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

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.