Instrumental motion assessment, i.e., peg insertion, gains importance as an addition to rating procedures to determine the efficacy of therapeutic interventions and to serve as diagnostic tool in patients with Parkinson's disease (PD). One of their motor features is disturbed execution of alternating motions. The objectives of the study were to assess pronation and supination of the forearms and peg insertion with instruments and to correlate the outcomes with rating scores in PD patients. Controls and 27 idiopathic PD patients, taken off treatment for 12 h, were scored and performed on both devices. PD patients showed a reduced maximum velocity (v), performance interval and amplitude of diadochokinetic movements than controls. V outcomes showed closer associations to rated motor behavior, in particular akinesia and rigidity, and peg insertion results than amplitude, respectively periods of diadochokinetic motion sequences. This altered performance of diadochokinetic movements corresponds to the clinical motor symptoms of PD patients and differs from healthy controls.