Variability In Rapid Force Production Predicts Motor Symptom Severity In Parkinson’s Disease

Parkinson’s disease (PD) research needs objective measures to quantify the severity and progression of motor symptoms. Rapid isometric contractions reveal slowing and disorder in people with PD (PwPD). When healthy adults perform these contractions to various submaximal force levels, the rate of force development (RFD) has a strong linear relationship with the peak force (PF) of the force pulse. Some PwPD modulate RFD less effectively and the slope of the relationship between PF and RFD (RFD-SF) and its R2 provide information about PD changes in central control. However, it is unknown how these measures vary in PwPD and relate to motor symptom severity. PURPOSE: Aim 1 was to determine the range of RFD-SF and R2 in a sample of mild to moderate people with PD. Aim 2 was to quantify relationships between force measures and motor symptom severity. Aim 3 was to determine which force measure best predicts motor symptom severity. METHODS: This study included 44 PwPD (11 females, aged 67.5 ± 7.9 years, diagnosed 5.1 ± 3.9 years, Hoehn and Yahr stage 1-4) on their usual medications. Motor symptom severity was assessed using part III of the Movement Disorder Society-Unified Parkinson’s Disease Rating Scale (UPDRS). The subjects performed approximately 100 rapid isometric finger abduction force pulses to amplitudes between 20-80% of their maximal voluntary contraction force. Linear regression quantified the slope (RFD-SF) and R2 of the relationship between PF and peak RFD of each force pulse. Spearman’s correlations quantified relationships between UPDRS, RFD-SF and R2. Multiple linear regression was used to test whether RFD-SF or R2 significantly predicted UPDRS score. RESULTS: RFD-SF (median = 4.35, range [1.63, 17.21]) and R2 (median = 0.57, range [0.08, 0.85]) were highly variable. UPDRS correlated with RFD-SF (ρ = -0.37, p = 0.012) and R2 (ρ = -0.52, p < 0.001). The overall regression was significant (F = 10.56, p < 0.001). R2 significantly predicted UPDRS (β = -44.2, p < 0.001), and RFD-SF did not (β =0.02, p = 0.979). CONCLUSIONS: Though RFD-SF and R2 related to motor symptom severity, R2 exclusively explained the variance in motor symptoms. This measure was sensitive to disease status with abnormal values calculated for all but two subjects (R2 < 0.8), and provides a potential disease marker. This work was supported by Shake It Off, Inc., West Chester, PA