-task conditions, suggesting elevated attentional investment in posture under the affective manipulation. Finally, based on an analysis of the cross-correlation function between anterior-posterior COP time series and enveloped calf muscle activity, indications of tighter anticipatory neuromuscular control of posture were found for the height condition only. Our data suggest that affective and cognitive perturbations have qualitatively different effects on postural control, and thus are likely to be associated with different control processes, as evidenced by differences in neuromuscular regulation and attentional investment in posture.

Entropy of balance--some recent results

BACKGROUND

Entropy when applied to biological signals is expected to reflect the state of the biological system. However the physiological interpretation of the entropy is not always straightforward. When should high entropy be interpreted as a healthy sign, and when as marker of deteriorating health? We address this question for the particular case of human standing balance and the Center of Pressure data.

METHODS

We have measured and analyzed balance data of 136 participants (young, n = 45; elderly, n = 91) comprising in all 1085 trials, and calculated the Sample Entropy (SampEn) for medio-lateral (M/L) and anterior-posterior (A/P) Center of Pressure (COP) together with the Hurst self-similarity (ss) exponent alpha using Detrended Fluctuation Analysis (DFA). The COP was measured with a force plate in eight 30 seconds trials with eyes closed, eyes open, foam, self-perturbation and nudge conditions.

RESULTS

1) There is a significant difference in SampEn for the A/P-direction between the elderly and the younger groups Old > young. 2) For the elderly we have in general A/P > M/L. 3) For the younger group there was no significant A/P-M/L difference with the exception for the nudge trials where we had the reverse situation, A/P < M/L. 4) For the elderly we have, Eyes Closed > Eyes Open. 5) In case of the Hurst ss-exponent we have for the elderly, M/L > A/P.

CONCLUSIONS

These results seem to be require some modifications of the more or less established attention-constraint interpretation of entropy. This holds that higher entropy correlates with a more automatic and a less constrained mode of balance control, and that a higher entropy reflects, in this sense, a more efficient balancing.

Center-of-pressure regularity as a marker for attentional investment in postural control: a comparison between sitting and standing postures

Postural control is a highly automatized basic activity that requires limited attentional investments. These investments have been shown to increase from balancing experts to controls, and from controls to persons with impaired postural control. Such between-subject comparisons led to a proposed direct relation between the regularity of center-of-pressure (COP) fluctuations and the amount of attention invested in posture. This study aims to expand this relation to a within-subject comparison of conditions that differ in balance demands. Specifically, more regular COP fluctuations were expected for standing than sitting, as stimulus-response reaction-time studies showed that the required attentional demands are lower for sitting than standing. COP registrations were made for fifteen healthy young adults in seated and standing postures. COP regularity was quantified with sample entropy. As expected, COP fluctuations were found to be more regular for standing than sitting, as evidenced by significantly lower sample entropy values. These findings expand the relation between COP regularity and the amount of attention invested in posture to postural tasks that vary in balance demands. An assessment of COP regularity may thus not only be instrumental in the examination of attentional investment in posture in between-subject designs, but also for different postures in within-subjects designs.

Patterns of postural sway in high anxious children

Background

Current research suggests that elevated levels of anxiety have a negative impact on the regulation of balance. However, most studies to date examined only global balance performance, with little attention to the way body posture is organized in space and time. The aim of this study is to examine whether posturographic measures can reveal (sub)clinical balance deficits in children with high levels of anxiety.

Methods

We examined the spatio-temporal structure of the centre-of-pressure (COP) fluctuations in children with elevated levels of anxiety and a group of typically developing children while maintaining quiet stance on a force plate in various balance challenging conditions. Balance was challenged by adopting sensory manipulations (standing with eyes closed and/or standing on a foam surface) and using a cognitive manipulation (dual-tasking).

Results

Across groups, postural performance was strongly influenced by the sensory manipulations, and hardly by the cognitive manipulation. We also found that children with anxiety had overall more postural sway, and that their postural sway was overall less complex than sway of typically developing children. The postural differences between groups were present even in the simple baseline condition, and the group differences became larger with increasing task difficulty.

