Obesity in rats appears to impair normal muscle function, a finding that may help explain why it is hard to lose weight, U.S. researchers say. Rudolf J. Schilder, an American Physiological Society postdoctoral fellow in physiological genomics at Pennsylvania State College of Medicine, and colleagues examined whether normal mammalian skeletal muscle perceives the amount of weight it is carrying and if it makes physiological adjustments to compensate for more or less weight. The study, published in the Journal of Experimental Biology, used both healthy and genetically obese rats to determine how the expression of troponin T a gene that codes for a protein essential to muscle function varied as rats gained weight. The researchers demonstrated troponin T expression varied with body weight during normal growth. Then they artificially increased the body weight of one group of rats by 30 percent using a custom-made weighted vest. The externally applied weight caused a shift in the muscle troponin T expression, matching that of animals whose actual body weight was 30 percent higher. However, troponin T expression did not respond to a similar increase in body weight in the obese rats. "These results may explain why muscle strength and locomotion are impaired in obese humans, and hence perhaps why it is so difficult to lose excess weight and recover from obesity," Schilder says.