Neuroscience Research 92 (2015) 39–45 Contents lists available at ScienceDirect Neuroscience Research jo ur nal homepage: www.elsevier.com/locate/neures Motor imagery of voluntary muscle relaxation induces temporal reduction of corticospinal excitability Kouki Kato a,d,∗ , Jun Watanabe b , Tetsuro Muraoka c , Kazuyuki Kanosue a a Faculty of Sport Sciences, Waseda University, Tokorozawa, Japan b Graduate of Sport Sciences, Waseda University, Tokorozawa, Japan c College of Economics, Nihon University, Tokyo, Japan d Japan Society for the Promotion of Science, Tokyo, Japan a r t i c l e i n f o Article history: Received 5 August 2014 Received in revised form 7 October 2014 Accepted 23 October 2014 Available online 1 November 2014 Keywords: Motor imagery Relaxation TMS Inhibition MEP a b s t r a c t Voluntary muscle relaxation is an “active process” requiring cortical activation. However, cortical activa- tion during motor imagery of muscle relaxation has not been well understood. The purpose of this study was to clarify time-dependent changes in corticospinal excitability during the imagery of muscle relax- ation. Ten participants imagined volitional muscle relaxation from an imagined pinching with their right index finger and thumb in response to an auditory cue. Transcranial magnetic stimulation was applied at the left primary motor area of the first dorsal interosseous (FDI) muscle at different time intervals after the auditory cue. Motor evoked potentials (MEPs) were recorded from the right hand and forearm muscles. The MEP amplitudes of the FDI and the synergist temporally decreased after the auditory cue as compared with those present in the resting condition. Our finding indicates that motor imagery of muscle relaxation induces a temporal reduction of the corticospinal excitability related to the targeted muscle. © 2014 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved. 1. Introduction Motor imagery is the mental representation of action with- out any overt movement and has been shown to have beneficial effects for improving sports skill and promoting neurological reha- bilitation (Isaac, 1992; Driskell et al., 1994; Holmes and Calmels, 2008). For example, motor imagery of the fifth digit’s abduction over a 4 week period induced an enhancement of muscle strength similar to that of actual physical training (Yue and Cole, 1992). During motor imagery, brain activity increases in regions simi- lar to those that are activated during normal motor execution. Such increases include the primary motor cortex (M1), the sup- plemental motor area (SMA), the premotor cortex and the parietal cortex (Ehrsson et al., 2003; Hanakawa et al., 2003; Mizuguchi et al., 2013a,b). Furthermore, a series of studies utilizing transcra- nial magnetic stimulation (TMS) provide a consensus that imagery of muscle contraction increases corticospinal excitability above res- ting levels for the target muscle (Kasai et al., 1997; Li et al., 2004; ∗ Corresponding author at: Graduate School of Sport Sciences, Waseda University, 2-579-15 Mikajima, Tokorozawa, Saitama 359-1192, Japan. Tel.: +81 4 2947 6826; fax: +81 4 2947 6826. E-mail address: kouki-nagoya@fuji.waseda.jp (K. Kato). Stinear and Byblow, 2004; Fourkas et al., 2006; Mizuguchi et al., 2009). Time-dependent changes in corticospinal excitability dur- ing motor imagery of phasic movement have also been reported. This suggests that motor imagery could have a dynamic effect on corticospinal excitability similar to that seen during an actual contraction (Hashimoto and Rothwell, 1999). During the initiation of simple contraction imagery, corticospinal excitability gradually increases before contraction onset in a manner similar to that of an actual contraction (Kumru et al., 2008). In daily life, although human movements consist of both mus- cle contraction and relaxation, much of the literature on motor imagery has been focused on muscle contraction. In athletes, musi- cians, neurological patients, and many everyday activities, proper muscle relaxation is absolutely necessary for the production of efficient and polished movements and actions (Lelli et al., 1991; Inzelberg et al., 1995; Sakurai and Ohtsuki, 2000; Buccolieri et al., 2004; Yoshie et al., 2009; Fujii et al., 2009; Seo et al., 2009). Thus, imagery of muscle relaxation, just as occurs with the imagery of muscle contraction, would be expected to aid in the acqui- sition of fine motor control of body parts. To clarify the neural substrates involved in volitional muscle relaxation, neuroimag- ing and neurophysiological studies utilizing functional magnetic resonance imaging, magnetoencephalography, electroencephalog- raphy and electromyography (EMG) have been performed. These http://dx.doi.org/10.1016/j.neures.2014.10.013 0168-0102/© 2014 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved.