Description (Adapted from Application): The aim of this investigation is to examine lifetime variation in locomotory ability and the aerobic capacity to support locomotion. Lifetime variation will be considered with respect to age, gender, reproductive status, and febrile effects. The project will ascertain the extent of individual consistency in locomotor performance under different conditions, and whether poor locomotor ability during one part of the lifetime (e.g., young age) is predictive of debilitation at another (e.g., old age). It will also clarify how changes in aerobic capacity and lean mass affect locomotor ability. A rodent model, the house mouse (Mus musculus), will be used for studies of locomotion and aerobic capacity during exercise. Extensive longitudinal studies of locomotor performance, with appropriate controls, will be conducted that would be difficult to conduct with human subjects. There will be tests of locomotor ability (maximal sprint speed, maneuverability, etc.), aerobic capacity, and voluntary performance (which reflects motivation) for individuals of different age and status. One set of tests will compare aging effects on locomotor performance and aerobic capacity for males and females. Among females, there will be additional tests of effects of reproductive condition (non-reproductive, pregnant, lactation, post-reproductive) on locomotor performance. Lastly, there will be tests of the effect of short-term fever on locomotor ability and aerobic capacity. These studies will measure the extent of variation in locomotor responses during the lifetime, and whether future performance capabilities can be assessed from current abilities.