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 (
Parkinsonian rigidity has been thought to be constant through a full range of joint angle [
This study included 20 healthy elderly volunteers (15 men and 5 women, mean age of
Patients’ clinical details.
Patient | Age |
Gender | Disease |
UPDRS score | Medication* | ||
---|---|---|---|---|---|---|---|
Part III | Rigidity (R) | Rigidity (L) | |||||
1 | 79 | M | 4.5 | 30 | 2 | 1 | C 2 mg; C/L 25/250 mg |
2 | 85 | M | 1 | 21 | 1 | 1 | Pra 0.375 mg |
3 | 64 | M | 3 | 19 | 1 | 1 | Pra 1.5 mg; C/L 20/200 mg |
4 | 50 | F | 5.5 | 15 | 1 | 1 | Per 0.9 mg; B/L 62.5/250 mg; T 1 mg |
5 | 58 | F | 20 | 26 | 1 | 1 | B/L 50/200 mg; A 200 mg; T 4.5 mg |
6 | 76 | F | 8 | 39 | 2 | 2 | C/L 20/200 mg |
7 | 60 | M | 3.5 | 24 | 1 | 1 | Pra 1.375 mg |
8 | 77 | F | 4 | 29 | 1 | 1 | Pra 1.5 mg; Per 0.15 mg |
9 | 67 | M | 8 | 26 | 1 | 1 | Pra 3 mg; Per 0.9 mg; B/L 75/300 mg; S 2.5 mg |
10 | 70 | F | 4.5 | 43 | 1 | 1 | Per 0.75 mg; C/L 20/200 mg |
11 | 66 | M | 15 | 45 | 3 | 3 | Per 0.75 mg; B/L 100/400 mg; E 400 mg; S 5 mg; T 4 mg |
12 | 73 | F | 12.5 | 32 | 1 | 1 | Pra 1.5 mg; Per 1.05 mg; B/L 75/300 mg; S 5 mg |
13 | 58 | M | 9 | 28 | 2 | 3 | Pra 1.5 mg; Per 0.75 mg; C/L 15/150 |
14 | 67 | M | 8.5 | 39 | 2 | 1 | Per 0.75 mg; B/L 150/600; S 2.5 mg; T 2 mg |
15 | 67 | M | 5 | 53 | 2 | 2 | C 3 mg; C/L 15/150 mg |
16 | 78 | M | 5.5 | 35 | 2 | 2 | Pra 3 mg; B/L 75/300 |
17 | 47 | M | 9 | 24 | 1 | 2 | Pra 2 mg; C 2 mg; B/L 50/200; A 300 mg |
18 | 67 | M | 5.5 | 39 | 1 | 2 | Pra 1.5 mg; Per 0.6 mg; C/L 30/300 |
19 | 67 | F | 3 | 45 | 2 | 3 | No medication |
20 | 69 | M | 4 | 49 | 4 | 3 | B/L 25/100 |
21 | 72 | M | 5 | 49 | 2 | 3 | Pra 1.5 mg; B/L 125/500 |
22 | 60 | F | 8 | 18 | 2 | 3 | Pra 2 mg; C 1 mg; C/L 15/150 |
23 | 73 | M | 6 | 51 | 4 | 3 | Per 0.15 mg; B/L 62.5/250; A 125 mg; T 2 mg |
24 | 65 | F | 2.5 | 29 | 3 | 2 | Pra 0.75 mg; C/L 10/100; S 2.5 mg; D 200 mg |
Figure
Schematic diagram of muscle tone measurement system. This system consists of small 3-axis force sensors, a gyro sensor, and surface electrodes. Two force sensors are placed to sandwich the wrist joint with soft pads to measure the force along the
(1) The 10°–110° portion of the degree-torque characteristics curve was extracted for each of the flexion and extension movements in healthy subjects.
(2) A likelihood ratio test was used to compare the fit of two regression lines: one of which (the null model) is one line fitting with no cutoff point and the other (the alternative model) is two lines fitting with a specified cutoff point.
Methods of analysis of properties of elbow joint movements for each of the distal and proximal segments with the joint angle of “cutoff point” as their boundary. (a) One regression line was fitted to the data of the 10°–110° portion of the degree-torque characteristics curve obtained during extension of the elbow joint of a normal healthy subject. (b) Two regression lines were fitted to the data of the same subject. The extension of the elbow joint was divided into the proximal phase and distal phase at the angle of the elbow joint of “cutoff point.”
(3) In “significant cases,” the degree of joint angle that divided the regression line was taken as a “cutoff point” and a 95% confidence interval (CI) for the cutoff point was obtained.
(4) The 10°–110° portion of the degree-torque characteristics curve was extracted for each of the flexion and extension movements in healthy volunteers and PD patients. For both flexion and extension, the regression line was divided by the cutoff point, and the “elastic coefficient in the distal phase” was calculated from the slope of the regression line from 10° to the cutoff point and the “elastic coefficient in the proximal phase” from the slope of the regression line from the cutoff point to 110°.
