Movement Smoothness Differentiates Voluntary from Parkinsonian Bradykinesia

Abstract

Objective: While considered a key symptom, bradykinesia is not specific to Parkinson's disease (PD). Measuring movement smoothness may help distinguish PD-induced from volitional bradykinesia.Methods: Eight PD patients and 12 healthy subjects performed alternating, maximal speed, small and large elbow flexion-extension movements. Six of the healthy subjects also performed the task while matching the average speed of PD patients. From angular displacement, we derived speed, acceleration, jerk measures and the power spectrum of acceleration frequencies. Acceleration variability was evaluated using the Normalized Average Rectified Jerk (NARJ) and the fast-frequency to movement-frequency (FF/MF) ratio. Ratios of maximal velocities and accelerations in large to those in small movements (L/S velocity and acceleration ratios) were also measured.Results: NARJ in PD was 189 ± 17% of controls and 151 ± 14% of speed-matched controls (p=0.004; pairwise p=0.003, p=0.051 respectively) in large movements and 146 ± 11% of controls and 139 ± 11% of speed-matched controls (p=0.012; pairwise p=0.011, p=0.067 respectively) in small movements. FF/MF ratio in PD was 277 ± 45% of controls and 200 ± 32% of speed-matched controls (p=0.032; pairwise p=0.028, non-significant, respectively) in large movements and 613 ± 73% of controls and 246 ± 29% of speed-matched controls (p<0.001; pairwise p<0.001, p<0.001 respectively) in small movements. Time since diagnosis, but not age, was correlated with NARJ (p<0.05) and FF/MF ratio (p<0.01) for all movements. L/S ratios did not differentiate PD from speed-matched movements in the study sample.Conclusion: The two smoothness metrics, NARJ and FF/MF ratio, distinguished PD from volitional slowness and correlated with time since diagnosis. They are candidate physiological markers of PD-induced bradykinesia.