Stella F. Donker

Regularity of center-of-pressure trajectories depends on the amount of attention invested in postural control.

The influence of attention on the dynamical structure of postural sway was examined in 30 healthy young adults by manipulating the focus of attention. In line with the proposed direct relation between the amount of attention invested in postural control and regularity of center-of-pressure (COP) time series, we hypothesized that: (1) increasing cognitive involvement in postural control (i.e., creating an internal focus by increasing task difficulty through visual deprivation) increases COP regularity, and (2) withdrawing attention from postural control (i.e., creating an external focus by performing a cognitive dual task) decreases COP regularity. We quantified COP dynamics in terms of sample entropy (regularity), standard deviation (variability), sway-path length of the normalized posturogram (curviness), largest Lyapunov exponent (local stability), correlation dimension (dimensionality) and scaling exponent (scaling behavior). Consistent with hypothesis 1, standing with eyes closed significantly increased COP regularity. Furthermore, variability increased and local stability decreased, implying ineffective postural control. Conversely, and in line with hypothesis 2, performing a cognitive dual task while standing with eyes closed led to greater irregularity and smaller variability, suggesting an increase in the “efficiency, or “automaticity” of postural control”. In conclusion, these findings not only indicate that regularity of COP trajectories is positively related to the amount of attention invested in postural control, but also substantiate that in certain situations an increased internal focus may in fact be detrimental to postural control.

Dynamical structure of center-of-pressure trajectories in patients recovering from stroke

In a recent study, De Haart et al. (Arch Phys Med Rehabil 85:886–895, 2004) investigated the recovery of balance in stroke patients using traditional analyses of center-of-pressure (COP) trajectories to assess the effects of health status, rehabilitation, and task conditions like standing with eyes open or closed and standing while performing a cognitive dual task. To unravel the underlying control processes, we reanalyzed these data in terms of stochastic dynamics using more advanced analyses. Dimensionality, local stability, regularity, and scaling behavior of COP trajectories were determined and compared with shuffled and phase-randomized surrogate data. The presence of long-range correlations discarded the possibility that the COP trajectories were purely random. Compared to the healthy controls, the COP trajectories of the stroke patients were characterized by increased dimensionality and instability, but greater regularity in the frontal plane. These findings were taken to imply that the stroke patients actively (i.e., cognitively) coped with the stroke-induced impairment of posture, as reflected in the increased regularity and decreased local stability, by recruiting additional control processes (i.e., more degrees of freedom) and/or by tightening the present control structure while releasing non-essential degrees of freedom from postural control. In the course of rehabilitation, dimensionality stayed fairly constant, whereas local stability increased and regularity decreased. The progressively less regular COP trajectories were interpreted to indicate a reduction of cognitive involvement in postural control as recovery from stroke progressed. Consistent with this interpretation, the dual task condition resulted in less regular COP trajectories of greater dimensionality, reflecting a task-related decrease of active, cognitive contributions to postural control. In comparison with conventional posturography, our results show a clear surplus value of dynamical measures in studying postural control.

Children with cerebral palsy exhibit greater and more regular postural sway than typically developing children.

Following recent advances in the analysis of centre-of-pressure (COP) recordings, we examined the structure of COP trajectories in ten children (nine in the analyses) with cerebral palsy (CP) and nine typically developing (TD) children while standing quietly with eyes open (EO) and eyes closed (EC) and with concurrent visual COP feedback (FB). In particular, we quantified COP trajectories in terms of both the amount and regularity of sway. We hypothesised that: (1) compared to TD children, CP children exhibit a greater amount of sway and more regular sway and (2) concurrent visual feedback (creating an external functional context for postural control, inducing a more external focus of attention) decreases both the amount of sway and sway regularity in TD and CP children alike, while closing the eyes has opposite effects. The data were largely in agreement with both hypotheses. Compared to TD children, the amount of sway tended to be larger in CP children, while sway was more regular. Furthermore, the presence of concurrent visual feedback resulted in less regular sway compared to the EO and EC conditions. This effect was less pronounced in the CP group where posturograms were most regular in the EO condition rather than in the EC condition, as in the control group. Nonetheless, we concluded that CP children might benefit from therapies involving postural tasks with an external functional context for postural control.

Coordination Between Arm and Leg Movements During Locomotion

Abstract To evaluate the contrasting dynamical and biomechanical interpretations of the 2:1 frequency coordination between arm and leg movements that occurs at low walking velocities and the 1:1 frequency coordination that occurs at higher walking velocities, the authors conducted an experiment in which they quantified the effect of walking velocity on the stability of the frequency and phase coordination between the individual limb movements. Spectral analyses revealed the presence of 2:1 frequency coordination as a consistent feature of the data in only 3 out of 8 participants at walking velocities ranging from 1.0 to 2.0 km/h, in spite of the fact that the eigenfrequencies of the arms were rather similar across participants. The degree of interlimb coupling, as indexed by weighted coherence and variability of relative phase, was lower for the arm movements and for ipsilateral and diagonal combinations of arm and leg movements than for the leg movements. Furthermore, the coupling between all pairs of limb movements was found to increase with walking velocity, whereas no clear signs were observed that the switches from 2:1 to 1:1 frequency coordination and vice versa were preceded by loss of stability. Therefore, neither a purely biomechanical nor a purely dynamical model is optimally suited to explain these results. Instead, an integrative model involving elements of both approaches seems to be required.

Interlimb coordination in prosthetic walking: effects of asymmetry and walking velocity

The present study focuses on interlimb coordination in walking with an above-knee prosthesis using concepts and tools of dynamical systems theory (DST). Prosthetic walkers are an interesting group to investigate from this theory because their locomotory system is inherently asymmetric, while, according to DST, coordinative stability may be expected to be reduced as a function of the asymmetry of the oscillating components. Furthermore, previous work on locomotion motivated from DST has shown that the stability of interlimb coordination increases with walking velocity, leading to the additional expectation that the anticipated destabilizing effect of the prosthesis-induced asymmetry may be diminished at higher walking velocities. To examine these expectations, an experiment was conducted aimed at comparing interlimb coordination during treadmill walking between seven participants with an above-knee prosthesis and seven controls across a range of walking velocities. The observed gait patterns were analyzed in terms of standard gait measures (i.e., absolute and relative swing, stance and step times) and interlimb coordination measures (i.e., relative phase and frequency locking). As expected, the asymmetry brought about by the prosthesis led to a decrease in the stability of the coordination between the legs as compared to the control group, while coordinative stability increased with increasing walking velocity in both groups in the absence of a significant interaction. In addition, the 2:1 frequency coordination between arm and leg movements that is generally observed in healthy walkers at low walking velocities was absent in the prosthetic walkers. Collectively, these results suggest that both stability and adaptability of coordination are reduced in prosthetic walkers but may be enhanced by training them to walk at higher velocities.

Effects of Velocity and Limb Loading on the Coordination Between Limb Movements During Walking

The authors investigated the effects of velocity (increasing from 0.5 to 5.0 km/hr in steps of 0.5 km/hr) and limb loading on the coordination between arm and leg movements during treadmill walking in 7 participants. Both the consistency of the individual limb movements and the stability of their coordination increased with increasing velocity; the frequency coordination between arm and leg movements was 2:1 at the lower velocities and 1:1 at the higher velocities. The mass manipulation affected the individual limb movements but not their coordination, indicating that a stable walking pattern was preserved. The results differed qualitatively from those obtained in studies on bimanual interlimb coordination, implying that the dynamical principles identified therein are not readily applicable to locomotion.

