Muscle Relaxation of the Foot Reduces Corticospinal Excitability of Hand Muscles and Enhances Intracortical Inhibition

Abstract

The object of this study was to clarify the effects of foot muscle relaxation on activity in the primary motor cortex (M1) of the hand area. Subjects were asked to volitionally relax the right foot from sustained contraction of either the dorsiflexor (tibialis anterior; TA relaxation) or plantarflexor (soleus; SOL relaxation) in response to an auditory stimulus. Single- and paired-pulse transcranial magnetic stimulation (TMS) was delivered to the hand area of the left M1 at different time intervals before and after the onset of TA or SOL relaxation. Motor evoked potentials (MEPs) were recorded from the right extensor carpi radialis (ECR) and flexor carpi radialis (FCR). MEP amplitudes of ECR and FCR caused by single-pulse TMS temporarily decreased after TA and SOL relaxation onset, respectively, as compared with those of the resting control. Furthermore, short-interval intracortical inhibition (SICI) of ECR evaluated with paired-pulse TMS temporarily increased after TA relaxation onset. Our findings indicate that muscle relaxation of the dorsiflexor reduced corticospinal excitability of the ipsilateral hand muscles. This is most likely caused by an increase in intracortical inhibition.

The difference in hemodynamic responses between dominant and non-dominant hands during muscle contraction and relaxation: An fNIRS study

“…Our previous studies using behavioral data showed that force control is more difficult in muscle relaxation than in muscle generation [10]. Some previous studies using fMRI, TMS, and electroencephalography (EEG) reported that neural activation in the M1, supplementary motor area (SMA), PM, dorsolateral prefrontal cortex (DLPFC), and anterior cingulate cortex (ACC) was associated with muscle relaxation [11–14]. However, to the best of our knowledge, no previous studies focused on the differences in neural activation of ipsi- or contralateral hemispheres between dominant and non-dominant hands during muscle relaxation.…”

Brain Activity Underlying Muscle Relaxation

“…We also used paired-pulse TMS to investigate SICI for the forearm muscles during ipsilateral ankle relaxation. The results revealed that SICI in the M1 forearm region increased during relaxation as compared to that of the resting condition in the ankle (Figure 3; Kato et al, 2016b). This increase in SICI was observed even when the amplitude of the test MEP during relaxation was adjusted to the same amplitude level as that in the resting condition.…”

“…As a result, corticospinal excitability of the pronated forearm extensor was temporally suppressed during relaxation of the ankle dorsiflexor. Likewise, excitability of the forearm flexor was suppressed during the planterflexor’s relaxation (Figure 3, Kato and Kanosue, 2016a; Kato et al, 2016b). Therefore, we suggest that muscle relaxation of the foot dorsiflexor produces a state in the hand muscles such that a required contraction is difficult.…”

Interlimb coordination from a psychological perspective

“…Neural crosstalk is considered a possible neuromuscular constraint for movements 41,42) . It is well known that the activity (contraction and relaxation of muscles) of one limb affects not only the corticospinal (or motoneuron) excitability of the muscles employed in the activity, but also the excitability of the resting muscles of other limbs [99][100][101][102][103][104][105][106][107][108][109] . This kind of influence from the activity of one limb to the resting muscles in other limbs is most pronounced between the homologous muscles 110) , but often spread to the non-homologous muscles 111,112) .…”