Theta burst stimulation of human primary motor cortex degrades selective muscle activation in 1 the ipsilateral arm 2 3

27 This study investigated whether repetitive transcranial magnetic stimulation (TMS) delivered as 28 continuous theta burst stimulation (cTBS) to left M1 degraded selective muscle activation in the 29 contralateral and ipsilateral upper limb in healthy participants. Contralateral motor evoked 30 potentials (cMEPs) were elicited in left and right biceps brachii (BB) before either elbow flexion 31 or forearm pronation. A neurophysiological index, the excitability ratio (ER), was computed 32 from the relative size of BB cMEPs before each type of movement. Short interval intracortical 33 inhibition (SICI) was assessed in cMEPs of right BB with paired-pulse TMS of left M1. 34 Ipsilateral MEPs (iMEPs) and silent periods (iSPs) were measured in left BB with single-pulse 35 TMS of left M1. Low intensity cTBS was expected to suppress corticospinal output from left 36 M1. A sham condition was also included. Real but not sham cTBS caused increases in BB ER 37 bilaterally. In the right arm ER increased because BB cMEPs before flexion were less facilitated, 38 while cMEPs in the pronation task were unaffected. This was accompanied by an increase in left 39 M1 SICI. In the left arm ER increased because BB cMEPs before pronation were facilitated, but 40 were unaffected in the flexion task. There was also facilitation of left BB iMEPs. These changes 41 in the left arm are consistent with inappropriate facilitation of left BB alpha-motoneurons 42 (αMNs) prior to pronation. This is the first demonstration that cTBS of M1 can alter excitability 43 of neurons controlling ipsilateral proximal musculature, and degrade ipsilateral upper limb motor 44 control, providing evidence that ipsilateral and contralateral M1 shape the spatial and temporal 45 characteristics of proximal muscle activation appropriate for the task at hand. 46 47