Rapid repetitive tapping, like the Foot Tap Test (FTT), slows with age, but the underlying mechanisms are unknown. Purpose: 1. Test the hypotheses that greater performance variability, increased muscle coactivation, and slowed muscle contractile speeds are related to lower foot tap count (FTC) in older adults; 2. Examine the relationship between FTC and physical function in older adults, using the advanced SPPB-A. Methods: 18 (25.0±3.1years, 9F, mean±SD), and 28 (73.4±4.9, 14F) adults were recruited; the later were divided into Higher (HFO) and Lower (LFO) Functioning based on SPPB-A score. Participants performed 10s of rapid tapping (FTT) while seated. A MATLAB program was used to calculate FTC and variability of the intertap-interval (COV-ITI). Contractile speed (rates of force development and relaxation, RFD and RFR) of the dorsiflexor muscles were determined using ballistic contractions. Electromyography was recorded on the tibialis anterior, soleus, and coactivation was calculated based on their agreement. Results: LFO had a lower FTC than Young and HFO (45.9±7.0taps, 54.4±7.5, 53.1±5.7, respectively; p=0.003), and lower COV-ITI than Young (12.3±5.9%, 20.5±8.1, respectively, p=0.009). No associations were found between COV-ITI and FTC. Coactivation was higher in Young than HFO (0.568±0.209, 0.321±0.129) and was negatively related to FTC in older (r2=0.274, p=0.005). The LFO had a slower RFD than Young and FTC was positively related to maximal RFD (r2=0.345, p=0.001) and RFR (r2=0.162, p=0.038) in older adults. In older, FTC was related to SPPB-A (r2=0.329, p=0.001). Conclusions: Greater coactivation during the FTT and slower force development may negatively affect FTC in older adults.