Arrhythmokinesis is evident during unimanual not bimanual finger tapping in Parkinson’s disease

BACKGROUND: Arrhythmokinesis, the variability in repetitive movements, is a fundamental feature of Parkinson’s disease (PD). We hypothesized that unimanual repetitive alternating finger tapping (AFT) would reveal more arrhythmokinesis compared to bimanual single finger alternating hand tapping (SFT), in PD. METHODS: The variability of inter-strike interval (CV(ISI)) and of amplitude (CV(AMP)) during AFT and SFT were measured on an engineered, MRI-compatible keyboard in sixteen PD subjects off medication and in twenty-four age-matched controls. RESULTS: The CV(ISI) and CV(AMP) of the more affected (MA) and less affected (LA) sides in PD subjects were greater during AFT than SFT (P < 0.05). However, there was no difference between AFT and SFT for controls. Both CV(ISI) and CV(AMP) were greater in the MA and LA hands of PD subjects versus controls during AFT (P < 0.01). The CV(ISI) and CV(AMP) of the MA, but not the LA hand, were greater in PDs versus controls during SFT (P < 0.05). Also, AFT, but not SFT, detected a difference between the MA and LA hands of PDs (P < 0.01). CONCLUSIONS: Unimanual, repetitive alternating finger tapping brings out more arrhythmokinesis compared to bimanual, single finger tapping in PDs but not in controls. Arrhythmokinesis during unimanual, alternating finger tapping captured a significant difference between both the MA and LA hands of PD subjects and controls, whereas that during a bimanual, single finger tapping task only distinguished between the MA hand and controls. Arrhythmokinesis underlies freezing of gait and may also underlie the freezing behavior documented in fine motor control if studied using a unimanual alternating finger tapping task.