Neural mechanisms for grip relaxation are relatively unknown and understudied, as compared to mechanisms for grip initiation. Yet, termination of motor activity is as important as initiation in daily function. This knowledge gap presents incomplete understanding of neural control of hand function and its impairment with aging and neurologic disorders.
The purpose of this dissertation was to identify and examine neural mechanisms of grip relaxation in healthy young adults, with aging, and in chronic stroke survivors. A series of experiments in healthy young adults showed that the relaxation from a maximum power grip was mediated by increase in the short-interval intracortical inhibition (SICI). The role of spinal motor excitability modulation for grip relaxation was refuted, in contrast to previous literature for the leg muscle. These data from healthy young adults suggest that the grip relaxation time is a cortically mediated active process. Additionally, these studies also showed that the neural mechanism of grip relaxation is comparable for the dominant and the nondominant hand in healthy young adults. The next step was to identify any delays in relaxing from a grip in healthy older adults.
Assessment of the effects of aging on the role of SICI showed that the delayed grip relaxation time in older adults was accompanied by reduced modulation of SICI for grip relaxation. The cortical silent period and H reflex did not explain delays in grip relaxation observed in older adults.
Another series of experiments showed that the chronic stroke survivors and age-matched control adults demonstrated comparable modulations of SICI, cortical silent period, corticomotor excitability, and H reflex. Yet, the paretic hand of the stroke survivors was significantly delayed in relaxing from a grip.
Correlation and regression analysis showed that the stroke-related delayed grip relaxation time may be explained by increased spasticity, reduced somatosensation, paretic grip weakness relative to the nonparetic, strength of the corticospinal connections and interhemispheric inhibition. An intervention aimed to modulate cortical excitability and interhemispheric inhibition, Active Passive Bilateral Therapy, was employed but was found to be not effective in modulating grip relaxation time and interhemispheric inhibition after a one-time 20-minute session, warranting a longer treatment time.
In summary, this dissertation investigated neural mechanisms of grip relaxation and contributes to the general body of knowledge regarding neural control of hand movements.