Abstract
When a person attempts intended finger movements, unintended finger movement also occur, a phenomenon called “enslaving”. Given that motor imagery (MI) and motor execution (ME) share a common neural foundation, we hypothesized that the enslaving effect on the spinal motor neuron excitability occurs during MI. To investigate this hypothesis, electromyography (EMG) and F-wave analysis were conducted in 11 healthy male volunteers. Initially, the EMG activity of the left abductor digiti minimi (ADM) muscle during isometric opposition pinch movement by the left thumb and index finger at 50% maximal effort was compared with EMG activity during the Rest condition. Next, the F-wave and background EMG recordings were performed under the Rest condition, followed by the MI condition. Specifically, in the Rest condition, subjects maintained relaxation. In the MI condition, they imagined isometric left thenar muscle activity at 50% maximal voluntary contraction (MVC). During ME, ADM muscle activity was confirmed. During the MI condition, both F-wave persistence and the F-wave/M-wave amplitude ratio obtained from the ADM muscle were significantly increased compared with that obtained during the Rest condition. No difference was observed in the background EMG between the Rest and MI conditions. These results suggest that MI of isometric intended finger muscle activity at 50% MVC facilitates spinal motor neuron excitability corresponding to unintended finger muscle. Furthermore, MI may induce similar modulation of spinal motor neuron excitability as actual movement.
Highlights
Motor imagery (MI) is defined as a dynamic mental state that reproduces a specific motor action within working memory without any overt movement (Decety and Grèzes, 1999; Guillot et al, 2012)
The root mean square (RMS) value of the left abductor digiti minimi (ADM) muscle during the motor execution (ME) condition was higher than that during the Rest condition [Wilcoxon signed-rank test, z = −2.93, p < 0.01, r = 0.625, (1−β) ≈ 100%, Figure 3]
F-wave amplitude during the MI condition was higher than that during the Rest condition [Wilcoxon signed-rank test, z = −2.93, p < 0.01, r = 0.625, (1−β) ≈ 100%, Table 2]
Summary
Motor imagery (MI) is defined as a dynamic mental state that reproduces a specific motor action within working memory without any overt movement (Decety and Grèzes, 1999; Guillot et al, 2012). Taniguchi et al (2008) and Hara et al (2010) investigated the effect of MI on the spinal motor neuron excitability using F-waves, whereas Kasai et al (1997) and Hashimoto and Rothwell (1999) used the Hoffmann reflex (H-reflex) to assess the spinal motor neuron excitability. We investigated whether MI and action observation of cyclic thumb opposition movements at a frequency of 1 Hz affect spinal motor neuron excitability using F-waves (Bunno and Suzuki, 2020). As a result, combined action observation and MI failed to facilitate spinal motor neuron excitability significantly over that of action observation alone This previous result suggested that MI of cyclic thumb opposition movements may have difficulty to increase spinal motor neuron excitability. The conflicting findings among these previous studies may be explained by methodological differences and further studies are required
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