Bilateral responses of trapezius muscle to transcranial magnetic stimulation in normal subjects

Introduction: Motor evoked potentials (MEPs) can be elicited in trapezius and other axial musclesby ipsilateral transcranial magnetic stimulation (TMS).The purpose of this study is to compare therelative amplitudes and latencies of ipsilateral versus contralateral responses to TMS from trapeziusmuscle of normal subjects.Materials and Methods: MEPs were bilaterally recorded during complete relaxation and voluntarycontraction from upper trapezius muscle in 40 healthy right handed subjects by TMS in right and lefthemispheres. Base-to-peak amplitude and latency of MEP were measured for each muscle on bothsides.Results: At rest, a contralateral response was obtained in 45 trapezius muscles. Ipsilateralresponses were observed in 11 muscles (latency contra-lateral and ipsilateral 9.8ms and 15.5 ms,respectively, P <0.001). During contraction ipsilateral MEPs could be evoked in 37 muscles.Contralateral MEPs were obtained in 76 experiments on 40 subjects (latency contralateral andipsilateral 8.5ms and 12.6 ms, respectively, P <0.001).Conclusion: MEPs can be readily evoked by TMS over the ipsilateral motor cortex in a variety ofproximal muscles, such as trapezius. Ipsilateral MEPs have a later onset and lower amplitude thancontralateral responses. It is considered that oligosynaptic connections may be a common feature ofaxial muscles, such as trapezius. These patterns of connections are discussed in relation to thecontrasting bilateral functional roles of this muscle.