Biomedical subjects

A D Kuo

Publications and source records attributed to A D Kuo.

13 recordsLinked to original sources

PubMed2002

Contributions of altered sensation and feedback responses to changes in coordination of postural control due to aging.

We used multivariate kinematics and joint torque measurements during dynamic posturography to determine the relative contributions of changes in overall control gain, relative weighting of sensors, and noise-like effects on posture control in the elderly. Our results show that sway coordination and amplitude both change with age, but that changes in overall feedback gains do not explain these differences. We propose that increased sway of elderly subjects in platform sway-referenced conditions is due to sensory noise or decreased ability to detect small motions of the platform, while increased sway during visual sway-referencing is due to re-weighting of the various sensors.

Adult↗

PubMed2001

Mechanical and metabolic determinants of the preferred step width in human walking.

We studied the selection of preferred step width in human walking by measuring mechanical and metabolic costs as a function of experimentally manipulated step width (0.00-0.45L, as a fraction of leg length L). We estimated mechanical costs from individual limb external mechanical work and metabolic costs using open circuit respirometry. The mechanical and metabolic costs both increased substantially (54 and 45%, respectively) for widths greater than the preferred value (0.15-0.45L) and with step width squared (R(2) = 0.91 and 0.83, respectively). As predicted by a three-dimensional model of walking mechanics, the increases in these costs appear to be a result of the mechanical work required for redirecting the centre of mass velocity during the transition between single stance phases (step-to-step transition costs). The metabolic cost for steps narrower than preferred (0.10-0.00L) increased by 8%, which was probably as a result of the added cost of moving the swing leg laterally in order to avoid the stance leg (lateral limb swing cost). Trade-offs between the step-to-step transition and lateral limb swing costs resulted in a minimum metabolic cost at a step width of 0.12L, which is not significantly different from foot width (0.11L) or the preferred step width (0.13L). Humans appear to prefer a step width that minimizes metabolic cost.

Biomechanical Phenomena↗

PubMed2001

A simple model of bipedal walking predicts the preferred speed-step length relationship.

We used a simple model of passive dynamic walking, with the addition of active powering on level ground, to study the preferred relationship between speed and step length in humans. We tested several hypothetical metabolic costs, with one component proportional to the mechanical work associated with pushing off with the stance leg at toe-off, and another component associated with several possible costs of forcing oscillations of the swing leg. For this second component, a cost based on the amount of force needed to oscillate the leg divided by the time duration of that force predicts the preferred speed-step length relationship much better than other costs, such as the amount of mechanical work done in swinging the leg. The cost of force/time models the need to recruit fast muscle fibers for large forces at short durations. The actual mechanical work performed by muscles on the swing leg appears to be of relatively less importance, although it appears to be minimized by the use of short bursts of muscle activity in near-isometric conditions. The combined minimization of toe-off mechanical work and force divided by time predicts the preferred speed-step length relationship.

Biomechanical Phenomena↗

PubMed2000

Active control of lateral balance in human walking.

We measured variability of foot placement during gait to test whether lateral balance must be actively controlled against dynamic instability. The hypothesis was developed using a simple dynamical model that can walk down a slight incline with a periodic gait resembling that of humans. This gait is entirely passive except that it requires active control for a single unstable mode, confined mainly to lateral motion. An especially efficient means of controlling this instability is to adjust lateral foot placement. We hypothesized that similar active feedback control is performed by humans, with fore-aft dynamics stabilized either passively or by very low-level control. The model predicts that uncertainty within the active feedback loop should result in variability in foot placement that is larger laterally than fore-aft. In addition, loss of sensory information such as by closing the eyes should result in larger increases in lateral variability. The control model also predicts a slight coupling between step width and length. We tested 15 young normal human subjects and found that lateral variability was 79% larger than fore-aft variability with eyes open, and a larger increase in lateral variability (53% vs. 21%) with eyes closed, consistent with the model's predictions. We also found that the coupling between lateral and fore-aft foot placements was consistent with a value of 0.13 predicted by the control model. Our results imply that humans may harness passive dynamic properties of the limbs in the sagittal plane, but must provide significant active control in order to stabilize lateral motion.

