Grip relaxation

1. Grip relaxation • Terminating voluntary grip activity in a timely manner is important for hand function (Seo 2009). • Slow movement with aging (Fozard 1994) impacts not only function but also safety, as slow reaction to danger can lead to injury. • To relax a grip, young adults’ brain generates short-interval intracortical inhibition, which may control the spinal excitability and facilitate muscle relaxation (Motawar 2012). • The effect of aging on neural function for grip relaxation is unknown. Understanding changes in neural function for relaxation can help develop interventions to assist grip relaxation in elderly. Increased intracortical inhibition Stable spinal excitability Muscle relaxation

2. Objective + Approach • The objective was to examine intracortical inhibition and spinal excitability for a major grip muscle in healthy young vs. old adults. • Intracortical inhibition and spinal excitability during grip relaxation using well-accepted methods • Transcranial magnetic stimulation (Brain inhibition) • Nerve stimulation (spinal excitability) 0.2mV 0.02s Transcranial magnetic stimulation (TMS) of hand area Motor cortex Auditory cue to relax Auditory cue to grip Stimulation 0.2 0.1 0 10 12 6 0 2 4 8 EMG, mV Time, s Grip relaxation time

3. Less brain inhibition with aging Failure to increase brain inhibition during grip relaxation Elevated spinal excitability Delayed grip relaxation Spinal excitability modulation, % Grip relaxation time, ms Insufficient intracortical inhibition Increased spinal excitability Delayed muscle relaxation