Parkinsonian Rigidity Shows Variable Properties Depending on the Elbow Joint Angle

Parkinsonian rigidity has been thought to be constant through a full range of joint angle. The aim of this study was to perform a detailed investigation of joint angle dependency of rigidity. We first measured muscle tone at the elbow joint in 20 healthy subjects and demonstrated that an angle of approximately 60° of flexion marks the division of two different angle-torque characteristics. Then, we measured muscle tone at the elbow joint in 24 Parkinson’s Disease (PD) patients and calculated elastic coefficients in flexion and extension in the ranges of 10°–60° (distal) and 60°–110° (proximal). Rigidity as represented by the elastic coefficient in the distal phase of elbow joint extension was best correlated with the UPDRS rigidity score ( ). A significant difference between the UPDRS rigidity score 0 group and 1 group was observed in the elastic coefficient in the distal phase of extension ( ), whereas no significant difference was observed in the proximal phase of extension and in each phase of flexion. Parkinsonian rigidity shows variable properties depending on the elbow joint angle, and it is clearly detected at the distal phase of elbow extension. 1. Introduction Parkinsonian rigidity has been thought to be constant through a full range of joint angle [1]. However, such definition is based on the subjective observation, and it is unclear whether rigidity varies with joint angles or not. We have successfully analyzed the components of rigidity perceived by physicians in routine clinical practice in a systematic way [2]. As a result, we found that rigidity consists of elastic coefficients (elasticity) and difference of bias (difference in torque measurements for extension and flexion) and that higher values of either or both of these components are associated with greater amounts of rigidity. The elastic coefficients in flexion and extension of the elbow joint were estimated by extracting the 10°–110° portion of the degree-torque characteristics curve and using the slope of the regression line relative to the entire data. Further analysis of the data of healthy subjects demonstrated that the 60° elbow joint angle marks the division of the two properties of the elastic coefficients. We considered the elbow-joint elastic coefficients as a combination of two elastic components with different properties, instead of considering them as having one single property over the entire range of joint angles. We first determined an optimal degree that divides the elastic coefficients of the elbow joint into two properties in healthy individuals and then divided