Timing and sequencing of human arm trajectories: Normal and abnormal motor behaviour

Using the dynamic optimization approach to the description of arm trajectories, the present paper examines trajectory planning principles underlying the generation of sequential arm movements in neurologically normal and Parkinsonian subjects. The paper discusses a possible scheme for sequence generation involving the superposition of temporally overlapping trajectory units. Evidence for the feasibility of this scheme is drawn from a recent study of arm tracking responses to double-step stimuli. The paper also discusses a possible criterion according to which basic strokes can be identified, and it examines to what extent the difficulties that Parkinson's disease patients have in generating motor sequences emerge from their inability to preplan a simple stroke or movement chunk as a single unit.