Conclusion

The pattern of postural sway suggests that balance is overall less stable and more attention demanding in children with anxiety than typically developing children. The findings provide further evidence for a neuro-behavioral link between psychopathology and the effectiveness of postural control.

Sway regularity reflects attentional involvement in postural control: effects of expertise, vision and cognition

We examined the time varying (dynamic) characteristics of center-of-pressure (COP) fluctuations in a group of 14 preadolescent dancers and 16 age-matched non-dancers. The task involved maintaining balance for 20s with eyes open or eyes closed, and with or without performing an attention demanding cognitive task (word memorization). The main finding was that the time-dependent structure of the COP trajectories of dancers exhibited less regularity than that of non-dancers, as evidenced by a higher sample entropy (decreased statistical regularity). COP irregularity also increased during secondary task performance but decreased during standing with eyes closed. The combined findings indicate that the degree of attentional involvement in postural control - as reflected in the COP dynamics - varies along an automaticity continuum, and is affected by relatively stable subject characteristics (expertise) and more transient factors related to the attentional requirements of the task at hand.

Athletic skill level is reflected in body sway: a test case for accelometry in combination with stochastic dynamics

Recent studies on postural control have shown that the variability of body sway during quiet standing may provide valuable information to characterize changes in postural control due to age, pathology, skill and task. The aim of the present study was to determine - as spade work for possible clinical applications - whether body sway measured with a three-axial accelerometer at the trunk can differentiate between three healthy young populations that differ in athletic skill level. The three groups in question (group size: n=22) consisted of regular bachelor students, physical education students and physical education students specialized in gymnastics. Data were recorded during tandem stance with eyes open or closed and while standing on foam. The acceleration time-series were analysed in anteriorposterior and mediolateral direction. Differences in postural control were quantified in terms of variability, spectral properties and stochastic dynamical measures, i.c., regularity (sample entropy, long-range correlations) and local stability (largest Lyapunov exponent). The results were clear-cut. Standing with eyes closed and on foam increased variability. Compared to standing with eyes open, standing with eyes closed resulted in less regular sway patterns but with greater local stability, whereas standing on foam had an opposite effect. With greater gymnastic skills, acceleration time-series were less variable, less regular and more stable. These results imply that quantifying the stochastic-dynamical structure of postural sway using ambulant accelerometry may provide a useful diagnostic tool.

Neuromechanical tuning of nonlinear postural control dynamics

Postural control may be an ideal physiological motor task for elucidating general questions about the organization, diversity, flexibility, and variability of biological motor behaviors using nonlinear dynamical analysis techniques. Rather than presenting "problems" to the nervous system, the redundancy of biological systems and variability in their behaviors may actually be exploited to allow for the flexible achievement of multiple and concurrent task-level goals associated with movement. Such variability may reflect the constant "tuning" of neuromechanical elements and their interactions for movement control. The problem faced by researchers is that there is no one-to-one mapping between the task goal and the coordination of the underlying elements. We review recent and ongoing research in postural control with the goal of identifying common mechanisms underlying variability in postural control, coordination of multiple postural strategies, and transitions between them. We present a delayed-feedback model used to characterize the variability observed in muscle coordination patterns during postural responses to perturbation. We emphasize the significance of delays in physiological postural systems, requiring the modulation and coordination of both the instantaneous, "passive" response to perturbations as well as the delayed, "active" responses to perturbations. The challenge for future research lies in understanding the mechanisms and principles underlying neuromechanical tuning of and transitions between the diversity of postural behaviors. Here we describe some of our recent and ongoing studies aimed at understanding variability in postural control using physical robotic systems, human experiments, dimensional analysis, and computational models that could be enhanced from a nonlinear dynamics approach.

