Validity of the apple iPhone^®/iPod Touch^® as an accelerometer-based physical activity monitor: A proof-of-concept study

Background: The popularity of smartphones has led researchers to ask if they can replace traditional tools for assessing free-living physical activity. Our purpose was to establish proof-of-concept that a smartphone could record acceleration during physical activity, and those data could be modeled to predict activity type (walking or running), speed (km•h-1), and energy expenditure (METs). Methods: An application to record and e-mail accelerations was developed for the Apple iPhone^®/iPod Touch^®. Twentyfive healthy adults performed treadmill walking (4.0 km•h ^-1 to 7.2 km•h^-1) and running (8.1 km•h ^-1 to 11.3 km•h^-1) wearing the device. Criterion energy expenditure measurements were collected via metabolic cart. Results: Activity type was classified with 99% accuracy. Speed was predicted with a bias of 0.02 km•h^-1 (SEE: 0.57 km•h^-1) for walking, -0.03 km•h^-1 (SEE: 1.02 km•h^-1) for running. Energy expenditure was predicted with a bias of 0.35 METs (SEE: 0.75 METs) for walking, -0.43 METs (SEE: 1.24 METs) for running. Conclusion: Our results suggest that an iPhone/iPod Touch can predict aspects of locomotion with accuracy similar to other accelerometer-based tools. Future studies may leverage this and the additional features of smartphones to improve data collection and compliance.