An activity monitor using four accelerometers (two on the trunk and one on each thigh) can distinguish up to 20 everyday postures with high sensitivity and specificity [27]. These include various forms of sitting, standing and lying, as well as general movement, walking, ascending and descending stairs, cycling and running. This monitor has been validated on normal subjects, amputees, failed back surgery patients, congestive heart failure patients and in psychophysiological studies [28–30]. Validation studies have also been performed in a home setting [30].