Detection of the erect position in the freely-moving human: sensor characteristics, reliability, and validity

The present report systematically examined a means to electronically detect the erect position in the human in the natural setting. The detector was based on pressure changes in a glycerin filled tube attached to a subject's leg, and it unobtrusively and continuously measured the relative vertical distance between the hip and leg. Initial experiments established the reliability of the sensor system as a function of 1. different sizes of the tubing, 2. different amounts of air in the glycerin and 3. different ambient temperatures (6 degrees-32 degrees C). Then, in a laboratory study of normal adults, the detector was seen to discriminate sitting from standing and (when activity data were included) these two behaviors, in turn, from walking. The detector also accounted for significant differences in HR seen in the standing, as opposed to the sitting, position. In addition, when subjects carried the detector during their daily activities and provided information about their activities using an experience sampling procedure, sitting was discriminated from standing and walking with acceptable diagnostic characteristics. Thus, sitting was detected with a sensitivity of 86.1% (correct detection of all occasions when sitting actually occurred). a positive predictive value of 92.6% (occasions that the detector was right when it indicated sitting), and a negative predictive value of 80.7% (occasions that the detector was right when it indicated sitting). Finally, we demonstrated in two additional ways the direct benefit of our detector in behavioral studies in the natural environment. First, with the detector, we could confirm that a subject had performed simple activities and errands while not under close supervision. Second, cigarette smoking in the natural environment was shown to increase HR, but only when the subjects were sitting. It was concluded that our detector can be effectively applied to the identification of the sitting vs. the erect position in humans in the natural setting, and that this information may be necessary to interpret behavioral and physiological effects seen in such subjects.