Amplitude- and velocity-dependency of rigidity measured at the wrist in Parkinson's disease

Abstract

Objective: Quantify the effects of increased amplitude and rate of muscle stretch on parkinsonian rigidity.

Methods: Eighteen subjects with Parkinson's disease participated in this study. Subjects' tested hand was passively displaced through 60° and 90° ranges of wrist flexion and extension at velocities of 50°/s and 280°/s in both treated and untreated conditions. Joint angular position, resistance torque, and surface electromyography (EMG) of the wrist flexors and extensors were recorded. Rigidity was quantified by normalized work scores and normalized angular impulses for flexion and extension, separately. Reflex responses of stretched and shortened muscles were quantified by mean EMG and EMG ratio. A series of ANOVAs was performed to determine the effect of amplitude, velocity and medication on selected variables.

Results: Both work scores and angular impulses revealed that the larger displacement amplitude and the higher velocity were associated with significantly greater rigidity, increased EMG ratio and mean EMG of stretched muscles. Dopaminergic medication was not associated with a reduction in rigidity.

Conclusions: Parkinsonian rigidity is modulated by the amplitude and rate of muscle stretch.

Significance: These findings shed light on the biomechanical underpinnings and physiological characteristics of rigidity and may inform clinical rigidity assessment in Parkinson's disease.

Figure 1

A representative sample of joint position (dotted) and joint torque (solid) signals recorded from one subject during the 60° range of imposed flexion and extension movements at 50°/s. Angular impulses and work scores were calculated for the periods of flexion and extension while the inertial components of torque, denoted by brackets, were omitted from the analysis.

Figure 2

Comparison of torque-angle traces from a subject with PD in the OFF-MED state during passive movements in the 60° SLOW (A), 60° FAST (B), 90° SLOW (C) and 90° FAST (D) conditions. As indicated on the figure panel A, the lower portion of the torque-angle traces in each panel represents the flexion movement and the upper portion the extension movement.

Figure 3

Mean normalized work scores for total, flexion and extension phases through 60° (A) and 90° (B) ranges of motion at 50°/s (SLOW) and 280°/s (FAST) obtained from all subjects in both medication states. The 90° range of motion was consistently associated with greater rigidity scores compared to the 60° range of motion. The same pattern held true for the faster velocity compared to the slow one. Error bars: standard deviation.

Figure 4

Mean normalized angular impulse for total rigidity as well as rigidity for the flexion and extension phases of movement at 50°/s (SLOW) and 280°/s (FAST) with 60° (A) and 90° range of motion from all participants in the OFF-MED and ON-MED states. The faster velocity was associated with greater total rigidity as well as greater flexion components of rigidity. The 90° range of motion was associated with greater rigidity scores compared to the 60° range of motion. Error bars: standard deviation.

Figure 5

Representative joint position and EMG tracings of the wrist flexors and extensors from a subject with PD during the wrist extension movement at 50°/s with 60° range of motion. In reflex was recorded in the wrist flexors. Dopaminergic medication greatly diminished the amplitude of these phenomena in the ON-MED condition (B). The onset of movement is indicated by the vertical line. Top panel: joint position (°); middle panel: average EMG of wrist flexor muscles; lower panel: averaged EMG of wrist extensor muscles.

Figure 6

Mean EMG values of stretched and shortened muscles and EMG ratios during passive wrist flexion when the wrist was flexed through a range of motion equal to 60° (dark) and 90° (white) during the SLOW (A) and FAST conditions (B) in the OFF- and ON-MED states. Error bars: standard deviation.

Figure 7

Mean EMG values of stretched and shortened muscles and EMG ratios during passive wrist extension when the wrist was extended through a range of motion equal to 60° (dark) and 90° (white) during the SLOW (A) and FAST conditions (B) in the OFF- and ON-MED states. Error bars: standard deviation.