BACKGROUND MUSCLE STATE CHANGES CORTICOSPINAL EXCITABILITY DURING MOTOR IMAGING OF MUSCLE RELAXATION AND CONTRACTION

Y. Takenaka¹, H. Matsumoto¹, T. Suzuki², K. Sugawara²

¹Kanagawa University of Human Services, Ph.D. Course, Yokosuka, Japan, ²Kanagawa University of Human Services, Physical Therapy, Yokosuka, Japan

Background: During daily living activities, smooth motor performance under voluntary control is achieved through the facilitatory and inhibitory neural mechanisms that drive the related muscles. A full description of facilitatory and inhibitory motor control mechanisms in the central nervous system is required to provide suggestions for constructing a training methodology with effective learning exercises and for improving smooth muscle contraction and relaxation performance in rehabilitation and sports. Motor imagery (MI) is advantageous in motor control experiments because the actions to be performed can be composed of internal simulation in working memory without any actual motor output or muscle contraction. In addition to actual motor practice, MI has been used as a complementary motor practice and is useful for improving performance in rehabilitation and sports training. We determined changes in corticospinal excitability for muscle contraction and relaxation during MI, and our main focus was on the contrast between muscle relaxation and contraction.

Purpose: The goal of this study was to understand the motor control mechanisms necessary for muscle relaxation in MI.

Methods: Thirty healthy individuals participated in this study (age range 20–27 years, 10 male). There were 24 participants in Experiment 1 (age range 20–25, 9 male), 16 in Experiment 2 (age range 20–27 years, 6 male), and 10 in both experiments. We investigated the specificity of excitability changes in the cortical motor areas involved in muscle contraction and relaxation using MI. We induced MI using a reaction time task paradigm and used two MI conditions for both contraction and relaxation of the right first dorsal interosseous (FDI) muscle. The motor-evoked potentials induced by transcranial magnetic stimulation were recorded after the go signal in the FDI. In addition, two background factors were used in each MI: static weak muscle contraction (Experiment 1) and resting state (Experiment 2).

Results: The results showed that when muscle contraction was maintained in the background, there was no increase in excitability in the contraction MI; however, a decrease in the relaxation MI excitability compared to the control was observed. When the muscles were at rest during MI, there was an increase in excitability in the contraction MI and a transient increase in excitability in the relaxation MI compared to the control condition.

Conclusions: These results suggest that the excitability of contraction MI is characterized by a continuous increase in excitability, whereas the excitability of relaxation MI is characterized by a transient increase followed by a decrease. Our findings also suggested that sensory information about muscle contraction in the background condition may be necessary for muscle relaxation.

Implications: To improve smooth motor performance, we must focus on not only muscle contraction but also muscle relaxation. While MI might be a useful compensatory way to improve motor performance, physiotherapists should consider the background factors of MI, such as the muscle contraction state.

Funding acknowledgements: This work was supported by JSPS KAKENHI Grant Number 20K11287.

Keywords:
Corticospinal excitability
Motor imagery
Muscle relaxation

Topics:
Neurology

Did this work require ethics approval? Yes

Institution: Kanagawa University of Human Services

Committee: Office of Research Ethics at Kanagawa University of Human Services

Ethics number: No. 7-20-27