Intermittent Feedback-Control Strategy for Stabilizing Inverted Pendulum on Manually Controlled Cart as Analogy to Human Stick Balancing.

The stabilization of an inverted pendulum on a manually controlled cart (cart-inverted-pendulum; CIP) in an upright position, which is analogous to balancing a stick on a fingertip, is considered in order to investigate how the human central nervous system (CNS) stabilizes unstable dynamics due to mechanical instability and time delays in neural feedback control. We explore the possibility that a type of intermittent time-delayed feedback control, which has been proposed for human postural control during quiet standing, is also a promising strategy for the CIP task and stick balancing on a fingertip. Such a strategy hypothesizes that the CNS exploits transient contracting dynamics along a stable manifold of a saddle-type unstable upright equilibrium of the inverted pendulum in the absence of control by inactivating neural feedback control intermittently for compensating delay-induced instability. To this end, the motions of a CIP stabilized by human subjects were experimentally acquired, and computational models of the system were employed to characterize the experimental behaviors. We first confirmed fat-tailed non-Gaussian temporal fluctuation in the acceleration distribution of the pendulum, as well as the power-law distributions of corrective cart movements for skilled subjects, which was previously reported for stick balancing. We then showed that the experimental behaviors could be better described by the models with an intermittent delayed feedback controller than by those with the conventional continuous delayed feedback controller, suggesting that the human CNS stabilizes the upright posture of the pendulum by utilizing the intermittent delayed feedback-control strategy.

Postural flexibility during quiet standing in healthy elderly and patients with Parkinson's disease.

Postural instability is one of the predominant symptoms of Parkinson's disease (PD). Despite its significant impact on the deterioration in quality of life in PD patients, mechanistic causes of the instability have not been clarified. Joint inflexibility at ankle and hip joints might be such a major cause, leading to small variability in the center of pressure (CoP) during quiet stance. However, this conjecture is still controversial. Thus, quantitative characterization of CoP patterns during quiet stance in PD patients remains a matter of research. Here we performed a linear discriminant analysis for CoP data in PD patients and age-matched healthy elderly during quiet stance, and showed that CoP variations in PD patients and those in healthy elderly could be well distinguished with an accuracy of about 90%, to which appropriately selected sway indices characterizing aspects of power spectrum for the CoP variations contributed. Specifically, major factors responsible for the discrimination were all associated with increase in the power at a high-frequency band (near and over 1 Hz) along with reduction at the low-frequency regime (lower than about 0.7 Hz). Then, the power-ratio, defined as the relative spectral power in a band around 1 Hz, was examined, since the power in this band reflects postural sway with anti-phase coordinated motions of the ankle and hip joints. We showed that the power-ratio values were significantly smaller in the PD patients than those in the healthy subjects. This difference as well as the results of the linear discriminant analysis suggest joint inflexibility in PD patients, particularly at hip joint, which diminished anti-phase coordination between trunk and lower extremity, leading to postural instability in PD patients.

Intermittent appearances of saddle-type hyperbolic dynamics during human stick balancing on a manually controlled cart.

Stabilization of an inverted pendulum on a manually controlled cart (cart-inverted pendulum; CIP), analogous to human fingertip stick balancing, is considered to get insights of how the human central nervous system stabilizes unstable dynamics. We explore a possibility that a type of intermittent control strategy proposed for human postural control might also be applicable to the CIP task, i.e., whether a transient contracting dynamics along a stable manifold of a saddle-type equilibrium of the non-controlled inverted pendulum is exploited intermittently. To this end, we measured task performances during CIP balancing from several experimental subjects. Intermittent appearances of hyperbolicity as typical characteristics reflecting the intermittent control strategy were examined in the recorded motion data using phase space analysis and wavelet analysis. We show that skilled subjects tend to exhibit those characteristics, suggesting that they stabilize upright posture of the stick by utilizing the intermittent control strategy.

Parkinsonian Rigidity Depends on the Velocity of Passive Joint Movement.

Background. It has been long believed that Parkinsonian rigidity is not velocity-dependent based on the neurological examination. However, this has not been verified scientifically. Methods. The elbow joints of 20 Parkinson's disease patients were passively flexed and extended, and two characteristic values, the elastic coefficient (elasticity) and the difference in bias (difference in torque measurements for extension and flexion), were identified from a plot of the angle and torque characteristics. Flexion and extension were done at two different velocities, 60°/s and 120°/s, and a statistical analysis was performed to determine whether the changes in these characteristic values were velocity-dependent. Results. The elastic coefficient was not velocity-dependent, but the difference in bias increased in a velocity-dependent manner (P = 0.0017). Conclusions. The features of rigidity may differ from the conventional definition, which states that they are not dependent on the velocity of joint movement.

4D human body posture estimation based on a motion capture system and a multi-rigid link model.

Human motion analysis in various fields such as neurophysiology, clinical medicine, and sports sciences utilizes a multi-rigid link model of a human body for considering kinetics by solving inverse dynamics of a motion, in which a motion capture system with reflective markers are often used to measure the motion, and then the obtained motion are mapped onto the multi-rigid link model. However, algorithms for such a mapping from spatio-temporal positions of the markers to the corresponding posture of the model are not always fully disclosed. Moreover, a common difficulty for such algorithms is an error caused by displacements of the markers attached on the body surface, referred to as the skin motion error. In this study, we developed a simple algorithm that maps positions of the markers to the corresponding posture of a rigid link model, and examined accuracy of the algorithm by evaluating quantitatively differences between the measured and the estimated posture. We also analyzed the skin motion error. It is shown that magnitude of the error was determined not only by the amplitude of the skin motion, but also by the direction of the marker displacement relative to the frame of reference attached to each segment of the body.