Characteristics of growth-inducing exercise.

growth-inducing exercise in hamsters were studied by examining (a) dependence of this phenomenon on a specific activity device, (b) its species-specificity, and (c) features of the running pattern which produce optimal growth acceleration. Acceleration of growth by exercise occurs in hamsters running on either horizontal discs or in vertical wheels, and in gerbils running in rotating wheels, and is therefore neither device-or species-specific phenomenon. In contrast to the two rodent species demonstrating increases in the rate of weight gain at levels of voluntary activity between 10,000 and 30,000 RPD, rats running on either activity device, and gerbils and ground squirrels running on horizontal discs, generated low levels of activity and no evidence of increased somatic growth. At weight ranges associated with maximal acceleration of growth by disc exercise, hamsters ran at moderately high speeds of between 35 and 51 cm/sec for up to one hour without a pause. Their total daily activity exceeded 15,000 RPDs (5.9-8.5 km) and lasted between 5 and 10 hours. Prolonged voluntary activity at relatively low speed constitutes a sufficient condition for acceleration of somatic growth in two rodent species. Hamsters Gerbils Rats Ground squirrels Activity patterns Activity discs Rotating wheels THE relationship between physical activity and somatic growth is not well understood. Under some circumstances, exercise in animals and man promotes protein synthesis in skeletal [I0,11] and heart muscle [21], muscle hypertrophy [9, 12, 13], and growth of skeleton [6]. In the rat, on the other hand, exercise is associated with reduced food intake, weight loss [8,22] and no change in somatic growth [24]. A clear example of exercise-induced somatic growth was recently described in golden hamsters [1]. Hamsters which have attained the relatively slower adult rates of growth, display increased somatic and skeletal growth [4] accompanied by hyperphagia [ I] and increased secretion of growth hormone [3] during, and immediately following, the exposure to voluntary exercise on horizontal activity discs. The present study examined whether (1) growth acceleration by exercise in hamsters was dependent on the specific type of activity device used, (2) increased growth was a response to exercise peculiar to golden hamsters, and (3) running activity eliciting greatest growth exhibited a characteristic pattern. The first two questions were addressed, respectively, by studying growth rates of hamsters running in two types of activity devices, a horizontal disc, and a rotating wheel, and by examining the weight changes of three other species of rodent …