Biomechanical Phenomena↗

PubMed1999

EquiTest modification with shank and hip angle measurements: differences with age among normal subjects.

The Sensory Organization Test protocol of the EquiTest system (NeuroCom International, Clackamas Oregon) tests utilization of visual, vestibular, and proprioceptive sensors by manipulating the accuracy of visual and/or somatosensory inputs during quiet stance. In the standard Sensory Organization Test, both manipulation of sensory input (sway-referencing) and assessment of postural sway are based on ground reaction forces measured from a forceplate. The purpose of our investigation was to examine the use of kinematic measurements to provide a more direct feedback signal for sway-referencing and for assessment of sway. We compared three methods of sway-referencing: the standard EquiTest method based on ground reaction torque, kinematic feedback based on servo-controlling to shank motion, and a more complex kinematic feedback based on servo-controlling to follow position of the center of mass (COM) as calculated from a two-link biomechanical model. Fifty-one normal subjects (ages 20-79) performed the randomized protocol. When using either shank or COM angle for sway-referencing feedback as compared to the standard EquiTest protocol, the Equilibrium Quotient and Strategy Score assessments were decreased for all age groups in the platform sway-referenced conditions (SOT 4, 5, 6). For all groups of subjects, there were significant differences in one or more of the kinematic sway measures of shank, hip, or COM angle when using either of the alternative sway-referencing parameters as compared to the standard EquiTest protocol. The increased sensitivities arising from use of kinematics had the effect of amplifying differences with age. For sway-referencing, the direct kinematic feedback may enhance ability to reduce proprioceptive information by servo-controlling more closely to actual ankle motion. For assessment, kinematics measurements can potentially increase sensitivity for detection of balance disorders, because it may be possible to discriminate between body sway and acceleration and to determine the phase relationship between ankle and hip motion.

Adult↗

PubMed1998

Effect of altered sensory conditions on multivariate descriptors of human postural sway.

Multivariate descriptors of sway were used to test whether altered sensory conditions result not only in changes in amount of sway but also in postural coordination. Eigenvalues and directions of eigenvectors of the covariance of shnk and hip angles were used as a set of multivariate descriptors. These quantities were measured in 14 healthy adult subjects performing the Sensory Organization test, which disrupts visual and somatosensory information used for spatial orientation. Multivariate analysis of variance and discriminant analysis showed that resulting sway changes were at least bivariate in character, with visual and somatosensory conditions producing distinct changes in postural coordination. The most significant changes were found when somatosensory information was disrupted by sway-referencing of the support surface (P = 3.2 x 10(-10)). The resulting covariance measurements showed that subjects not only swayed more but also used increased hip motion analogous to the hip strategy. Disruption of vision, by either closing the eyes or sway-referencing the visual surround, also resulted in altered sway (P = 1.7 x 10(-10)), with proportionately more motion of the center of mass than with platform sway-referencing. As shown by discriminant analysis, an optimal univariate measure could explain at most 90% of the behavior due to altered sensory conditions. The remaining 10%, while smaller, are highly significant changes in posture control that depend on sensory conditions. The results imply that normal postural coordination of the trunk and legs requires both somatosensory and visual information and that each sensory modality makes a unique contribution to posture control. Descending postural commands are multivariate in nature, and the motion at each joint is affected uniquely by input from multiple sensors.

Adult↗

PubMed1998

Multivariate changes in coordination of postural control following spaceflight.

