Abstract:
To investigate ipsilateral neural connectivity from the lower limb motor cortex to spinal motoneurons, transcranial magnetic stimulation (TMS) was employed to produce a medial to lateral (M – L) current within the lower limb motor cortex in 22 neurologically healthy subjects. Surface electromyography (EMG) was recorded from the tibialis anterior (TA), vastus lateralis (VL), and soleus (SOL) muscles of both left and right legs. Amplitudes of TMS-induced motor evoked potentials (MEPs) were calculated in the EMG recorded from contralateral and ipsilateral muscles. Fourteen of these subjects completed two behavioural assessments; a footedness assessment, and a knee and ankle visuo-motor tracking task. The strength of footedness and subjects’ ability to accurately perform the visuo-motor tracking tasks was assessed in subjects with and without evidence of ipsilateral connectivity. The results indicate approximately 40% of the muscles tested in the 22 subjects had evidence of ipsilateral connectivity. Of the three muscles examined, the VL more frequently revealed evidence of ipsilateral connectivity, and this evidence was more often revealed in muscles on the left side of the body. Subjects with ipsilateral connectivity to muscles on the left side of the body were more strongly right footed. This was taken to mean that they were more likely to use the left leg as a stance limb whilst manipulating or avoiding objects with the right leg and foot. The visuo-motor tracking task revealed that subjects with evidence of ipsilateral connectivity only with muscles on the left side of the body performed antiphase tracking less accurately than subjects without evidence of ipsilateral connectivity. Subjects without evidence of ipsilateral connectivity tended to perform both inphase and antiphase tracking more accurately compared with subjects with evidence of ipsilateral connectivity to one or both legs.