Intermittent theta burst stimulation (iTBS) is a form of repetitive transcranial magnetic stimulation (TMS) that can increase corticomotor excitability in distal upper limb muscles, but the effect on the more proximal biceps is unknown. The study objective was to determine the effect of iTBS on corticomotor excitability of the biceps brachii in non-impaired individuals. Ten individuals completed three sessions, and an additional ten individuals completed one session in a secondary study; each session included sham and active iTBS. Resting and active motor thresholds (RMT, AMT) were determined prior to sham and active iTBS. Motor evoked potentials (MEPs) in response to single pulse TMS served as our measure of corticomotor excitability. In our primary cohort, MEPs were recorded with biphasic stimulation to accurately capture the same neurons affected by biphasic iTBS. MEPs were recorded at an intensity of 120% of RMT, or for instances of high RMTs, 100% of the maximum stimulator output (MSO), at baseline, and 10, 20, and 30 minutes after iTBS. MEPs were normalized by the maximum voluntary isometric muscle activity. In the secondary, MEPs were recorded with monophasic stimulation, which increased our ability to record MEPs at 120% of RMT. Linear mixed effects models were used to determine the effect of iTBS on normalized MEPs (nMEPs), with analyses to evaluate the interaction of the biceps AMT:RMT ratio as a measure of corticomotor conductance. Change in nMEPs from baseline did not differ for the active and sham conditions (p = 0.915 ) when MEPs were assessed with biphasic stimulation. With MEPs assessed by monophasic stimulation, there was an increase in biceps nMEPs after active iTBS, and no change in nMEPs after sham. Our results suggest that when RMTs are expected to be high when measured with biphasic stimulation, monophasic stimulation can better capture changes in MEPs induced by iTBS, and biphasic stimulation appears limited in its ability to capture changes in biceps MEPs in nonimpaired individuals. In both cohorts, increased corticomotor excitability after iTBS occurred when the biceps AMT:RMT ratio was high. Thus, the AMT:RMT ratio may be a predictive measure to evaluate the potential for iTBS to increase biceps corticomotor excitability.

中文翻译：

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间歇性θ爆发刺激对未受损个体肱二头肌皮质运动兴奋性的影响。


间歇性θ爆发刺激（iTBS）是重复经颅磁刺激（TMS）的一种形式，可以增加远端上肢肌肉的皮质运动兴奋性，但对更近端二头肌的影响尚不清楚。研究目的是确定 iTBS 对未受损个体肱二头肌皮质运动兴奋性的影响。十个人完成了三个疗程，另外十个人在二次研究中完成了一个疗程；每个会话包括假 iTBS 和主动 iTBS。在假手术和活动 iTBS 之前确定静息和活动运动阈值（RMT、AMT）。对单脉冲 TMS 的运动诱发电位（MEP）作为我们测量皮质运动兴奋性的指标。在我们的主要队列中，通过双相刺激记录 MEP，以准确捕获受双相 iTBS 影响的相同神经元。 MEP 的记录强度为 RMT 的 120%，或者对于高 RMT 的情况，记录最大刺激器输出 (MSO) 的 100%、基线以及 iTBS 后 10、20 和 30 分钟。 MEP 通过最大自愿等长肌肉活动进行标准化。在二级中，MEP 是通过单相刺激记录的，这提高了我们以 RMT 的 120% 记录 MEP 的能力。使用线性混合效应模型来确定 iTBS 对标准化 MEP (nMEP) 的影响，并通过分析评估二头肌 AMT:RMT 比率的相互作用，作为皮质运动传导性的衡量标准。当使用双相刺激评估 MEP 时，在活动条件和假条件下，nMEP 相对于基线的变化没有差异 (p = 0.915)。通过单相刺激评估 MEP，活动 iTBS 后二头肌 nMEP 增加，而假手术后 nMEP 没有变化。 我们的结果表明，当使用双相刺激测量时预计 RMT 较高时，单相刺激可以更好地捕获 iTBS 诱导的 MEP 变化，而双相刺激捕获未受损个体二头肌 MEP 变化的能力似乎有限。在这两个队列中，当二头肌 AMT:RMT 比率较高时，iTBS 后皮质运动兴奋性就会增加。因此，AMT:RMT 比率可能是评估 iTBS 增加二头肌皮质运动兴奋性潜力的预测指标。