Postural and gait instabilities in astronauts returning from spaceflight are thought to result from in-flight adaptation of central nervous system processing of sensory inputs from the vestibular, proprioceptive, and visual systems. We hypothesized that reorganization of posture control relying on these multiple inputs would result in not only greater amounts of sway, but also changes in interjoint coordination. We tested this hypothesis by examining the multivariate characteristics of postural sway and comparing the postural control gain used for maintenance of upright stance during the altered sensory conditions of the Sensory Organization Test (EquiTest, Neurocom Intl.). We used the covariance of hip and ankle kinematics as a measure of joint motion and interjoint coordination, and then utilized discriminant analysis to further examine these characteristics in a group of 10 first-time astronauts. In five of the six conditions, the most important difference was an increased relative utilization of the hip strategy, which would not be evident using conventional balance measures such as peak or root-mean-square sway. This finding was supported by indications of increased hip torque gains relative to lower extremity and neck motion in at least four conditions (p < 0.05). In contrast, ankle torque gains to these motions did not appear to change. These results suggest that after spaceflight, astronauts exhibit significant multivariate changes in multijoint coordination, of which increased sway is only one component. These changes are consistent with reweighting of vestibular inputs and changes in control strategy in a multivariable control system.

Adult↗

PubMed1998

Optimization-based differential kinematic modeling exhibits a velocity-control strategy for dynamic posture determination in seated reaching movements.

We proposed a velocity control strategy for dynamic posture determination that underlay an optimization-based differential inverse kinematics (ODIK) approach for modeling three-dimensional (3-D) seated reaching movements. In this modeling approach, a four-segment seven-DOF linkage is employed to represent the torso and right arm. Kinematic redundancy is resolved efficiently in the velocity domain via a weighted pseudoinverse. Weights assigned to individual DOF describe their relative movement contribution in response to an instantaneous postural change. Different schemes of posing constraints on the weighting parameters, by which various motion apportionment strategies are modeled, can be hypothesized and evaluated against empirical measurements. A numerical optimization procedure based on simulated annealing estimate the weighting parameter values such that the predicted movement best fits the measurement. We applied this approach to modeling 72 seated reaching movements of three distinctive types performed by six subjects. Results indicated that most of the movements were accurately modeled (time-averaged RMSE < 5 degrees) with a simple time-invariant four-weight scheme which represents a time-constant, inter-joint motion apportionment strategy. Modeling error could be further reduced by using less constrained schemes, but notably only for the ones that were relatively poorly modeled with a time-invariant four-weight scheme. The fact that the current modeling approach was able to closely reproduce measured movements and do so in a computationally advantageous way lends support to the proposed velocity control strategy.

Animals↗

PubMed1998

A least-squares estimation approach to improving the precision of inverse dynamics computations.

A least-squares approach to computing inverse dynamics is proposed. The method utilizes equations of motion for a multi-segment body, incorporating terms for ground reaction forces and torques. The resulting system is overdetermined at each point in time, because kinematic and force measurements outnumber unknown torques, and may be solved using weighted least squares to yield estimates of the joint torques and joint angular accelerations that best match measured data. An error analysis makes it possible to predict error magnitudes for both conventional and least-squares methods. A modification of the method also makes it possible to reject constant biases such as those arising from misalignment of force plate and kinematic measurement reference frames. A benchmark case is presented, which demonstrates reductions in joint torque errors on the order of 30 percent compared to the conventional Newton-Euler method, for a wide range of noise levels on measured data. The advantages over the Newton-Euler method include making best use of all available measurements, ability to function when less than a full complement of ground reaction forces is measured, suppression of residual torques acting on the top-most body segment, and the rejection of constant biases in data.

Acceleration↗

PubMed1995

An optimal control model for analyzing human postural balance.

The question posed in this study is whether optimal control and state estimation can explain selection of control strategies used by humans, in response to small perturbations to stable upright balance. To answer this question, a human sensorimotor control model, compatible with previous work by others, was assembled. This model incorporates linearized equations and full-state feedback with provision for state estimation. A form of gain-scheduling is employed to account for nonlinearities caused by control and biomechanical constraints. By decoupling the mechanics and transforming the controls into the space of experimentally observed strategies, the model is made amenable to the study of a number of possible control objectives. The objectives studied include cost functions on the state deviations, so as to control the center of mass, provide a stable platform for the head, or maintain upright stance, along with a cost function on control effort. Also studied was the effect of time delay on the stability of controls produced using various control strategies. An objective function weighting excursion of the center of mass and deviations from the upright stable position, while taking advantage of fast modes of the system, as dictated by inertial parameters and musculoskeletal geometry, produces a control that reasonably matches experimental data. Given estimates of sensor performance, the model is also suited for prediction of uncertainty in the response.

