Many patients with physical disabilities are limited by poor endurance. This could be due to the level of deconditioning, the disease or injury, and/or the demands of the activities they must perform. Occupational therapists historically have been involved in prescribing activity and teaching energy conservation. The purpose of this paper is to (1) review briefly the study of energy cost and its significance to occupational therapy practice and (2) to describe the Physiological Monitored Evaluation (PME). The PME can be used to assess the demands of activities on patients by measuring the heart rate, blood pressure, and symptomatic responses to activities. This procedure can be used to obtain baseline measurements to compare and document progress, set safe guidelines for activity so that the patient may increase endurance without becoming "overstressed", and collect data for research.