The interest in this area has increased along with evidence that non-exercise activity and the associated energy expenditure (non-exercise activity thermogenesis, or NEAT) is associated with resistance to obesity (, ). The running wheel is commonly used in studies of obesity and energy balance as a surrogate for general activity-that is, the tendency of a rodent to be more or less active. Most rodents readily run in wheels, and it has become an uncomplicated, easily quantifiable measure of physical activity (). Measurement of physical activity in laboratory rodents has enabled the widespread use of the running wheel. Given the impact of wheel-running behavior on numerous overlapping systems that influence behavior and physiology, this review outlines the need for careful design and interpretation of studies that utilize running wheels as a means for exercise or as a measurement of general physical activity. We contend that wheel-running behavior represents factors in addition to rodents' tendency to be physically active, engaging additional neural and physiological mechanisms which can then independently alter energy balance and behavior.
As it relates to energy balance, wheel running engages several brain systems-including those related to the stress response, mood, and reward, and those responsive to growth factors-that influence energy balance indirectly. These behavioral processes include defensive behavior, predatory aggression, and depression- and anxiety-like behaviors. We contend that wheel-running activity should be considered a behavior in and of itself, reflecting several underlying behavioral processes in addition to a rodent's general, spontaneous activity. In fact, the presence of running wheels alters several aspects of energy balance, including body weight and composition, food intake, and energy expenditure of activity. While wheel running and physical activity in the absence of a wheel commonly correlate in a general sense, in many specific aspects the two do not correspond. Here, we will begin by systematically reviewing how running wheel availability affects physical activity and other aspects of energy balance in laboratory rodents.
There is no consensus on the nature of wheel-running activity or its underlying causes, however. Running wheels are commonly employed to measure rodent physical activity in a variety of contexts, including studies of energy balance and obesity.
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