I've recently read a couple of papers on exercise and mitochondria (see e.g., http://www.sciencedirect.com/science/article/pii/S0891584907008349), in which state 4 and state 3 respiration rates and ROS production are assessed in vitro after exercise has been performed (i.e., rat heart mitochondria are isolated). My question: given that it takes 1 hr. for the mitochondria to be removed from the animal and isolated, that additional (saturating) substrate is added before respiration parameters are measured, and that the medium is saturated to the ambient oxygen concentration (much higher than in vivo), is this type of experiment really valid to assess mitochondrial function in vivo in active animals? Opinions and/or pointers to resources that clarify how useful these measurements are would be helpful. Thanks! J.