Decreasing perceived optic flow rigidity increases postural sway

Optic flow simulating self-motion through the environment can induce postural adjustments in observers. Some studies investigating this phenomenon have used optic flow patterns increasing in speed from center to periphery, whereas others used optic flow patterns with a constant speed. However, altering the speed gradient of an optic flow stimulus changes the perceived rigidity of such a stimulus. Optic flow stimuli that are perceived as rigid can be expected to provide a stronger sensation of self-motion than non-rigid optic flow, and this may well be reflected in the amount of postural sway. The current study, therefore, examined, by manipulating the speed gradient, to what extent the rigidity of an optic flow stimulus influences posture along the anterior–posterior axis. We used radial random dot expanding or contracting optic flow patterns with three different speed profiles (single-speed, linear speed gradient or quadratic speed gradient) that differentially induce the sensation of self-motion. Interestingly, most postural sway was observed for the non-rigid single-speed optic flow pattern, which contained the least self-motion information of the three profiles. Moreover, we found an anisotropy in that contracting optic flow produced more postural sway than expanding optic flow. In addition, the amount of postural sway increased with increasing stimulus speed, but for contracting optic flow only. Taken together, the results of the current study support the view that visual and sensorimotor systems appear to be tailored toward compensating for rigid optic flow stimulation.

Sensory Influences on Interlimb Coordination During Gait

This chapter addresses the role of afferent feedback and reflexes in the regulation of interlimb coordination in animals and humans with a focus on locomotion. From the work on cats it is known that the rhythmic muscle activities during gait are generated by specialized neural circuits located in the spinal cord (the so-called central pattern generator, “CPGs”). These CPGs are coordinated by neurons, which interconnect both sides or which transmit information between the cervical and lumbar spine. It is argued that afferent input, especially load-related information, plays a major role in shaping the resulting coordination of these CPGs. Induced changes are seen not only with a general loading of the animal but also with the selective loading of a given limb. Such principles also apply to human locomotion. Studies on infants have shown that basic coordination patterns exist, very similar to those found in the cat. The effects of afferents (notably those related to load and to hip position) play an important role in phase transitions, much as was described in feline models. In adults, the role of proprioceptive afferents was studied by muscle vibration (selective stimulus for la afferents) and by adding load (activating mainly Ib afferents). When applied during gait, tendon vibration has little effect on intra - and interlimb coordination. In contrast, load manipulations produce more profound changes. During gait, the loading of one of the limbs induces adaptations in inter-limb coordination in the 3 remaining limbs, thereby providing rhythm constancy (stable cadence). This is in line with other evidence indicating that the coordination between arm and leg movements is quite robust across various types of locomotion, suggesting a strong coupling between both homologous and non-homologous limbs.

Illusory Motion of the Motion Aftereffect Induces Postural Sway

It remains an open question, however, whether this perception-action cycle is the result of direct visual stimulation only, or whether postural adjustments also occur when the motion of the visual stimulus is illusory. Here, we show that the latter is the case. Prolonged viewing of visual motion results in neural adaptation, and subsequent viewing of a stationary stimulus normally results in illusory motion in the opposite direction, a famous phenomenon known as the motion aftereffect (MAE; Anstis, Verstraten, & Mather, 1998). Surprisingly, this sequence of stimulation also causes postural sway in the direction consistent with the perceived illusory motion. Control test patterns that do not generate an MAE after identical adaptation do not induce sway. This suggests that the visuo-vestibular interactions that govern postural control are not influenced by visual stimulation per se, but can be modulated by an illusory motion signal (e.g., the internal neural signal responsible for the MAE).

Identification of human balance control in standing

The goal was to identify the contribution of intrinsic mechanical properties of the muscular-skeletal system and the reflex gains in controlling balance during standing. The combination of balance perturbations experiments and closed loop identification schemes made it possible to identify the reflex loop gains and intrinsic mechanical properties in various environmental conditions. Human balance responses were studied by placing subjects on a movable support base while keeping their eyes open or closed. EMG, body motion and the ground reaction forces were recorded. From the platform perturbation data the frequency response functions of the controller dynamics and the admittance of the balance control system were estimated with a non-parametric closed loop identification technique. Using a parametric model of balance control and a fitting procedure the sum of the intrinsic stiffness and neural position feedback gain, the neural time delay, the neural velocity feedback gain and the intrinsic damping were uniquely identified. The results show that subjects balancing on a randomly moving platform in the forward-backward direction applied a minimal stiffness strategy. The required damping to avoid oscillations was mainly due to neural velocity feedback rather than to intrinsic damping.