The document discusses the ActivPal, an objective monitor used to collect activity data. It is a small, thigh-worn device that can continuously collect data for 7 days. The ActivPal uses algorithms to classify posture (sitting/lying vs upright) and stepping based on static acceleration from gravity and dynamic acceleration from movement. It provides measures of time spent sitting/lying, standing, walking and step count. Studies show the ActivPal has excellent reliability and validity for measuring physical activity in populations ranging from children to older adults.