Strength training of a limb muscle on one side of the body produces functionally and clinically meaningful increases in motor output of the same muscle on the opposite side of the body. Such adaptations occur without muscle hypertrophy, implying a role for the ipsilateral motor cortex and the spinal cord in force transfer. The goal of this project is to determine the interhemispheric and spinal mechanisms of force transfer in healthy young adults. We hypothesize that the effects of unilateral practice on interhemispheric inhibition (IHI) and on the excitability of the contralateral corticospinal projections are linked, and these effects are graded according to the nature of muscle activation. In a series of experiments on healthy adults age 18 to 60, we will characterize the motor cortical inhibitory and excitatory mechanisms of interhemispheric practice-induced transfer of muscle force. We will use a longitudinal design that will include three exercise groups. In 20 sessions over 4 weeks, subjects will strengthen their first dorsal interosseus (FDI) muscle of the dominant hand either with voluntary, electrically stimulated, or imagined contractions. The design also includes three control groups, a sham-stimulated group, a repetitive transcranial magnetic stimulation (1HzTMS) group, and a no-intervention group. Before and after the training programs, we will determine the changes in the outcome measures, including voluntary and electrical stimulation evoked force of the trained and the untrained FDI. Measures of excitability of the involved and uninvolved motor cortex, assessed with TMS, and measures of spinal cord excitability, assessed with peripheral nerve stimulation, are the additional outcome measures. The results of these studies will substantially increase our understanding of central nervous system (CNS) control of voluntary movement in health and disease.
- INCLUSION CRITERIA: Male and female volunteers ages 18-60 years old will qualify for this study. The subject population will be equitable and we will strive to enroll volunteers from all ethnic backgrounds. EXCLUSION CRITERIA: 1. orthopedic impairments of the upper extremities 2. neurological impairments, including current or past peripheral or central nervous system dysfunction 3. being on drugs that may lower the seizure threshold 4. having a pacemaker, an implanted medication pump, a metal plate in the skull, metal objects in the eye or skull (for example after brain surgery or shrapnel wounds) 5. consumption of more than moderate amounts of alcohol or caffeine