Abstract
Human postural sway during stance arises from coordinated multi-joint movements. Thus, a sway trajectory represented by a time-varying postural vector in the multiple-joint-angle-space tends to be constrained to a low-dimensional subspace. It has been proposed that the subspace corresponds to a manifold defined by a kinematic constraint, such that the position of the center of mass (CoM) of the whole body is constant in time, referred to as the kinematic uncontrolled manifold (kinematic-UCM). A control strategy related to this hypothesis (CoM-control-strategy) claims that the central nervous system (CNS) aims to keep the posture close to the kinematic-UCM using a continuous feedback controller, leading to sway patterns that mostly occur within the kinematic-UCM, where no corrective control is exerted. An alternative strategy proposed by the authors (intermittent control-strategy) claims that the CNS stabilizes posture by intermittently suspending the active feedback controller, in such a way to allow the CNS to exploit a stable manifold of the saddle-type upright equilibrium in the state-space of the system, referred to as the dynamic-UCM, when the state point is on or near the manifold. Although the mathematical definitions of the kinematic- and dynamic-UCM are completely different, both UCMs play similar roles in the stabilization of multi-joint upright posture. The purpose of this study was to compare the dynamic performance of the two control strategies. In particular, we considered a double-inverted-pendulum-model of postural control, and analyzed the two UCMs defined above. We first showed that the geometric configurations of the two UCMs are almost identical. We then investigated whether the UCM-component of experimental sway could be considered as passive dynamics with no active control, and showed that such UCM-component mainly consists of high frequency oscillations above 1 Hz, corresponding to anti-phase coordination between the ankle and hip. We also showed that this result can be better characterized by an eigenfrequency associated with the dynamic-UCM. In summary, our analysis highlights the close relationship between the two control strategies, namely their ability to simultaneously establish small CoM variations and postural stability, but also make it clear that the intermittent control hypothesis better explains the spectral characteristics of sway.
Highlights
The major origin of human postural sway during quiet stance has been considered to stem from rotational body motion around the ankle joints (Winter et al, 1998)
In order to model human quiet stance, we considered a double inverted pendulum model working in the sagittal plane (Figure 2A), where mi (i = {L, HAT}) and li are the mass and lengths of the lower (L) and upper (HAT) links, hL is the distance from the ankle joint to the center of mass (CoM) of the lower link, and hHAT is the distance from the hip joint to the CoM of the upper link. θ a and θ h represent, respectively, the tilt angle of the lower link from the upright position, referred to as the ankle joint angle, and the hip joint angle defined as the angle between the upper and the lower links
We considered two types of uncontrolled manifolds during human quiet stance based on a double inverted pendulum model of the human body
Summary
The major origin of human postural sway during quiet stance has been considered to stem from rotational body motion around the ankle joints (Winter et al, 1998). Aramaki et al (2001) measured ankle and hip joint motion during quiet stance and reported that ranges of angular rotation, velocity, and acceleration of the hip joint angle are comparable with, or even greater than, those of the ankle joint They revealed that angular acceleration of the ankle and hip joints is negatively correlated with each other at a specific ratio, suggesting that such specific coordination might reflect active control of the central nervous system (CNS) in minimizing the acceleration of the center of mass (CoM) position of the whole body. Sasagawa et al (2009) simultaneously measured ankle and hip joint motion with the ground reaction force during quiet stance, and compared two different estimates of the CoM acceleration, one obtained as a linear combination of the ankle and hip joint accelerations and the other as the horizontal component of the ground reaction force divided by the body mass. They demonstrated that the two estimates are well matched, i.e., the latter estimate is characterized by the former with specific weight coefficients, implying that hip joint motions make substantial contributions to neural control during quiet stance
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