This study's objective was to investigate regional cerebral blood flow (rCBF) within the primary motor cortex (M1) and to compare it with thresholds of transcranial magnetic stimulation (TMS) and electromyographic recordings during exertion of different force levels with the right index finger. Quantitative electromyographic recordings, TMS, and positron emission tomography scans were performed while five and six volunteers, respectively, pressed a Morse key repetitively or with constant force with the right hand at five different force levels: 5, 10, 20, 40, and 60% of the individual's maximum voluntary contraction (MVC). Although at 5% MVC muscle activity was restricted to the first dorsal interosseus muscle, superficial finger flexors, and extensors, there was progressive involvement of proximal muscles during finger flexion with increasing force. rCBF increased logarithmically in the contralateral M1 with increasing force. In ipsilateral M1, rCBF decreased at 5% MVC and then increased logarithmically at higher force levels. TMS thresholds in the contralateral hemisphere declined logarithmically to reach a plateau at high force levels. The threshold in the ipsilateral hemisphere decreased slightly at high force levels. The logarithmic increase of rCBF and decrease of TMS thresholds in the contralateral hemisphere suggest related underlying physiological phenomena; increased cortical synaptic activity and increased excitability. It suggested that the pronounced ipsilateral rCBF alterations reflect transcallosal inhibition and are more prominent during repetitive movements (as used in the positron emission tomography study) than during the generation of a constant force (as exerted during TMS).
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