42 strains of the BXD family (BXD43 – BXD103) and both parental strains (C57BL/6 and DBA/2) were born and raised at the EPFL in Switzerland prior to inclusion in this study. For each strain, 10 male animals were born and then separated evenly into two cohorts at 8 weeks of age: 5 animals per strain on a chow diet (6% kcal/fat, 20% protein, 74% carbohydrate) and 5 animals per strain on high fat diet (60% kcal/fat, 20% protein, 20% carbohydrate). Animals adjusted to the diet for 8 weeks, and then an intensive phenotyping metabolic phenotyping protocol was followed from 16 to 24 weeks of age (respiration, cold tolerance, oral glucose response, VO2max exercise, voluntary exercise, basal activity). Animals were communally housed until the last 5 weeks of the experiment, when the animals could rest. Animals were fasted overnight prior to sacrifice, which occurred between 9am and 11am after isoflurane anesthesia and perfusion. The quadriceps were the last tissue frozen in liquid nitrogen during the sacrifice, about 15 minutes after perfusion. Quadriceps were taken by cutting laterally at the knee.