NTT has, for the first time in the world, shown that one of the main causes of variability in human movement is fluctuation in the timing of muscle activity commands sent from the brain to the muscles. The findings were published on March 2 in the academic journal Neural Networks.
The results reveal that variability in human movement is more closely associated with fluctuations in the timing of muscle activation than with fluctuations in muscle activation size, which has been the conventional explanation.
Provided by NTT
When a person moves their body, commands are sent from the brain to the muscles, which then engage in activity to produce movement. However, a certain amount of variability always occurs in movement, making it impossible to move the body exactly as intended. Understanding the mechanism behind this variability is a key topic in research on brain motor control, and it was previously thought that the primary cause was fluctuation in the amplitude of muscle activity.
In contrast, this NTT study focused on the "timing of commands sent from the brain to the muscles," a factor that had been overlooked in previous research. The researchers set up experimental tasks using several types of arm movements that each require a different kind of muscle coordination—reaching movements, periodic movements, and circular movements. They analyzed in detail the relationship between movement variability and muscle activity based on simultaneously measured data.
To explore variability in reaching movement position, 15 participants were asked to perform a reaching movement 50 times, gripping a robotic handle used for motion measurement and bending their elbow to move their hand from a start position to a target position.
Although the same movement was repeated, clear "variability (variability in end position)" was observed trial by trial. The activity of the agonist and antagonist muscles involved in elbow movement was measured using electrodes attached to the surface of the skin. It was observed that the agonist, which accelerates the hand, and the antagonist, which decelerates it, activated alternately.
In addition to trial-by-trial fluctuation in the amplitude of these muscle activities, fluctuation also occurred in the timing of activity (i.e., the time taken to reach peak activity). Analysis of the relationship between reaching movement variability and muscle activity revealed that variability in hand position was not associated with fluctuation in muscle activity amplitude but was significantly associated with fluctuation in muscle activity timing.
Next, the researchers investigated whether the same trend could be seen in periodic movements such as drumming. Previous research has shown that periodic movements are executed by a different neural control mechanism than that of reaching movements. Both arms were measured in order to examine whether differences in motor performance between the dominant and non-dominant hands were related to fluctuation in muscle activity timing.
Fifteen participants (all right-handed) were asked to grip a fixed robotic handle and use either their left or right arm to tap with a constant force in time with a rhythm presented by a metronome.
The results once again showed that in periodic movements, variability in force was not associated with fluctuation in muscle activity amplitude but with fluctuation in muscle activity timing. It was also suggested that fluctuation in muscle activity timing may be one factor contributing to the greater variability in force output seen in the non-dominant hand.
Furthermore, the same experiment was conducted for circular movements, which require particularly extensive coordination among muscles even among periodic movements, and the same results were obtained as for reaching and periodic movements.
Going forward, NTT plans to combine behavioral experiments with brain activity measurement and theoretical research to identify the brain regions and neural representations that control muscle activity timing, and to advance neuroscientific understanding of a wide range of phenomena related to movement variability including motor skill learning, ataxia caused by disease, hand dominance, motor development, and age-related changes. The company also aims to apply the findings obtained to a wide range of fields such as sports and medicine.
Journal Information
Publication: Neural Networks
Title: Minimizing command timing variability is a key factor in skilled actions
DOI: 10.1016/j.neunet.2026.108759
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.