Table 2.

The proposed best-practice protocols for the validation of wearable-derived VO_2max

Domain	Variable	Protocol consideration	Reporting consideration
Target population	Population	If purpose is to validate wearable-derived VO2max for the general healthy population, a broad heterogeneous sample should be used If purpose is to use wearables in specific clinical applications, validation should be performed in homogenous samples	Report the inclusion/exclusion criteria defining the target population and recruitment methodology and provide basic demographic information (e.g., age, height, weight, or BMI)
	Age	Validation protocols targeting a general healthy population should include the main age ranges: children (< 12 years), adolescents and adults (13–64 years), and older adults	Average and range of sample age should be reported
	Sex	Include an equal sample of males and females within the study	The number of female and male participants should be reported
	Sample size	For those studies aimed at testing the accuracy of a given device, a sample size calculation should be performed based on the previously published data according to Lu et al.[43]. If no previous data are available or this is not the focus of the evaluation, we advise to include a minimum of 15 participants per age group according to previously published recommendations on wearables-derived health measures [19, 20]	Describe the sample size calculation if included If sample size calculation is not feasible, cite previous literature supporting the inclusion of a recommended sample size Describe the flow of sample size recruited and analyzed
Reference standard	Indirect calorimetry	The gold standard for the assessment of VO2max is a maximal graded exercise test, performed in laboratory conditions with indirect calorimetry [7] Any brand of metabolic cart is accepted when reporting validity and reliability, as well as measuring both VO2 and VCO2 during expiration The metabolic cart should be properly calibrated before the VO2max assessment according to manufacturer’s instructions	Indicate if indirect calorimetry was used Report the metabolic cart used, the type of recording technology (e.g., breath-by-breath), and whether the metabolic cart used is valid and reliable Describe the calibration process of the metabolic cart
Index measure	Wearable devices	Consumer wearables should be worn in ecological body locations in accordance with the manufacturer’s instructions. If wrist worn, a maximum of 2 devices per wrist should be used at the same time, with placement being randomly counterbalanced between participants Wearable devices can measure HR with PPG and/or chest-strap technology, and this may have an impact on the VO2max estimation	Report the placement of the device and information on order of placement if more than one wrist worn device is used Specify whether HR was recorded with PPG on wrist/arm (or others) or chest-strap technology
Testing protocols and conditions for both reference and index measure	Maximal graded exercise testing with indirect calorimetry	The accepted protocol to assess VO2max is a maximal graded exercise testing evaluated in laboratory conditions Maximal test requires participants to perform to the point of volitional fatigue, and at least two accepted criteria are recommended to ensure that participants are reaching the maximum effort during the tests. The ACSM proposes several maximum-effort criteria that can be used [7] A verification phase after the maximal test is recommended to compare both VO2max results. Schaun [55] provides an update of the literature on how to perform this verification phase Any type of exercise testing is accepted (e.g., walking, running, or biking) as long as it adapts to the type of activity in which the consumer wearable is intended to be validated In populations unable to perform maximal test, submaximal exercise-based equations might be an alternative to predict VO2max, since overall these have demonstrated a moderate to strong relationship with maximal tests. However, authors should select the most appropriate equation for their target population [9, 70]	Report whether maximal or submaximal exercise test is being used. In the case of submaximal test, provide a rationale of its implementation and specify the exercise-based equations used In maximal exercise test, report the need for reaching volitional fatigue and indicate the maximum-effort criteria included (at least two criteria) Report the type of exercise testing used as well as its characteristics (e.g., increase in the ramp inclination in treadmill tests or power increase in cycle-ergometer tests)
	Standardized conditions before the reference and index measure	Participants should not consume a significant caloric uptake at least 2 h before the exercise test No caffeine, similar stimulants, or alcohol should be consumed 24 h before the exercise test No intensive sports activities should be performed 48 h before the exercise test Participants should not take any medication that may alter the normal HR response to a maximal exercise The exercise test should begin with at least 2–3 min warm-up	Report the standardized conditions followed by participants Describe the warm-up characteristics
	Wearable device set up	Follow the manufacturer’s instructions for the VO2max estimation protocol Provide all the information required by the device, since in some cases this is used to improve the VO2max estimation If the device has the option to select a specific exercise mode (i.e., indoor running, cycling, walking, etc.), choose the mode that best reflects the activity that is going to be performed In those wearable devices using GPS data, it is recommended to perform the test outdoor to ensure a proper GPS connection	Report the device model and version Report what demographic details are input into the device per participant for initiation Report what mode (if any) is used during each activity (i.e., indoor running, cycling, walking, etc.) If GPS is used, indicate that the satellite connection was checked before the exercise test
Data processing	Indirect calorimetry processing	If a time average is used to reduce variability in the indirect calorimetry data, typically this should be between 15 and 30 s [26] If a breath average is used, a 15-breath running average is recommended [26] Confirm that the maximum-effort criteria were met when interpreting the VO2max values	Report the time-averaged or breath-averaged sampling used Report whether maximal or peak VO2 is being assessed Detail the data processing conducted in the VO2max interpretation
	Time interval between evaluations	If resting conditions are used for wearable VO2max estimation, no time interval is needed before the reference VO2max test is performed If the wearable test involves exercising, between 24 and 48 h is recommended to ensure an effective muscle recovery. If the maximal test is evaluated first, a time interval between 48 and 72 h is recommended [7]	Report the time interval between both assessments
Statistical analysis	Statistical tests	To assess device accuracy, the following statistical tests should be performed: 1. Bland–Altman with limits of agreement 2. Least product regression of the difference against the means 3. MAPE Subgroup analysis is encouraged if sample size allows. (e.g., sex, age category, ethnicity, BMI)	Include Bland–Altman plots for a visual inspection of the validity results Binary conclusions about the validity of the device should not be made if a formal sample size analysis has not been conducted

ACSM American College of Sports Medicine, BMI body mass index, HR heart rate, MAPE mean absolute percentage error, PPG photoplethysmography, VO_2max maximal oxygen consumption