Mitochondria are the powerplants of your cells, churning away to turn food into ATP, the common currency of cellular energy. They are also a factor in aging, via the creation of damaging free radicals (such as reactive oxygen species) as a side-effect of their chemical processing. Mitochondrial uncoupling is much as it sounds; a feedback mechanism in which processing is disconnected from ATP production; energy from food goes elsewhere, as heat for example. Because this affects free radical production, it seems to be important in tissue aging: "Faster aging is predicted in more active tissues and animals because of greater reactive oxygen species generation. Yet age-related cell loss is greater in less active cell types, such as type II muscle fibers. Mitochondrial uncoupling has been proposed as a mechanism that reduces reactive oxygen species production and could account for this paradox between longevity and activity. ... These results reject respiration rate as the sole factor impacting the tempo of cellular aging. Instead, they support mild uncoupling as a mechanism protecting mitochondrial function and contributing to the paradoxical longevity of the most active muscle fibers."