Fig. 1.

a Photograph of mechanical rigidity device. b Time series of force and displacement during stimulation at 5 Hz from subject 7 (UPDRS—clinical rigidity score—1). c Superimposed torque–angular displacement cycles during stimulation at 5 Hz. d Schematic of analytical method for determining rigidity coefficients in flexion and extension. A single displacement cycle is shown from stimulation at 5 Hz (blue line) with linear regression of mid-cycle phase (dashed lines 50 % of cycle) for both flexion and extension (red line). The gradient of the regression line is taken as the rigidity for that cycle