Many epidemiological studies with durations ranging from months to decades have shown that past participation in physical activity has a protective effect on incident functional difficulty and walking difficulty among older adults. The analytical techniques including logistic and proportional hazard regressions used in most of the longitudinal prospective studies assumed that the protective effect on the risk of incident difficulty was constant throughout the study period. However, even with the same physical activity participation levels as in the past, individuals' risk of walking difficulty may still vary, which suggest that one's participation level may have weaker effects on temporally distal outcomes. Therefore, this study examined the effects of past record of physical activity on incident walking difficulty in two-year intervals over a six-year period. Participants were 2,137 older adults who had no walking difficulty at the baseline survey in the Longitudinal Study of Aging (Mean Age=76.63, SD=5.27). Walking difficulty was defined as reporting difficulty on both walking ten steps without resting and walking a quarter mile. The effect of physical activity at baseline on incident walking difficulty was hypothesized to decrease over the subsequent follow-ups while controlling for possible confounding variables. A discrete-time hazard model with proportional assumptions for all explanatory variables (constrained model) was compared with a model that the proportional assumption for physical activity was released (unconstrained model). Results showed that the model fit for the unconstrained model was significantly better than the constrained model (Δ-2LL=79.73, Δdf=2, p<.001). The hazard odds ratios of physical activity were .56 (95%CI=.48-.66), .66 (95%CI=.54-.80), and .99 (95%CI=.81-1.21) at two, four, and six years after the baseline of the study, respectively. The predicted incident rates at these three subsequent time points were 14%, 15%, and 24% for a typical person who had not reported walking difficulty at the previous time point. Without further information on the recent participation levels, past activity levels six years ago did not have a significant effect on incident walking difficulty for those who have not reported difficulty after four years. Relatively short-term protective effects of physical activity on incident walking difficulty were supported in this study. Hence, continued participation in physical activity is highly recommended for older adults to enhance or maintain the mobility as well as the quality of life. Keyword(s): aging/older adult issues, exercise/fitness/physical activity, research