Validity and reliability issues in objective monitoring of physical activity.

Major technical advances have occurred within the last 10 years in the field of physical activity monitoring. The biggest one is real-time data acquisition, and the development of computer microchips that allow vast amounts of information to be stored and later recalled. It is evident that no single motion sensor will provide an accurate estimate of energy expenditure across all activities. Future directions for research include the use of combined instruments such as an accelerometer plus questionnaire, multiple motion sensors, or dual HR-motion sensor technology.