Background/Study Context: Much has been written regarding age-related changes in sensory and motor functions, general slowing of the nervous system, and deficiencies in inhibition. Few studies, however, have attempted to define how each of these factors may contribute to poorer accuracy of motor performance with aging. The purpose of this study was to examine whether these changes were best explained by speed of task or stimulus-response compatibility. Methods: Twenty-four younger (M=19.5; 18-22 years) and older (M=72.5; 65-82 years) adults used knee movement to track a computer-generated disc along a computer-generated sinusoidal wave that either moved at 50 or 70cm/s. Stimulus-response compatibility consisted of leg and disc movement in the same direction and stimulus-response incompatibility consisted of leg and disc movement in the opposite direction. Performance was analyzed using a mixed-design analysis of variance (ANOVA). Results: Younger adults performed better than older adults in all conditions. Magnitudes of error between the stimulus-compatible and stimulus-incompatible conditions were greater for the old group compared to the young group. Both of these findings were consistent with the hypotheses. Inconsistent with the hypothesis, speed of task did not contribute to age-related differences in accuracy of motor performance in either cognitive load conditions. Conclusion: Differences in performance could be attributed to age-related changes in selective inhibition. Future research should focus on examining the potential consequences of decreased inhibition among older adults when completing various activities of daily living and what interventions might mitigate these consequences.