Neuromotor control in handwriting and drawing: introduction and overview.

In this introduction to the Special Issue on Neuromotor Control in Handwriting and Drawing, an overview is given of the themes and sub-themes covered by these proceedings of the eighth Biennial Conference of the International Graphonomics Society (IGS) on handwriting and other graphic tasks, held in Genoa, Italy, 24-28 August, 1997. Section overviews are given for Neuromuscular and Biophysical Models; Learning and Cognitive Models; and Neuropsychological Issues. Moreover, to guide researchers in the field, this introduction also provides a summary list of major publications following the preceding IGS conferences.

Neural network learning of robot arm impedance in operational space.

Impedance control is one of the most effective control methods for the manipulators in contact with their environments. The characteristics of force and motion control, however, is determined by a desired impedance parameter of a manipulator's end-effector that should be carefully designed according to a given task and an environment. The present paper proposes a new method to regulate the impedance parameter of the end-effector through learning of neural networks. Three kinds of the feed-forward networks are prepared corresponding to position, velocity and force control loops of the end-effector before learning. First, the neural networks for position and velocity control are trained using iterative learning of the manipulator during free movements. Then, the neural network for force control is trained for contact movements. During learning of contact movements, a virtual trajectory is also modified to reduce control error. The method can regulate not only stiffness and viscosity but also inertia and virtual trajectory of the end-effector. Computer simulations show that a smooth transition from free to contact movements can be realized by regulating impedance parameters before a contact.

A novel mechatronic system for measuring end-point stiffness: mechanical design and preliminary tests.

Measuring arm stiffness is of great interest for many disciplines from biomechanics to medicine especially because modulation of impedance represents one of the main mechanism underlying control of movement and interaction with external environment. Previous works have proposed different methods to identify multijoint hand stiffness by using planar or even tridimensional haptic devices, but the associated computational burden makes them not easy to implement. We present a novel mechanism conceived for measuring multijoint planar stiffness by a single measurement and in a reduced execution time. A novel mechanical rotary device applies cyclic radial perturbation to human arm of a known displacement and the force is acquired by means of a 6-axes commercial load cell. The outcomes suggest that the system is not only reliable but allows obtaining a bi-dimensional estimation of arm stiffness in reduced amount of time and the results are comparable with those reported in previous researches.

Multijoint arm stiffness during movements following stroke: implications for robot therapy.

Impaired arm movements in stroke appear as a set of stereotypical kinematic patterns, characterized by abnormal joint coupling, which have a direct consequence on arm mechanics and can be quantified by the net arm stiffness at the hand. The current available measures of arm stiffness during functional tasks have limited clinical use, since they require several repetitions of the same test movement in many directions. Such procedure is difficult to obtain in stroke survivors who have lower fatigue threshold and increased variability compared to unimpaired individuals. The present study proposes a novel, fast quantitative measure of arm stiffness during movements by means of a Time-Frequency technique and the use of a reassigned spectrogram, applied on a trial-by-trial basis with a single perturbation. We tested the technique feasibility during robot mediated therapy, where a robot helped stroke survivors to regain arm mobility by providing assistive forces during a hitting task to 13 targets covering the entire reachable workspace. The endpoint stiffness of the paretic arm was estimated at the end of each hitting movements by suddenly switching of the assistive forces and observing the ensuing recoil movements. In addition, we considered how assistive forces influence stiffness. This method will provide therapists with improved tools to target the treatment to the individual's specific impairment and to verify the effects of the proposed exercises.

Subtle upper limb impairment in asymptomatic multiple sclerosis subjects.

We evaluated upper limb function in multiple sclerosis (MS) subjects (11 clinically definite MS patients and seven clinically isolated syndrome (CIS) subjects), with a normal upper limb standard neurological examination. Subjects performed center-out reaching movements under visual control, with and without vision of the hand. Their movements were recorded through a digitizing tablet. Motor performance was also related to lesion load, estimated from magnetic resonance imaging (MRI). We found that in MS and CIS subjects, under the hand vision condition, movements were significantly less smooth, and had a less symmetric speed profile. However, the observed impairment did not correlate with MRI findings. This result may be interpreted as evidence of a compensatory strategy, elicited by subtle alterations in sensorimotor control.

Can muscle stiffness alone stabilize upright standing?

A stiffness control model for the stabilization of sway has been proposed recently. This paper discusses two inadequacies of the model: modeling and empiric consistency. First, we show that the in-phase relation between the trajectories of the center of pressure and the center of mass is determined by physics, not by control patterns. Second, we show that physiological values of stiffness of the ankle muscles are insufficient to stabilize the body "inverted pendulum." The evidence of active mechanisms of sway stabilization is reviewed, pointing out the potentially crucial role of foot skin and muscle receptors.

Human hand impedance characteristics during maintained posture.

The present paper examines human hand impedance characteristics, including inertia and viscosity as well as stiffness, in multi-joint arm movements. While a subject maintains a given hand location, small external disturbances are applied to his hand by a manipulandum. The corresponding force-displacement vectors are measured and sampled over time in order to estimate the hand impedance by means of a second-order linear model. The experimental results in different subjects and hand locations are summarized as follows: (1) the estimated inertia matrices of the human hand well agrees with computed values using a two-joint arm model, (2) spatial variations of the stiffness ellipses are consistent with the experimental results of Mussa-Ivaldi et al. (1985), (3) hand stiffness and viscosity increase with the grip force of the subject, and (4) viscosity and stiffness ellipses tend to have similar orientation. The accuracy of the impedance estimation method is validated with a mechanical spring-mass system with known parameters.

Computational implications of modeling grasping as a form of (multiple-parallel) reaching.

Although it is true that the specific research on grasping has been dominated in recent years by the canonical transport + grip model originally formulated by Jeannerod (1984), still one can find in the research on reaching a number of links and anticipations to the new view on grasping made explicit by the authors of the target article. This paper reviews some of the relevant concepts and outlines a modeling framework that aims at biological plausibility.

Internal models in the control of posture.

The aim of the paper is to investigate the application of control schemes based on "internal models" to the stabilization of the standing posture. The computational complexities of the control problems are analyzed, showing that muscle stiffness alone is insufficient to carry out the task. The paper also re-visits the concept of the cerebellum as a Smith's predictor.