Sugawara K1, Tanabe S2, Suzuki T1
1Kanagawa University of Human Services, Division of Physical Therapy, Faculty of Health and Social Work, Yokosuka, Japan, 2Fujita Health University, Faculty of Rehabilitation, School of Health Sciences, Toyoake, Japan
Background: Not only contraction, but also a voluntary muscle relaxation is responsible for smooth motor control in daily activities; therefore, it is important to understand the neural mechanisms of muscle relaxation.
Purpose: This study investigated how central processes are engaged in an immediate onset of muscle relaxation following a voluntary muscle contraction and how the relaxation can be modified. A peripheral afferent input by electrical stimulation (ES) is suggested to lead to cortical reorganization and induce changes in cortical excitability, which may result in compensatory and novel motor functions. Thus, motor cortex excitability and how it can be modified by ES was investigated upon the onset of voluntary muscle relaxation immediately following a period of voluntary tonic muscle contraction.
Methods: Twenty-three healthy participants were divided into 3 groups (control, N=7, tibialis anterior (TA), N=8 and gastrocnemius (gastro) group, N=8). The task was to execute constant ankle dorsiflexion at 20% of maximal voluntary contraction (MVC) and to release upon hearing an auditory “GO” signal. ES was applied to TA or gastro for a random time between 3-5 seconds and terminated simultaneously to the “GO” signal.TMSwas applied during the task at random time intervals after the “GO” signal (range 20-150 ms) and the MEPs were recorded from the TA and gastro. The timing of the signals and stimulations were randomized for the subjects using Lab VIEW, ver.7.1. (National Instruments Corp.; USA). Relaxation reaction times (RRTs) were calculated as the time taken from the “GO” signal to the point when 50% decline of the 20% MVC force output was reached, measured by force transducer signals. The timing of TMS was presented as the percentage of the RRT (time from the “GO” signal to the TMS relative to the RRT) and the MEP control ratio was compared for the three groups in bins of timing of TMS. The RRT and the relationship between the timing of TMS and the MEP control ratio were compared across three groups.
Results: Motor cortex excitability of TA was significantly greater in the TA group than in the stimulated antagonist muscle (gastro) and control group, while the relaxation time of TA was shorter in the gastro group than in control and the stimulated agonist muscle (TA) group. Motor cortex excitability of gastro did not show statistical differences between the groups. Applying TMS at 30% of RRT in the gastro group yielded significantly stronger effects in TA than any other time section.
Conclusion(s): The ES lead to a reduction of reaction time in the antagonist muscle and facilitated motor cortex excitability of TA.
Implications: The results suggest that terminating a muscle contraction triggers a transient neurophysiological mechanism of increased cortical motor excitability in the period prior to muscle relaxation, facilitated by the ES. The effect was specific for the location of the ES application (agonist or antagonist). The results might contribute to designing new methods of facilitating recovery of motor function in rehabilitation for neurological disorders (e.g. stroke) or reducing muscle stiffness in musculoskeletal disorders.
Keywords: Muscle relaxation, motor cortex excitability, electrical stimulation
Funding acknowledgements: This work was supported by JSPS KAKENHI Grant Number 17K01520.
Purpose: This study investigated how central processes are engaged in an immediate onset of muscle relaxation following a voluntary muscle contraction and how the relaxation can be modified. A peripheral afferent input by electrical stimulation (ES) is suggested to lead to cortical reorganization and induce changes in cortical excitability, which may result in compensatory and novel motor functions. Thus, motor cortex excitability and how it can be modified by ES was investigated upon the onset of voluntary muscle relaxation immediately following a period of voluntary tonic muscle contraction.
Methods: Twenty-three healthy participants were divided into 3 groups (control, N=7, tibialis anterior (TA), N=8 and gastrocnemius (gastro) group, N=8). The task was to execute constant ankle dorsiflexion at 20% of maximal voluntary contraction (MVC) and to release upon hearing an auditory “GO” signal. ES was applied to TA or gastro for a random time between 3-5 seconds and terminated simultaneously to the “GO” signal.TMSwas applied during the task at random time intervals after the “GO” signal (range 20-150 ms) and the MEPs were recorded from the TA and gastro. The timing of the signals and stimulations were randomized for the subjects using Lab VIEW, ver.7.1. (National Instruments Corp.; USA). Relaxation reaction times (RRTs) were calculated as the time taken from the “GO” signal to the point when 50% decline of the 20% MVC force output was reached, measured by force transducer signals. The timing of TMS was presented as the percentage of the RRT (time from the “GO” signal to the TMS relative to the RRT) and the MEP control ratio was compared for the three groups in bins of timing of TMS. The RRT and the relationship between the timing of TMS and the MEP control ratio were compared across three groups.
Results: Motor cortex excitability of TA was significantly greater in the TA group than in the stimulated antagonist muscle (gastro) and control group, while the relaxation time of TA was shorter in the gastro group than in control and the stimulated agonist muscle (TA) group. Motor cortex excitability of gastro did not show statistical differences between the groups. Applying TMS at 30% of RRT in the gastro group yielded significantly stronger effects in TA than any other time section.
Conclusion(s): The ES lead to a reduction of reaction time in the antagonist muscle and facilitated motor cortex excitability of TA.
Implications: The results suggest that terminating a muscle contraction triggers a transient neurophysiological mechanism of increased cortical motor excitability in the period prior to muscle relaxation, facilitated by the ES. The effect was specific for the location of the ES application (agonist or antagonist). The results might contribute to designing new methods of facilitating recovery of motor function in rehabilitation for neurological disorders (e.g. stroke) or reducing muscle stiffness in musculoskeletal disorders.
Keywords: Muscle relaxation, motor cortex excitability, electrical stimulation
Funding acknowledgements: This work was supported by JSPS KAKENHI Grant Number 17K01520.
Topic: Human movement analysis; Musculoskeletal; Neurology
Ethics approval required: Yes
Institution: Kanagawa University of Human Services
Ethics committee: Ethics Committee of Kanagawa University of Human Services
Ethics number: No.7-8
All authors, affiliations and abstracts have been published as submitted.