(5) The correlation between the UPDRS rigidity score and each value of the elastic coefficient for each of flexion and extension movements was evaluated. A pairwise comparison was made between score 0 group and score 1 group. The elastic coefficients in extension and flexion were normalized using the mass of the subject’s body weight since these are dependent on the subject’s muscle mass [
(6) We investigated whether a differentiation between healthy subjects and PD patients can be made with logistic discriminant analysis using two values of the elastic coefficient for each of the flexion and extension movements.
Spearman’s correlation coefficients with 95% CI were calculated to assess a degree of relationship between variables, and Wilcoxon rank sum test was used for pairwise comparisons. It could not be assumed that the raw data obtained were homoscedastic and normally distributed.
An analysis was performed for each of the 260 measurements in flexion and extension for healthy subjects. This analysis indicated that it is preferable for all the data to be approximated by two lines. The cutoff point was identified as 58.1° in flexion (95% CI: 55.3° to 60.9°) and 61.1° in extension (95% CI: 59.2° to 62.9°). The 95% CI for both cutoff points included 60°, and the cutoff points for both flexion and extension were determined as 60°.
Then, in healthy subjects and PD patients, the elastic coefficients in the distal phase (10°–60°) and proximal phase (60°–110°) were calculated with the cutoff point of 60° for 4 measurements each in flexion and extension. The correlation between these values and the UPDRS rigidity score was evaluated (see Figures
Results for elastic coefficients in elbow flexion and extension for each of distal and proximal phase. Comparison between the elastic coefficient in (a) the distal phase of flexion (10–60°), (b) the proximal phase of flexion (60–110°), (c) the distal phase of extension (10–60°), and (d) the proximal phase of extension (60–110°) and the UPDRS rigidity score. The elastic coefficient in the distal phase of extension showed the best correlation (
A logistic discriminant analysis was performed, taking into account the calculated elastic coefficients, age, gender, and side (right or left), to investigate the feasibility of screening for PD. Among the 4 calculated elastic coefficients, those in the distal phase of extension, proximal phase of extension, and proximal phase of flexion were selected, and the analysis was performed with the addition of age. The results showed that healthy individuals and PD patients could be differentiated with 78.8% sensitivity, 83.3% specificity, and 81.5% correct classification rate, suggesting that it may be possible to perform screening for PD with high specificity.
As reported by Amis et al., moment arms vary substantially with joint angles in flexion and extension movements of the elbow joint in healthy individuals [
In the present analysis, we obtained 4 values of the elastic coefficients by calculating the coefficients divided into two blocks. Among them, the elastic coefficient in the distal phase of extension was best correlated with the UPDRS rigidity score. The combination that yielded the highest specificity in screening for PD was the elastic coefficients in the distal phase of extension, proximal phase of flexion, and proximal phase of extension. These results suggest that rigidity is clearly detected at the phase where the flexor and extensor muscle groups at the elbow joint are stretched from the middle and that rigidity is more pronounced in extension than in flexion. The Parkinsonian rigidity has been thought to be constant through a full range of joint angle [
So far, rigidity has been rated as a score of 4 in the UPDRS rigidity score in individuals with limited joint mobility, and attention has been drawn to the distal phase of extension. This level of rigidity can be easily perceived by the human senses. At the level of score 2 or 3, difference in torque measurements for flexion and extension (difference of bias) is likely to be the major feature value, as described in our previous report. In our previous study, distinction between score 0 and 1 rigidity could be made by using continuous activity on the surface electromyogram at the maximum extension position of the muscle as a feature value; however, the results of the present study demonstrated that the feature value exists in the elastic coefficient in the distal phase of extension, which supports the findings of our previous study.
The judgment of UPDRS rigidity score 1 (slight rigidity) seems to be based on the elastic component in the distal phase of extension. Further investigation for difference in muscle tone between rigidity and spasticity will be needed because it has been reported that Parkinsonian rigidity also has a spastic component which is described as “rigospasticity” [
Our study demonstrated that Parkinsonian rigidity shows variable properties depending on the elbow joint angle, and it is clearly detected at the distal phase of elbow extension.
The scientific sensing technology has revealed that a feature to be noted is common to both the weakest and strongest rigidity. If such a simple and compact measurement system can be used at the bedside, new avenues will be opened for understanding the pathogenesis of the abnormality of muscle tone (including spasticity) and assessing treatment effects.
The authors express their thanks to Ms. Rie Yoshida, Neurology Department of Osaka University Graduate School of Medicine, for data analysis in this study. This study was supported by the Program for Promotion of Fundamental Studies in Health Sciences of the National Institute of Biomedical Innovation (NIBIO). This study was supported by JSPS KAKENHI 23700551.