Tadashi Kashima

Trajectory formation based on physiological characteristics of skeletal muscles

Abstract. Human arm trajectories in natural unrestricted reaching movements were studied. They have particular properties such that a hand path is a rather simple straight or curved line, and a tangential velocity profile of hand is bell-shaped. Also these properties are invariant, independent of movement duration and hand-held load. In this study, trajectory formation is investigated on the basis of physiological characteristics of skeletal muscles, and a criterion prescribed by a derivative of isometric muscle torque is proposed. Subsequently, optimal trajectories are formulated under various conditions of movement to account for a planning strategy of human arm trajectories. In addition to such a theoretical approach, human arm trajectories are experimentally observed by a measuring system which provides a visual sensor and a target tracking device, enabling totally unrestricted movements. Then, optimal trajectories are quantitatively evaluated in comparison with experimental data in which essential properties of human arm trajectories are demonstrated. These results support the idea that human arm trajectories are planned in order to minimize the proposed criterion which is determined from physiological aspects. Finally, the physiological advantages of human arm trajectories are discussed with regard to the analysis of observed and optimal trajectories.

Analysis of a muscular control system in human movements

Abstract. In this work, we have studied a muscular control system under experimental conditions for analyzing the dynamic behavior of individual muscles and theoretical considerations for elucidating its control strategy. Movement of human limbs is achieved by joint torques and each torque is specified as the sum of torques generated by muscle forces. The behavior of individual muscles is controlled by the neural input which is estimated by means of an electromyogram (EMG). In this study, the EMGs for a flexor and an extensor are measured in elbow joint movements and the dynamic behavior of individual muscles is analyzed. As a result, it is verified that both a flexor and an extensor are activated throughout the entire movement and that the activation of muscles is controlled above a specific limit independent of the hand-held load. Subsequently, a system model for simulating elbow joint movements is developed which includes the muscle dynamic relationship between the neural input and the isometric force. The minimum limit of muscle activation that has been confirmed in experiments is provided as a constraint of the neural input and the criterion is defined by a derivative of the isometric force of individual muscles. The optimal trajectories formulated under these conditions are quantitatively compared with the experimentally observed trajectories, and the control strategy of a muscular control system is studied. Finally, a muscular control system in multi-joint arm movements is discussed with regard to the comparative analysis between observed and optimal trajectories.

Trajectory formation based on a human arm model with redundancy

Human arm trajectory formations have been widely investigated to understand how we control the trajectories in the central nervous system. The trajectory formations proposed in most of studies are for now limited to a plane motion, and the arm model used for analyses are mostly restricted within three degrees of freedom. On the contrary, the structure of the human arm has redundancy for locating a hand to any position in a three dimensional space. This study presents the trajectory formation applicable to a redundant arm model which is capable of moving in a three dimensional space. The criterion is defined by the angular jerk for all joint angles, and a constraint is provided for the motion of the elbow joint in order to exhibit the hand velocity profile of the human arm trajectory. This trajectory formation is applied to a redundant arm model of which structure is similar to a human arm. The trajectories produced under the proposed trajectory formation are evaluated by the trajectories observed in a three dimensional space as well as in a sagittal plane. Consequently, the relevance of the trajectory formation for demonstrating human arm trajectories has been discussed.

Trajectory control of biomimetic robots for demonstrating human arm movements

This study describes the trajectory control of biomimetic robots by developing human arm trajectory planning. First, the minimum jerk trajectory of the joint angles is produced analytically, and the trajectory of the elbow joint angle is modified by a time-adjustment of the joint motion of the elbow relative to the shoulder. Next, experiments were conducted in which gyro sensors were utilized, and the trajectories observed were compared with those which had been produced. The results showed that the proposed trajectory control is an advantageous scheme for demonstrating human arm movements.

An optimal control model of a neuromuscular system in human arm movements and its control characteristics

The joint torque which sets human limbs into motion is generated by a separate group of muscles provided for each joint. As the activation of each muscle is determined by a neural input, a neuromuscular system controlling all muscles has to be considered in order to understand human movements. In this study, an optimal control model of a neuromuscular system is investigated, and its control characteristics are examined. First, the dynamic and mechanical properties of a muscle are examined, and a neuromuscular system is formulated mathematically. Second, a performance criterion for the optimal control model is defined in order to characterize the dynamic behavior of the neuromuscular system, and a mathematical procedure for producing optimal trajectories is represented. Third, optimal trajectories in human arm movements are produced under various conditions of movement, and these trajectories are compared with experimentally observed ones. It is then verified that the optimal trajectories demonstrate human arm movements well. Finally, the behavior of individual muscles in various movements is examined quantitatively by means of simulation results, and the control characteristics of the human neuromuscular system are investigated.

Control of biomimetic robots based on analysis of human arm trajectories in 3D movements

In this study, a biomimetic robot arm with joint redundancy movable in a three-dimensional space is taken into consideration. The basic trajectories for controlling all joints are formulated under the minimum angular jerk criterion. Then, a time adjustment of the joint motion of the elbow relative to the shoulder is provided for representing specific properties of joint angular trajectories during a movement. Here, a systematical scheme for formulating the human-like trajectory has been developed by use of a direct kinematics. As the angular trajectories of all joints can be formulated in the proposed manner, the hand trajectory can be uniquely produced once the initial and final postures of the arm and a movement duration are given. The trajectories under the proposed scheme are produced by utilizing the same movement conditions observed by experiments. Then, performance for reproducing human-like trajectories has been evaluated under the comparative analysis between the observed and the produced trajectories.

Experimental and theoretical analysis of human arm trajectories in 3D movements

In this study, experiments of three-dimensional (3D) arm movements have been conducted, and the positions of marks provided on a shoulder, an elbow, and a hand are observed. In addition to the investigation of such trajectories in a 3D space, these trajectories are projected to a sagittal, a frontal, and a transverse planes. Then, specific features in these planes are investigated, and detailed properties of human arm trajectories have been uncovered. Next, kinematical features of a human arm during a movement have been analyzed on the basis of behavior of joint angles. Here, a kinematical arm model with joint redundancy is defined, and the kinematics of the model is reconstructed from the measured trajectories. Subsequently, the trajectories of all joint angles during a movement are obtained using the inverse kinematics, and their properties are analyzed in detail. The result shows that the angular trajectories are remarkably similar to those which are produced under the minimum angular jerk criterion.

Kinematic properties of human arm reaching movements in a three-dimensional space

In this study, kinematic properties of human arm reaching movements have been analyzed by use of experimental results of arm trajectories observed in a three-dimensional (3D) space. In the beginning, hand paths obtained by the experiments are kinematically analyzed to pursue their linearity, and we successfully specify a plane on which a hand moves. In the next place, the hand speed profile is calculated by use of position data observed by the experiment in a 3D space. Besides, the hand speed profile is also analytically produced under the minimum jerk criterion with respect to the displacement along the hand path. These observed and produced trajectories are compared, and the similarity of two trajectories has been demonstrated. As a result of the analyses for path and the speed profile of a hand, kinematic properties of human arm trajectories have been identified.