Postural control and perceptive configuration: influence of expertise in gymnastics

The purpose of the present experiment was to investigate how postural adaptations to the perceptive configuration are modified by specific gymnastics experience. Two groups, one expert in gymnastics and the other non-expert, had to maintain the erected posture while optical flow was imposed as follows: 20s motionless, 30s approaching motion, and 20s motionless. The centre of pressure and head displacements were analysed. The postural adaptations were characterised by the variability of movements for the flow conditions and by the postural latencies for the flow transitions. The results showed that the gymnasts tended to minimise their body movements and were more stationary (head) but not more stable (COP) than the non-gymnasts. These results suggest that gymnastics experience develops a specific postural adaptability relative to the perceptive configuration. We conclude that a specific postural experience could be considered as an intrinsic constraint, which leads to modification in the patterns of functional adaptation in the perceptive motor space.

Influence of embedding parameters and noise in center of pressure recurrence quantification analysis

Recurrence quantification analysis (RQA) can extract the dynamics of postural control from center of pressure (CoP) data by quantifying the system's repeatability, complexity, and local dynamic stability through several variables. Computation of these variables requires the selection of suitable embedding parameters for state space reconstruction (i.e. time delay and embedding dimension); however, it is unclear how the parameters influence RQA variables when examining noisy CoP data. This study evaluated the sensitivity of RQA variables to embedding parameter values and noise level, and assessed methods of selecting embedding parameters for CoP data. Five healthy male subjects maintained quiet stance for 30s while the anterior-posterior CoP was measured. The effect of noise was evaluated by adding uniform white noise of increasing amplitude to the raw CoP signal. The magnitude of all RQA variables decreased with increasing noise amplitude for all subjects. A sensitivity analysis was performed by systematically altering the embedding parameters for the raw data with and without a selected level of added noise. The key result was that, for all subjects, the RQA variables were sensitive to the embedding parameter values and the level of noise in the CoP data. Finally, the performance of false nearest neighbors and average displacement algorithms for choosing embedding parameters was evaluated. Both methods gave clear and consistent results for all subjects with either raw or noisy data. The results suggest that careful selection of embedding parameters is essential when using RQA to examine postural control based on noisy CoP data.

Posturographic testing and motor learning predictability in gymnasts

PURPOSE

One aim of this study was to find if there was a difference between balance and stability between elite level gymnasts and non-gymnasts. Another aim was to find if there was a relationship between dynamic posturographic scores associated with sway fatigue or adaptability and the ability to learn new gymnastic routines. The ultimate aim of the study was to improve gymnastic performance while reducing the probability of injury.

METHODS

Computer dynamic posturography (CDP) provided stability scores, fatigability ratios and adaptation ratios in elite level gymnasts and non-gymnasts controls. Relationships between the postural integrity of gymnasts and non-gymnasts were calculated. The gymnasts were trained in a novel gymnastic routine and performance outcomes were compared to the CDP outcomes.

RESULTS

Tests of postural stability have shown that gymnasts have greater postural stability than non-gymnasts. Gymnasts whose adaptability scores were higher were able to learn and perform new motor routines better than those with lower adaptability scores or high fatigability ratios.

CONCLUSIONS

While gymnasts have greater postural integrity than do non-gymnasts, CDP can identify individuals whose ability to perform new motor activities might be impaired. Methodology to improve functional stability not associated with the motor task may contribute to increased sports performance and decreased probability of injury.

Deterministic center of pressure patterns characterize postural instability in Parkinson's disease

Static posturographic recordings were obtained from six Parkinson's patients and six age-matched, healthy control participants. The availability of vision and visuo-spatial cognitive load were manipulated. Postural sway patterns were analyzed using recurrence quantification analysis (RQA), which revealed differences in center of pressure (COP) dynamics between Parkinson's and control participants. AP COP trajectories for the Parkinson's group were not only significantly more variable than for the control group, but also exhibited distinct patterns of temporal dynamics. The visual manipulation did not differentially affect the two groups. No cognitive load effects were found. The results are generally consistent with the hypothesis that pathological physiological systems exhibit a tendency for less flexible, more deterministic dynamic patterns.