Use of Transcranial Magnetic Stimulation to Measure Muscle Activation and Response to Exercise

These dissertation experiments test the use of transcranial magnetic stimulation (TMS), a neurophysiological measurement tool that can measure changes following exercise interventions, in the first dorsal interosseous (FDI) muscle. In chapter two, a force-based TMS measure known as the twitch interpolation technique was tested as a method to estimate muscle activation in the FDI. Chapter two describes two experiments that investigate whether these force-based TMS measures were reproducible, sensitive to change, and valid methods of estimating muscle activation in the FDI. 14 participants were tested in the first experiment, and 6 participants were tested in the second experiment. These experiments compared force-based TMS measures to peripheral nerve stimulation (PNS) over a range of voluntary force levels. The twitch interpolation technique was reproducible in the FDI using TMS and PNS. However, twitch interpolation of the FDI lacked sensitivity and validity when measured with TMS and PNS due to anatomical, physiological, and technical limitations. Chapter three focused on using TMS measures based on traditional electromyogram (EMG) recordings. These EMG-based TMS measures were used to measure changes in corticomotor excitability, intracortical inhibition, and intracortical facilitation following moderate and high intensity treadmill walking. Twenty-two participants exercised for 30 minutes on two, non-consecutive days, with the intensity targeted to 65% and 80% of age-predicted maximum heart rate. Following moderate intensity treadmill walking, corticomotor excitability increased as measured by the motor evoked potential (MEP) amplitude, slow-acting intracortical inhibition increased as demonstrated by a lengthened cortical silent period (CSP) duration, and short-latency intracortical facilitation (SICF) increased. Following high intensity walking, corticomotor excitability decreased as demonstrated by increased stimulus intensity required to elicit a 1 mV MEP, slow-acting intracortical inhibition decreased as measured by decreased long-latency intracortical inhibition (LICI), and SICF decreased. There were no changes in short-latency intracortical inhibition (SICI) following either walking intensity. The apparent contrast between intensities could be due to U-shaped relationships between exercise intensity and specific neurotransmitter activation patterns, cortisol, or cerebral blood flow.