Kanna Uno

A novel muscle synergy extraction method to explain the equilibrium-point trajectory and endpoint stiffness during human upper-limb movements on a horizontal plane

Based on the idea of synergy to explore the building blocks of movements, this study focused on the muscle space for reaching movements by human upper limbs on a horizontal plane to estimate the relationship among muscle synergies, equilibrium-point (EP) trajectories, and endpoint stiffness in two ways: (1) a novel estimation method that analyzes electromyographic signals under the concept of agonist-antagonist (A-A) muscle pairs and (2) a conventional estimation method that uses mechanical perturbations. The experimental results suggest that (1) muscle activities of reaching movements by human upper limbs are represented by only three functional muscle synergies; (2) each muscle synergy balances the coacti-vations of A-A muscle pairs; (3) two of the muscle synergies are invariant bases that form an EP trajectory described in polar coordinates centered on a shoulder joint, where one is a composite unit for radial movement and the other is for angular movement; and (4) the third muscle synergy is the invariant basis for additional adjustment of the endpoint stiffness and has some influence on the direction and size of the endpoint stiffness ellipse.

Tacit representation of muscle activities during coordination training: Muscle synergy analysis to visualize motor enhancement in virtual trajectory of multi-joint arm movement

The tacit representation of muscle coordination has been a major topic of research on motor control since Bernsteins pioneering work. To unravel the mechanisms underlying voluntary movements, we investigated the electromyography signals of six muscles in a non-dominant upper limb during fast spiral movements on a horizontal plane. We considered muscle synergy to be a coordination index that we defined as the balance among co-activations of agonist-antagonist muscle pairs; it is a composite unit related to adjusting the impedance across joints. The virtual trajectory is a time series and is a succession of equilibrium points at the endpoint; it can be represented by the weights for the muscle synergies. Muscle synergy analysis was performed for three healthy subjects before and after voluntary training for eight days. The results revealed that (1) the six muscle activities in a non-dominant upper limb during spiral tracing are explained by three muscle synergies representing the bases for the radial, argumental, and null movements, respectively, of a hand according to polar coordinates centered on the shoulder; (2) the three muscle synergy bases for movements hardly changed with voluntary training kinematics, whereas the kinematics assessment scores for all subjects greatly improved; and (3) the virtual trajectory drastically changed with motor enhancement, especially in the argument direction. When the subjects were asked to perform fast spiral tracing, the polished virtual trajectory formed a beautiful but slightly distorted spiral curve that rotated in the opposite direction of the kinematic trajectory. This may originate from dynamic compensation by the central nervous system. A central factor in motor skill acquisition must be learning a virtual trajectory by considering the dynamic effect of movement especially in the argument direction. Our results imply that virtual trajectories for movements can be learned with invariant bases using polar coordinates, i.e., muscle synergies.

Exploiting invariant structure for controlling multiple muscles in anthropomorphic legs: An inspiration from electromyography analysis of human pedaling

Motor redundancy is a fundamental problem inherent to the inverse problem of motor planning in a muscle-activated endoskeleton such as that of a human and an anthropomorphic musculoskeletal robot. This paper proposes a biologically inspired control framework for multiple redundant muscles, based on the invariant structure in the group of agonist-antagonist coactivation. The theoretical framework explains that the invariant structure represents the balance of the muscle mechanical impedances and reveals that the invariant structure is identical to the reference frame in the muscle space for motor representation at the endpoint. We tested our approach using a typical locomotion task - pedaling - and validated our approach by analyzing electromyography signals during human movement; we applied the extracted invariant structure to anthropomorphic musculoskeletal legs. Our study provides a novel insight into the interpretation of muscle synergies.

A feasibility study to assess intralimb coordination in stroke rehabilitation: two indices of mechanical impedance by coactivation of agonist muscles

Stroke rehabilitation requires intralimb coordination to achieve natural movement after recovery. Focusing on mechanical impedance by the coactivation of agonist muscles, we performed two experiments to assess the intralimb coordination of a post-stroke subject using two indices of the endpoint stiffness and muscle synergies. The results of the first experiment showed that the endpoint stiffness of a post-stroke subject during posture maintenance estimated from muscle synergy analysis resembled that estimated from the mechanical perturbation method. Based on the validity of proposed muscle synergy analysis shown in the first experiment, the results of the second experiment revealed that muscle activities of both the post-stroke and healthy subjects are composed of three muscle synergies in the circle-tracing task. These muscle synergies were invariant despite being determined from time-variant muscle activities; muscle synergies of the post-stroke subject before rehabilitation were different from those of the healthy subject. In addition, the muscle synergies of the post-stroke subject after rehabilitation resembled those of the healthy subject. It is assumed that the post-stroke subject regained appropriate muscle synergies (i.e., the balance of mechanical impedance) after rehabilitation. This study tested the feasibility for practical uses in the assessment, diagnosis, and interventions for stroke rehabilitation using two indices of muscle synergies and endpoint stiffness.

Pilot study on quantitative assessment of muscle imbalance: differences of muscle synergies, equilibrium-point trajectories, and endpoint stiffness in normal and pathological upper-limb movements.

This paper proposes a novel method for assessment of muscle imbalance based on muscle synergy hypothesis and equilibrium point (EP) hypothesis of motor control. We explain in detail the method for extracting muscle synergies under the concept of agonist-antagonist (AA) muscle pairs and for estimating EP trajectories and endpoint stiffness of human upper limbs in a horizontal plane using an electromyogram. The results of applying this method to the reaching movement of one normal subject and one hemiplegic subject suggest that (1) muscle synergies (the balance among coactivation of AA muscle pairs), particularly the synergies that contributes to the angular directional kinematics of EP and the limb stiffness, are quite different between the normal subject and the hemiplegic subject; (2) the concomitant EP trajectory is also different between the normal and hemiplegic subjects, corresponding to the difference of muscle synergies; and (3) the endpoint (hand) stiffness ellipse of the hemiplegic subject becomes more elongated and orientation of the major axis rotates clockwise more than that of the normal subject. The level of motor impairment would be expected to be assessed from a comparison of these differences of muscle synergies, EP trajectories, and endpoint stiffness among normal and pathological subjects using the method.

Functional Electrical Stimulation for Equilibrium-Point Control of Human Ankle Movement: Frequency Domain System Identification of Human Ankle Dynamics

This paper presents a novel method for creating an electrical stimulation pattern to control the equilibrium-point (EP) of human ankle movement. Focusing on the synergetic activation of agonist–antagonist (AA) muscles, the proposed method employs the ES-AA ratio (the ratio of the electrical stimulation levels for AA muscles) and the ES-AA sum (the sum of the electrical stimulation levels for AA muscles), which are based on the AA ratio (the ratio of the electromyography (EMG) voltage levels for AA muscles) and the AA sum (the sum of the EMG voltage levels for AA muscles) used in human movement analysis [1, 2]. The ES-AA ratio is related to the EP of the joint whereas the ES-AA sum is associated with mechanical stiffness of the joint. Using the AA concepts, we estimated the transfer function between the input ES-AA ratio (for the tibialis anterior (TA ) and gastrocnemius (GC)) and the force output of the endpoint in the ankle joint in an isometric environment by investigating the frequency characteristics, and finally found that the ankle-joint system was a second-order system with dead time in terms of the ES-AA ratio and foot force.Copyright