Variability in the Assessment of Cortical-Evoked Potentials Using Magnetic Stimulation

Transcranial Magnetic Stimulation (TMS) is a powerful technique used for the assessment of central motor function. Although several thousand papers have appeared over the last several years, there is scant data available regarding the variability and reliability of the data obtained using TMS. As we expect to pursue several studies using TMS, we have conducted an investigation to determine the reliability of cortical-evoked potentials using TMS. Cortical-evoked potentials were assessed in 14 subjects using a Cadwell magnetic stimulator. Subjects attended the laboratory on each of three days. Motor-evoked potentials (MEP) were assessed in three different stimulation conditions from both the biceps and the first dorsal interosseous (FDI) muscles. The preliminary results demonstrate very good reliability, as long as sufficient numbers of trials are used in each condition. We expect these results to provide considerable guidance in the design of future experiments using transcranial magnetic stimulation.

Fatigue Influences on Motor Unit Synchronization

Although several investigators have suggested that the frequency of motor unit synchronization (the near-simultaneous activation of two or more motor units) might increase with muscular fatigue, there is little evidence of this phenomenon. We have begun a project to explore the changes in synchronization when the first dorsal interosseous muscle is fatigued using isometric contractions.

Motor unit synchronization in weight-trained adults

Investigators have also suggested that the frequency of motor unit synchronization might be greater in individuals trained for high-force activities. We have begun to investigate this issue by measuring motor unit synchronization in weight lifters and in a group of control subjects during both low-force and high-force contractions.

Variability issues in motor unit discharge behavior in young and older adults

We have begun to explore the extent to which neural codes are present in motor unit discharge variability. Both force and motor unit action potentials are recorded during constant and varying force isometric contractions. We plan on using both traditional and nonlinear measures to identify the extent to which motor unit discharge activity displays alterations in variability as a result of age, force level, muscle group, fatigue, or exercise training status.