Generalized representation of handwriting: evidence of effector independence.

Previous analyses of writing data have provided qualitative evidence of temporal and spatial similarities across muscle effectors. In order to further explore the notion of effector independence in handwriting a single case study approach was adopted that employed quantitative analyses. A left-handed individual who lost his left hand and forearm in an automobile accident and wore a myoelectric arm was tested. After his accident he learned to write with the right hand developing this skill for 8 years. Once he was fitted with the myoelectric arm he began again to write with the left hand. Thus, the acquired writing patterns of both hands reflect two well-learned skills which utilize different muscle effectors. Wright's NAME-EQUATION paradigm (1990) was utilized which required the subject to write his name and the equation X + Y = Z with the dominant, non-dominant hand and the elbow of the dominant arm. A control subject was also tested. For the subject wearing a myoelectric arm (primary subject), kinematic and handwriting analysis revealed astonishing similarities between the different effectors tested. For the control subject, the obtained results showed that as the effectors changed so did the handwriting kinematics. The collective findings support the hypothesis that motor programs, the directives that guide skilled movement, can be generalized across different effectors, but only after considerable practice.