Variances of drawing arm movements between patients with Parkinson's disease and healthy
controls were compared. The aim was to determine whether differences in joint synergies
or individual joint rotations affect the endpoint (hand position) variance. Joint
and endpoint coordinates were measured while participants performed drawing tasks.
Variances of arm configurations and endpoints were computed and statistically analyzed
for 12 patients and 12 controls. The variance of arm movements for patients (both
for arm configuration and endpoint) was overall higher than that for the control group.
Variation was smaller for drawing a circle versus a square and for drawing with the
dominant versus the nondominant hand within both groups. The ratio of arm configuration
variances between groups was similar to the ratio of endpoint variances. There were
significant differences in the velocity, but not in the path lengths of movements
comparing the two groups. Patients presented less movement stability while drawing
different figures in different trials. Moreover, the similarity of the ratios suggests
that the ill-coordinated hand movement was caused by the error in the movements of
individual body parts rather than by the lack of intersegmental coordination. Thus,
rehabilitation may focus on the improvement of the precision of individual joint rotations.