This dissertation focuses on behavioral differences in dynamic grip force control at the edge of instability between healthy older adults and individuals with Parkinson’s disease (PD), and underlying neurophysiological mechanisms of dynamic grip force control in healthy young adults. By pushing the motor system to its limit of sensorimotor performance with a spring compression task, we aimed to develop more sensitive clinical measures, which detect 1) differences in motor severity between the hands in wellmanaged mild to moderate PD and 2) disease state from non-disease states, based on the ability to control dynamic grip force.

We measured the compression force level (F), and force variability at low (<4Hz, voluntary control, F_LF) and high frequency (4-12Hz, involuntary control, F_HF) bands for the more-affected and less-affected hand in PD and their relationship with motor severity, measured by Unified Parkinson’s Disease Rating Scale (UPDRS) motor examination. Our results revealed significantly lower F in the more-affected hand (p= 0.019), greater F_LF in the more-affected hand (p= 0.042), but no difference in F_HF between hands by a 10,000-iteration permutation test. The greater F_LF in the moreaffected hand was significantly correlated with the UPDRS motor scores (total motor, rho= -0.44, p= 0.04;​ hand only, rho= -0.52, p= 0.016), revealing the force variability decreased as motor severity increased. Because of greater heterogeneity of motor severity among PD participants, we further tested if the difference in force variability between two hands (ΔF_LF and ΔF_HF) was associated with symptom severity. The results showed a decrease in ΔF_LF as motor severity increased (total motor, rho= -0.46, p= 0.04). Interestingly, as non-hand motor symptoms (e.g. gait and balance) increased, ΔF_LF and ΔF_HF significantly decreased, which suggests that the measure of dynamic grip force control may also reflect systemic motor impairment.

As the measures of dynamic grip force control clearly revealed differences in control strategies between the more-affected and less-affected hand in well-managed mild to moderate PD, we used the same measures in healthy older adults to test how well these measures could distinguish disease state (PD) from non-disease state (control). We used percentile ranks for each hand in PD and Receiver Operating Characteristic (ROC) curves to test if force measures could be a potential diagnostic tool. Our results revealed that F_LF and F_HF were more sensitive to separate PD from the control group than F, using these methods. The F_LF in the less-affected hand showed that 13 out of 20 individuals were ranked at above 80th percentile with respect to these measures from the control group, and F_HF in the more affected hand showed 14 out of 20 individuals were ranked at above 80th percentile. The UPDRS motor scores for the individuals clustering above 80th percentile had little to no influence on the ranking of force variability with respect to force variability in healthy individuals. Our results of ROC curve showed that both F_LF and F_HF had good performance, revealing area values of 0.845 and 0.833 respectively, which indicates that F_LF and F_HF have a 84.5% and 83.3% chance of accurate diagnosis respectively. Therefore, measures of force variability might be a useful tool as an adjunct to current clinical diagnostic measures, considering these participants were well managed with medications.

Individuals with PD exhibit altered corticospinal excitability in primary motor cortex (M1). The greater force variability in PD might be associated with changes in corticospinal excitability in M1. In the healthy brain, bilateral activity of M1 was observed during unimanual dynamic force control tasks, however, there has been no investigation of the neurophysiological mechanisms underlying dynamic grip force control. Therefore, studying the corticospinal excitability in M1 ipsilateral to a task hand in healthy individuals will be helpful to understand neuropathological changes in PD. We measured 1) corticospinal excitability in the right M1 by motor evoked potentials (MEP) in the left first dorsal interosseous (FDI), 2) mirror EMG activity in the left FDI, and 3) ipsilateral silent period (ISP) in the right FDI, to determine if interhemispheric inhibition (IHI) would influence the control of dynamic grip force with different dexterity demands in three motor tasks (the unstable spring, stable spring, and dowel compression). We found a significant increase (almost twofold) in MEP during the unstable spring task, but not in the stable spring, compared to the dowel task. Modulation of corticospinal excitability in the right M1 was independent from the effect of IHI, revealing no changes in ISP among three tasks. We also found no correlations between MEP amplitudes and mirror EMG activity. This suggests that dynamic grip force control during stabilizing highly unstable objects may require fundamentally different neural mechanisms from other stable grip or isometric contraction tasks that have been used for previous studies of IHI. Furthermore, increasing corticospinal excitability in M1 ipsilateral to a task hand by unimanual dexterous task may be useful for neurorehabilitation for bilateral recovery in hemiparesis such as stroke and cerebral palsy.