Validity of the finger tapping test in Parkinson’s disease, elderly and young healthy subjects: is there a role for central fatigue?

[Abstract] Objective: The main goal of this work is to evaluate the validity of the finger tapping test (FT) to detect alterations in rhythm formation. Methods: We use FT to study the alterations in motor rhythm in three different groups: Parkinson’s patients, elderly healthy controls, and young healthy control subjects (HY). The test was performed in COMFORT and FAST tapping modes and repeated on two different days. Results: For the variables analyzed (frequency and variability) both modes were repeatable in all groups. Also, intra-class correlation coefficients showed excellent levels of consistency between days. The test clearly differentiated the groups in both FAST and COMFORT modes. However, when fatigue was analyzed, a decrease in the tapping frequency was observed in HY during the FAST mode only. The amplitude of motor evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) was early-potentiated but not delayed-depressed, both for COMFORT and FAST modes. This suggests that fatigue was not of cortico-spinal origin. Other forms of central fatigue are discussed. Conclusions: FT at FAST mode is not a valid test to detect differences in rhythm formation across the groups studied; fatigue is a confounding variable in some groups if the test is performed as fast as possible. Significance: COMFORT mode is recommended in protocols including the FT for evaluating rhythm formation