Introduction: One of the most common syndromes in Parkinson's disease (PD) is rigidity. Currently, an index is used to evaluate the level of PD by the clinical measurement of rigidity in the upper extremity. The index uses a subjective method called Unified Parkinson's Disease Rating System (UPDRS). The subjective nature of this method makes the influence of physician in the measurement of rigidity possible. Hence, the development of a new standard method based on objective indices is needed.
Materials and Methods: In this research, a new device was fabricated and used to measure the viscous and elastic indices and the range of motion during passive movement of elbow joint. The relation between each index and the level of illness was analyzed. The parameters were measured on 41 patients and 11 controls. The indices were extracted using Matlab-R14 software and the statistical analysis was performed using Spss-13.
Results: Although there were significant differences in both the viscous and elastic indices between the pair groups and also among the UPDRS groups, but better correlations of the viscous ones and UPDRS were found.  The range of motion by itself has no good correlation with the level of the disease.
Discussion and Conclusion: Based on the obtained results, it can be inferred that using viscous indices of rigidity may have an advantage over the elastic ones for the evaluation of Parkinson’s disease.
Upon conducting more trials and also considering the sub indices in different parts of the range of motion, the method used here may become a standard objective method for the evaluation of Parkinson's disease.