Validity of the iPhone M7 motion coprocessor to estimate physical activity during structured and free-living activities in healthy adults

Modern smartphones such as the iPhone contain an integrated accelerometer which can be used to measure body movement and estimate the volume and intensity of physical activity
Objectives: The primary objective was to assess the validity of the iPhone to measure step count and energy expenditure during laboratory-based physical activities. A further objective was to compare free-living estimates of physical activity between the iPhone and the Actigraph GT3X+ accelerometer.
Methods: Twenty healthy adults wore the iPhone 5S and GT3X+ in a waist-mounted pouch during bouts of treadmill walking, jogging, and other physical activities in the laboratory. Step counts were manually counted and energy expenditure was measured using indirect calorimetry. During two weeks of free-living, participants (n=17) continuously wore a GT3X+ attached to their waist and were provided with an iPhone 5S to use as they would their own phone.
Results: During treadmill walking, iPhone (703 ± 97 steps) and GT3X+ (675 ± 133 steps) provided accurate measurements of step count compared to the criterion method (700 ± 98 steps). Compared to indirect calorimetry (8 ± 3 kcal·min−1), the iPhone (5 ± 1 kcal·min−1) underestimated energy expenditure with poor agreement. During free-living, the iPhone (7990 ± 4673 steps·day-1) recorded a significantly lower (P < 0.05) daily step count compared to the GT3X+ (9085 ± 4647 steps·day-1).
Conclusions: The iPhone accurately estimated step count during controlled laboratory walking but records a significantly lower volume of physical activity compared to the GT3X+ during free living.