Running Forward

content in muscle fibers (Figure How does this process occur? Research done during the last 10 years has begun to answer this question at the molecular level (Figure Much of this work has relied on the study of genetically modified mice (eg, that depicted in Figure Rodents are natural-born endurance exercisers. Provided with in-cage running wheels, most mice will voluntarily run 5 to 8 km per night. Although this level of exercise does not precisely model the human condition, the ability to model endurance exercise in an organism amenable to genetic manipulation has proven invaluable. Conceptually, the adaptation process can be divided into mechanisms that sense endurance activity and mechanisms that carry out the desired adaptations (Figure