The Lines-of-Defense model postulates that older adults should engage in important health goals and behaviors for as long as possible and adjust them downwardly only when they become impossible to pursue. This process is thought to be supported by goal engagement and self-protective control strategies. We tested this model in a 4-year longitudinal study of 236 older adults by predicting the maintenance of physical activity using accelerometers. We hypothesized that older adults would exert shifts from more strenuous (e.g., vigorous and moderate intensity) to less strenuous (e.g., light intensity) physical activity over time. In addition, we expected that these processes would be supported by the use of health-specific control strategies. Multilevel modeling revealed that older adults experienced declines in moderate and vigorous physical activity but increases in light physical activity. Health engagement predicted an accelerated increase in light physical activity, and exerted substantial, but longitudinally decreasing, benefits for moderate physical activity. Health-related self-protection, by contrast, predicted the maintenance of vigorous physical activity over time. These results support the Lines-of-Defense model by demonstrating that control strategies can predict the maintenance of older adults' physical activity levels.