Force Amplitude and Force Duration in Parkinsonian Handwriting

Abstract

Previous research has revealed that Parkinsonian handwriting may be impaired more in terms of force amplitude than force timing. These findings suggest that the source of the impairment may be located at the level of motor-unit recruitment The present experiment demonstrates that an impairment of force amplitude or of force timing produce distinct handwriting distortions. To understand these distortions, it is assumed that pen movements may be decomposed into two movement axes. By simulations it can be shown that if force amplitude is manipulated the straight baseline of the handwriting pattern will become irregular. In contrast, if the force timing per axis is manipulated, sharp movement reversals (e.g., in cursive-script “u”) become loops. Accordingly, it was found that Parkinsonians, in comparison to age-matched controls, exhibit no problems with sharp movement reversals but evince problems in maintaining a straight baseline. The effects observed were robust with respect to task load (i.e., writing “eluhule” interspersed with “eludule”) and likewise, by alternating writing size (i.e., between normal and larger) and by writing speed (between normal and faster). Collectively, the results suggest a sequential organization of multi-joint movements in Parkinson’s disease patients handwriting.

This research was supported by grants from the USA National Institute of Neurological Diseases and Stroke (NS 17421) and American Parkinson’s Disease Association awarded to George E. Stelmach, and the Nijmegen University Research Pool (SW4/88). The researchers wish to thank the subjects from the Madison Area Parkinson Support Group for their motivated cooperation.