Influence of Motor Imagery of Isometric Opponens Pollicis Activity on the Excitability of Spinal Motor Neurons: A Comparison Using Different Muscle Contraction Strengths

Abstract

[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.

The Effect of Motor Imagery on Spinal Motor Neuron Excitability and Its Clinical Use in Physical Therapy

“…In previous work, MI under 10, 30, 50, and 70% MVC was shown to increase spinal motor neuron excitability [13,14]. The imagined muscle contraction strengths did not influence the facilitation amount of spinal motor neuron excitability [13,14].…”

“…We investigated spinal motor neuron excitability during MI of isometric thenar muscle activity under 10, 30, 50, 70, and 100% MVC [13,14,19]. As a result, MI of isometric thenar muscle activity can facilitate spinal motor neuron excitability.…”

“…In previous work, MI under 10, 30, 50, and 70% MVC was shown to increase spinal motor neuron excitability [13,14]. The imagined muscle contraction strengths did not influence the facilitation amount of spinal motor neuron excitability [13,14]. However, Cowley et al [20] suggested that the H-reflex amplitude during MI of plantar flexion under 100% MVC was significantly higher than that under 50% MVC.…”

“…From the results of our previous work [13,14,19], MI of isometric thenar muscle activity under 10, 30, 50, 70, and 100% MVC can facilitate spinal motor neuron excitability. Furthermore, the imagined muscle contraction strength is not involved in changes of spinal motor neuron excitability.…”

“…Alternatively, the influence of duration of MI on the changes of spinal motor neuron excitability is not apparent. In our previous work [10,13,14,19], participants were asked to perform MI for 1min. Thus, this research aimed to investigate the influence of MI for 5min on spinal motor neuron excitability by analyzing F-waves.…”