Criterion validity of a newly developed Apple Watch app ('MVPA') compared to the native Apple Watch 'activity' app for measuring criterion moderate intensity physical activity

Introduction
Many people fail to meet physical activity guidelines. One possible solution is wearable technology. Yet it is unclear if popular devices such as the Apple Watch can accurately measure intensity, which is a cornerstone of physical activity guidelines. We developed a bespoke Apple Watch app based on the use of relative intensity (%HRR), with the aim to determine if the bespoke app measures ‘moderate’ intensity more accurately compared to the native Apple Watch Activity app, using % oxygen consumption reserve (%VO₂R) as the criterion.

Methods
Seventy-four participants (18–65 years) attended the laboratory twice. During Visit 1, they completed medical screening, and measurements of resting heart rate, maximal oxygen consumption, and maximal heart rate on a motorised treadmill. During Visit 2, participants completed 5-minute treadmill bouts starting at 3.5 km.h⁻¹, increasing by 0.5 km.h⁻¹ until the bespoke app recorded 3 minutes at ≥40%HRR, and the native app recorded 5 minutes of ‘exercise’. Oxygen consumption and heart rate were recorded. Bayesian posterior distributions were used to compare the two apps.

Results
At the walking speed that the native app recorded exercise, the mean (95% HDI) %VO₂R was 33 (31-36)%, which is below moderate relative intensity (40%HRR). This is compared to a mean (95% HDI) of 43 (40-44)% for our bespoke app.

Conclusion
The bespoke app measured relative moderate intensity more accurately compared to the native app when compared to the %VO₂R criterion. Exercise guidelines and wearable devices should incorporate relative measures of physical activity to better individualise monitoring and prescription.