Biomechanical Phenomena↗

PubMed1993

A biomechanical analysis of muscle strength as a limiting factor in standing posture.

We developed a method for studying muscular coordination and strength in multijoint movements and have applied it to standing posture. The method is based on a musculoskeletal model of the human lower extremity in the sagittal plane and a technique to visualize, geometrically, how constraints internal and external to the body affect movement. We developed an algorithm to calculate the set of all feasible accelerations (i.e., the 'feasible acceleration set', or FAS) that muscles can induce. For the ankle, knee, and hip joints in the sagittal plane, this set is a polyhedron in three dimensions. Using the volume of the FAS as an indicator of overall mobility, we found that strengthening muscles on the posterior side (as opposed to the anterior) of the body would cause greater increases in mobility. Employing the experimental observations of others, we also found that acceleration constraints greatly reduce the range of feasible accelerations. We then defined a set of four basic acceleration vectors which, when used in various combinations, can produce the repertoire of postural movements. We used linear programming to find the maximum magnitudes of these vectors, and the sensitivity of these magnitudes to muscle strength, thereby delineating those muscles which, if strengthened, would cause the greatest increase in the body's ability to generate the basic acceleration vectors. For our particular model, those muscle groups were found to be hamstrings, tibialis anterior, rectus femoris, and gastrocnemius. These muscle groups would be of great importance in cases involving severely reduced muscle strength. This methodology may therefore be useful for purposes such as design of functional electrical stimulation controllers or exercises for persons at risk for falling.

Acceleration↗

PubMed1993

Human standing posture: multi-joint movement strategies based on biomechanical constraints.

We developed a theoretical framework for studying coordination strategies in standing posture. The framework consists of a musculoskeletal model of the human lower extremity in the sagittal plane and a technique to visualize, geometrically, how constraints internal and external to the body affect movement. The set of all feasible accelerations (i.e., the "feasible acceleration set" or FAS) that muscles can induce at positions near upright were calculated. We found that musculoskeletal mechanics dictate that independent control of joints is relatively difficult to achieve. When muscle activations are constrained so the knees stay straight, to approximate the typical postural response to perturbation, the corresponding subset of the feasible acceleration set greatly favors a combination of ankle and hip movement in the ratio 1:3 (called the "hip strategy"). Independent control of these two joints remains difficult to achieve. When near the boundary of instability, the orientation and shape of this subset show that the movement strategy necessary to maintain stability, without taking a step, is quite restricted. Hypothesizing that regulation of center-of-mass position is crucial to maintaining balance, we examined the feasible set of center-of-mass accelerations. When the knees must be kept straight, the acceleration of the center of mass is severely limited vertically, but not horizontally. We also found that the "ankle strategy", involving rotation about the ankles only, requires more muscle activation than the "hip strategy" for a given amount of horizontal acceleration. Our model therefore predicts that the hip strategy is most effective at controlling the center of mass with minimal muscle activation ("neural effort").

Ankle Joint↗

PubMed1991

Computational methods for analyzing the structure of cancellous bone in planar sections.

Conventional stereologic methods for expressing the orientation of anisotropic materials are limited to materials assumed to possess orthogonal directions of orientation. In many substances, including cancellous bone, this assumption is unsubstantiated. Presented here are two simple methods for characterizing the orientation of any anisotropic material within a plane. By modeling the substance as a series of lines oriented in particular directions, it is possible to arrive at either a "phase distribution" that expresses the degree of orientation distributed over a range of angles or a series of "primary orientations" that express the degree of orientation at a select number of angles, with an additional measure of the degree of isotropy. This characterization of anisotropy is highly dependent on such parameters as feature size, sample size, test line spacing, and test line width. Given the careful selection of these parameters, the new methods provide simple measures of orientation, which may prove useful in testing Wolff's trajectorial theory of the relationship between mechanical stresses and the orientation of cancellous bone.

Animals↗