To evaluate validity and reliability of consumer-grade optical heart-rate devices we aim to compare these devices with medical-grade ECG. The design includes an extended and practical validation protocol for consumer-grade optical heart rate devices in laboratory and ambulatory free-living conditions using a medical-grade ECG device as a criterion measure.

Candidate consumer-grade wearables are selected according to available data from manufacturers, available scientific literature and our team's own practical experience.

First, a mobile 12-lead ECG (Amedtec ECGpro, Aue, Germany) device is placed on the participant according to recommendations of medical societies and the manufacturer's instructions. Additionally, the participant wears four optical heart rate devices simultaneously. Each device will be placed according to the manufacturer's instructions, The devices will be randomly applied on the dominant or non-dominant wrist and to the dominant and non-dominant arm, respectively. All participants will be handed an instruction sheet and video how to place each device in case it is displaced and how to check that data collection is working.

The 13-min laboratory protocol will be performed in keeping with an expert statement of the INTERLIVE Network and starts with a session mimicking lifestyle activity that consists of: 1 minute of quiet sitting, 2 minutes of upper body activities, 1 minute of quiet sitting, 30 seconds of transition to treadmill, 2 minutes of walking at self-selected speed, 30 seconds of transition to chair, 2 minutes of seated typing, 30 seconds of transition, 2 minutes of full body daily living activities including vacuum cleaning and ironing, and 1 minute of quiet sitting.

After completion of the lifestyle activities test, participants proceed with the treadmill step test. The treadmill step test protocol starts with 1 minute of quiet standing, 3 minutes of (4 km/h), 30 seconds of rest, 3 minutes of running (6 km/h) and an increment of 2 km/h at each stage with 30 seconds of rest between each stage until volitional exhaustion. Capillary blood samples will be taken from the earlobe to assess blood lactate concentration after each stage of the exercise test. Dependent on physical fitness 3 to 8 blood lactate samples will be taken. A breath-by-breath spiroergometry device will measure gas exchange, ventilation and maximal oxygen uptake during the exercise test using a metabolic analyzer. After termination of the exercise test subjective exhaustion will be assessed by the BORG scale. The 12-lead ECG will allow ruling out clinically relevant coronary artery disease and/or arrhythmias, and identify maximum heart rates from which intensity zones will be derived. Test termination will be followed by a 5-min cool down period at walking speed (3 km/h) on the treadmill. Throughout the treadmill test the participant is secured with a safety belt to prevent any injuries in case of falling.

Directly after the 5-min cool down, exercise testing on the cycle ergometer starts, beginning with 1 minute of quiet sitting. Then load is increased until the participant exercises at low intensity (57-63% of maximum heart rate obtained in the treadmill test) which will be maintained for at least 2 minutes, followed by 1 minute of rest, 2 minutes of moderate intensity (64-76% of maximum heart rate), 1 minute of rest, 2 minutes of vigorous intensity (>77% of maximum heart rate) followed by 1 minute of cool-down. All activity intensity zones are defined according to medical guidelines.

For the free-living part the electrodes of an ambulatory 3-lead-ECG (Amedtec ECGpro, Aue, Germany) are placed according to the manufacturer's instructions. Additionally, the participant wears four optical heart rate devices simultaneously.

The measurement of all devices starts simultaneously between 8:00 and 11:00 a.m. and is recorded for 24 hours. The only instruction the participants get are to walk for six minutes on a flat terrain as fast as possible once in the afternoon. At the end of the 24-hour period, participants stop all optical heart rate devices and return the devices to the University Institute of Sports Medicine, Prevention and Rehabilitation. There, the lifestyle activities and cycle ergometry laboratory testing protocol of day 1 will be repeated for reliability assessment. Finally, participants will be asked to fill out the IDEA-Fast Technology Experience Questionnaire (ITEQ) questionnaire to obtain information on the usability of the devices.

The 3-lead Holter-ECG will be analyzed by a licensed cardiologist for relevant arrhythmias.

Inclusion Criteria:

• Age between 35 and 80 years
• All sexes
• Able to perform physical activity according to the study protocol
• Written informed consent

Exclusion Criteria:

• Acute infections and/or diseases
• Diseases limiting walking distance and/or physical activity outlined in the study protocol
• Atrial fibrillation
• Clinical signs of chronic venous insufficiency ≥ 1 at upper extremities
• Obesity WHO grade 2 or more (Body Mass Index >35 kg/m²)
• Skin wound or tattoo where wearables are placed
• Pregnancy