[Purpose] This study aimed to determine the differences in the excitability of spinal motor neurons during motor imagery of a muscle contraction at different contraction strengths. [Methods] We recorded the F-wave in 15 healthy subjects. First, in a trial at rest, the muscle was relaxed during F-wave recording. Next, during motor imagery, subjects were instructed to imagine maximum voluntary contractions of 10%, 30%, and 50% while holding the sensor of a pinch meter, and F-waves were recorded for each contraction. F-waves were recorded immediately and at 5, 10, and 15 min after motor imagery. [Results] Both persistence and F/M amplitude ratios during motor imagery under maximum voluntary contractions of 10%, 30%, and 50% were significantly higher than that at rest. In addition, persistence, F/M amplitude ratio, and latency were similar during motor imagery under the three muscle contraction strengths. [Conclusion] Motor imagery under maximum voluntary contractions of 10%, 30%, and 50% can increase the excitability of spinal motor neurons. The results indicated that differences in muscle contraction strengths during motor imagery are not involved in changes in the excitability of spinal motor neurons.

BACKGROUND: It is unclear whether mental simulation without actual muscle contraction associated with actual motion can increase the excitability of the spinal neural function. OBJECTIVE: To determine the best method for mental simulation without actual muscle contraction, we analyzed the F-wave of thenar muscles after stimulating the median nerve by motor imagery whilst holding the sensor of a pinch meter between the thumb and index finger and without holding the sensor. METHODS: Healthy volunteers (n = 11; mean age, 34 years) participated in this study after providing informed consent. We examined the F-wave of the left thenar muscles after stimulating the left median nerve at the wrist at rest and under holding and motor imagery conditions. For the motor imagery condition, the subjects were asked to establish 50% maximal voluntary contraction (MVC) of isometric contraction while holding the sensor between the thumb and index finger (motor imagery with the sensor condition) and without holding the sensor on another day (motor imagery without the sensor condition). RESULTS: The persistence and amplitude ratio of F/M during motor imagery with or without the sensor was better than that during relaxation. In particular, this ratio was significantly higher under the with sensor condition than under the without sensor condition. CONCLUSION: Movement preparation for a motor imagery task involving 50% MVC isometric contraction of the opponens pollicis is important.

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[Purpose] We often perform physical therapy using motor imagery of muscle contraction to improve motor function for healthy subjects and central nerve disorders. This study aimed to determine the differences in the excitability of spinal motor neurons during motor imagery of a muscle contraction at different contraction strengths. [Subjects] We recorded the F-wave in 15 healthy subjects. [Methods] In resting trial, the muscle was relaxed during F-wave recording. For motor imagery trial, subjects were instructed to imagine maximal voluntary contractions of 50% and 100% while holding the sensor of a pinch meter, and F-waves were recorded for each contraction. The F-wave was recorded immediately after motor imagery. [Results] Persistence and F/M amplitude ratio during motor imagery under maximal voluntary contractions of 50% and 100% were significantly higher than that at rest. In addition, the relative values of persistence, F/M amplitude ratio, and latency were similar during motor imagery under the two muscle contraction strengths. [Conclusion] Motor imagery under maximal voluntary contractions of 50% and 100% can increase the excitability of spinal motor neurons. Differences in the imagined muscle contraction strengths are not involved in changes in the excitability of spinal motor neurons.

To investigate the effect of physical therapy on F-wave characteristics in a patient with cerebrovascular disease, we tested the relationship between F-wave waveforms and the recovery of voluntary movement on the affected side. We measured F-waves of affected thenar muscles after stimulating the affected median nerve at the wrist in a left hemiplegia patient (female, 60 years old) with cerebrovascular disease. We analysed the F/M amplitude ratio and Fwave waveform values from 30 trials. F-wave waveform values were determined as follows. We calculated the correlation coefficient of each waveform and defined identifying F-waves as those with a correlation coefficient greater than 0.9. We determined the number of different wave forms from the number of identifying waveforms. F-waves were tested three times at 9, 52, and 70 months from sideration. The patient underwent physical therapy twice a week, with each session lasting 20 min. The F/M amplitude ratio gradually decreased and the number of F-wave waveform types gradually increased. Muscle tonus and voluntary movements of the affected arm gradually improved with physical therapy. In this patient with cerebrovascular disease, F-wave waveform values in her affected arm gradually increased as muscle tonus and voluntary movements of the arm improved.