Recommendations to reduce sedentary time and increase moderate-to-vigorous physical activity form the core of current public health strategies to improve population health. However, the population-level effect of such changes has not been fully described by accelerometer-based measures of physical activity. A previous accelerometer-based study of US adults estimated that 6.9% of deaths could be prevented annually if moderate-to-vigorous activity in the population increased by 10 minutes per day. This commentary addresses the quantification of the impact of physical activity on public health.