Why Exercise Does Not Work Equally Well for Everyone: Life-Course Conditions and the Limits of Precision Medicine

Advances in molecular profiling and digital health technologies have accelerated efforts to develop precision approaches to physical activity. Initiatives such as the Molecular Transducers of Physical Activity Consortium (MoTrPAC), alongside wearable-based monitoring, promise to tailor exercise prescriptions to individual biological responses. Yet a central paradox remains: the biological systems activated by exercise are the same systems implicated in chronic disease under conditions of psychosocial stress. Why do similar biological processes produce adaptive benefits in some contexts and pathological outcomes in others? This paper argues that precision approaches to exercise are limited by their focus on biological signals without sufficient account of the conditions under which those signals are generated and resolved. These signals are the temporal organization of physiological activation and recovery, and the life-course conditions we call "biography. Biography is not merely a modifier of exercise effects but a component of the mechanism itself. Because access to recovery, autonomy, and meaningful activity is socially stratified, the benefits of exercise are unevenly distributed, a limitation that increasingly granular molecular measurement alone cannot resolve.