The timing of prehensile movements in subjects with cerebral palsy

In this study, a paradigm is presented for the assessment of manual dexterity in subjects with cerebral palsy (CP) that divides the prehensile action into a 'time-to-contact' phase and a 'time-in-contact' phase. Two experiments were performed that determined the effect of object weight on the timing of both phases for the impaired hand and non-impaired hand of subjects with spastic hemiparesis (N = 14). In the first experiment, subjects had to reach for and lift a tube at their own preferred speed. The results showed that the prehensile deficit of the impaired limb is to a large degree manifested by a longer time spent in contact with the object before it was lifted. The time-in-contact phase was decreased after repeated lifts, suggesting that subjects with CP can control and modify force output in advance based on weight information from preceding lifts. In the second experiment speed of movement execution was stressed to examine whether the observed timing pattern of the first experiment is characteristic of prehensile movements of the paretic arm or represents a movement strategy adapted to the disorder. The results of the second experiment showed that subjects could comply with the instruction by reducing the absolute duration of both phases of the prehensile movement. Furthermore, the anticipation effects were eliminated to a large degree. In both experiments the time-in-contact phase was longer for the impaired limb. These results indicate a pathological constant in the time-in-contact phase for the impaired limb. This assumption is discussed in relation to the application of grip and lift forces during this phase. It is concluded that the paradigm is well suited for use in a practical setting as a simple and broad clinical test to assess the prehensile decrements of subjects with CP.