Kenta Tominaga

Task‐specific tremor in violinists: Evidence of coactivation in the 3 to 8 Hz frequency range

Task‐specific tremor in musicians severely impairs fine motor control. However, little is known about its pathophysiology. Here, we quantify electromyography (EMG) properties in primary bowing tremor—in particular, muscular coactivation—to determine whether primary bowing tremor affects a specific frequency range of coactivation.

Extraction and implementation of muscle synergies in neuro-mechanical control of upper limb movement

This work faces the redundancy problem, a central concern in robotics, in a particular force-producing task by using muscle synergies to simplify the control. We extracted muscle synergies from human electromyograph signals and interpreted the physical meaning of the identified muscle synergies. Based on the human analysis results, we hypothesized a novel control framework that can explain the mechanism of the human motor control. The framework was tested in controlling a pneumatic-driven robotic arm to perform a reaching task. This control method, which uses only two synergies as manipulated variables for driving antagonistic pneumatic artificial muscles to generate desired movements, would be useful to deal with the redundancy problem; thus, suggesting a simple but efficient control for human-like robots to work safely and compliantly with humans. Graphical Abstract

Losing dexterity: patterns of impaired coordination of finger movements in musician’s dystonia

Extensive training can bring about highly-skilled action, but may also impair motor dexterity by producing involuntary movements and muscular cramping, as seen in focal dystonia (FD) and tremor. To elucidate the underlying neuroplastic mechanisms of FD, the present study addressed the organization of finger movements during piano performance in pianists suffering from the condition. Principal component (PC) analysis identified three patterns of fundamental joint coordination constituting finger movements in both patients and controls. The first two coordination patterns described less individuated movements between the “dystonic” finger and key-striking fingers for patients compared to controls. The third coordination pattern, representing the individuation of movements between the middle and ring fingers, was evident during a sequence of strikes with these fingers in controls, which was absent in the patients. Consequently, rhythmic variability of keystrokes was more pronounced during this sequence of strikes for the patients. A stepwise multiple-regression analysis further identified greater variability of keystrokes for individuals displaying less individuated movements between the affected and striking fingers. The findings suggest that FD alters dexterous joint coordination so as to lower independent control of finger movements, and thereby degrades fine motor control.

Analysis of muscle coordination in human pedaling and implementation with a musculoskeletal robot

This paper describes a novel method for controlling a human-like musculoskeletal leg robot by implementing muscle coordination in human voluntary movement. To examine human motor control involving the ill-posed problem of redundant degrees-of-freedom (DOFs), we introduce the concept of the agonist-antagonist muscle pair ratio (A-A ratio) which represents the coordination of agonist-antagonist muscle pairs. Our concept enables us to extract kinematic information (instantaneous equilibrium posture) from the electromyography (EMG) signals. We show two patterns of muscle coordination extracted from the A-A ratio dataset during human pedaling, clarifying the motor functions of these patterns. Then, by transferring the muscle coordination of human pedaling to a human-like musculoskeletal leg robot with redundant DOFs of pneumatic artificial muscles, we enable the robot to achieve pedaling movement.

Kinematic Origins of Motor Inconsistency in Expert Pianists.

For top performers, including athletes and musicians, even subtle inconsistencies in rhythm and force during movement production decrease the quality of performance. However, extensive training over many years beginning in childhood is unable to perfect dexterous motor performance so that it is without any error. To gain insight into the biological mechanisms underlying the subtle defects of motor actions, the present study sought to identify the kinematic origins of inconsistency of dexterous finger movements in musical performance. Seven highly-skilled pianists who have won prizes at international piano competitions played a short sequence of tones with the right hand at a predetermined tempo. Time-varying joint angles of the fingers were recorded using a custom-made data glove, and the timing and velocity of the individual keystrokes were recorded from a digital piano. Both ridge and stepwise multiple regression analyses demonstrated an association of the inter-trial variability of the inter-keystroke interval (i.e., rhythmic inconsistency) with both the rotational velocity of joints of the finger used for a keystroke (i.e., striking finger) and the movement independence between the striking and non-striking fingers. This indicates a relationship between rhythmic inconsistency in musical performance and the dynamic features of movements in not only the striking finger but also the non-striking fingers. In contrast, the inter-trial variability of the key-descending velocity (i.e., loudness inconsistency) was associated mostly with the kinematic features of the striking finger at the moment of the keystroke. Furthermore, there was no correlation between the rhythmic and loudness inconsistencies. The results suggest distinct kinematic origins of inconsistencies in rhythm and loudness in expert musical performance.

Muscle synergy analysis of human adaptation to a variable-stiffness exoskeleton: Human walk with a knee exoskeleton with pneumatic artificial muscles

This paper presents a variable-stiffness knee exoskeleton to enhance human walking. The developed exoskeleton is an agonist-antagonist system with pneumatic artificial muscles (PAMs), and its equilibrium-joint angle and joint stiffness are separately controlled using on our concepts of the agonist-antagonist muscle-pair ratio (A-A ratio) and agonist-antagonist muscle-pair activity (A-A activity). We focus on human adaptation to the variable-stiffness assistance of the exoskeleton, and explore the stiffness control strategy of the device to reduce the subjects excess muscle activity. Muscle synergy analysis of A-A activity indicates that our stiffness-control approach based on EMG analysis leads to successful performance of the human musculoskeletal system with exoskeleton assistance.

Quantification of a secondary task-specific tremor in a violinist after a temporal lobectomy.

Task-specific tremors (TSTs) occur mainly during certain tasks and may be highly disabling. In this case study, we report on a 66-year-old violinist who developed a TST of the right arm only while playing the violin 4 weeks after a temporal lobectomy, which had been performed as a result of his temporal lobe epilepsy. Since a similar case, to our knowledge, has not been reported so far, our aim was to quantitatively assess and describe the tremor by measuring (a) the electromyography (EMG) activity of the wrist flexor and extensor as well as (b) an accelerometer signal of the hand. We found a tremor-related frequency of about 7 Hz. Furthermore, at a similar frequency of about 7 Hz, there was coherence between the tremor acceleration and EMG-activity of the wrist flexor and extensor as well as between the tremor acceleration and coactivation. The tremorgenesis remains unclear, and possible explanations can only be speculative.