Energy expenditure in males with mental retardation.

Previously, we reported that males with mental retardation (MR) (MR group) expended more energy than males without MR (control group) at a given work load. The precise physiological mechanisms for this difference remain unclear. Using the same set of subjects (23 age-, height-, and weight-matched male pairs, mean age: 36.3 y), we examined possible causes for the observed metabolic difference by monitoring physical movements and evaluating the metabolic capability of the skeletal muscles. In the supine position when no body movements were detected for any subjects, oxygen consumption (VO2) and heart rate (HR) were not markedly different between the MR and the control groups. By contrast, in the sitting and standing positions and during walking at 30 and 50 m/min, when significantly larger body movements were monitored in the MR group, VO2 and HR were significantly higher in this group than in the control group. Linear regression analyses, performed separately in the MR and control groups, revealed that the slope of the regression line of HR on relative exercise intensity (%VO2max) during walking, that of VO2 on walking speeds, and that of VO2 on the number of steps in the MR group were almost identical with those in the control group. These results suggest that the capability of skeletal muscles was not so different between the two groups. Thus, the high energy expenditure in the MR group was suggested to be a result of excessive body movements rather than an intrinsic incapability of skeletal muscles.