Proceedings of The Physiological Society
King's College London (2009) Proc Physiol Soc 14, PC19
Posture dependent changes in motor responses of ankle dorsi- and plantar flexors to transcranial magnetic stimulation
K. N. Mileva1, J. L. Bowtell1, A. R. Kossev2
1. Academy of Sport, FESBE, London South Bank University, London, United Kingdom. 2. Institute of Biophysics, Bulgarian Academy of Sciences, Sofia, Bulgaria.
The aim of this study was to investigate the corticospinal and intracortical excitability associated with ankle extensor and flexor muscle activation during postural tasks. Motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation (TMS) of the leg area of the motor cortex were recorded in tibialis anterior (TA) and soleus (SOL) muscles of 7 healthy male adults (mean±SEM, 36±4y, 181±3cm, 82±5kg) whilst in upright standing (110s) and static squat (330s). Five single suprathreshold (120% resting TA motor threshold) and five paired TMS pulses were delivered during each condition (short-interval intracortical inhibition, SICI, interstimulus interval, ISI=3ms; or intracortical facilitation, ICF, ISI=13ms). Data were analysed using 2-way repeated measures ANOVA, and post hoc tests with Holm-Sidak corrections where appropriate. During standing TA MEP responses to single TMS were not significantly different from SOL although background SOL EMG activation was more than 5 times higher than TA (523±127%, p=0.018, Fig 1). TA MEP amplitudes were significantly higher at ISI=13ms (80±24%, p=0.009) and lower at ISI=3ms (-33±15%, p=0.050) compared to SOL MEPs indicative of increased ICF and SICI to TA motoneurons during standing. Single TMS during squat evoked significantly higher MEP amplitudes in TA than in SOL (194±69%, p=0.008) although background SOL EMG activity was almost 4 times higher than in TA (402±152%, p=0.014). During squat, TA MEP amplitudes at both ISI=13ms and ISI=3ms were not significantly different from SOL. TA EMG background activity (42±9%, p=0.037), TA MEP amplitude (253±112%, p=0.002) and total area (254±106%, p=0.017) in response to single TMS were significantly higher during squat than standing. During squat compared to standing, MEPs in both muscles at ISI=3ms tended to be only slightly higher, whereas TA but not SOL MEPs at ISI=13ms were significantly smaller (-35±7%, p=0.007). Neither SOL background activity nor SOL MEP parameters were different between standing and squat. Excitability of the cortical representations of lower leg muscles appears to be muscle and task-specific and may be differentially modulated according to the postural role of the muscle. SICI and ICF networks can be modulated independently and as for arm and hand muscles, disinhibition appears to be a more important mechanism for plastic intracortical changes than increased facilitation.
Where applicable, experiments conform with Society ethical requirements