The Molecular Transducers of Physical Activity Consortium (MoTrPAC) was established to map out the molecular responses to acute and chronic exercise training, at multiple levels of molecular analysis. The MotrMito project is generating measurements of the mitochondrial response to exercise training, to assess whether certain molecular markers are associated with that mitochondrial response. We are using 31-phosphorus magnetic resonance spectroscopy (31P MRS) to measure the mitochondrial response to exercise training, among MoTrPAC participants at four sites (Pennington Biomedical, AdventHealth, University of Colorado Denver, and University of Texas San Antonio)..