Abstract: A wearable device for monitoring physical activity of a user, the wearable device being reversibly attachable to a chest strap and an arm strap. The wearable device comprises an ECG sensor arranged to collect ECG measurements of the user only when the wearable device is attached to the user's chest by the chest strap, and a PPG sensor arranged to collect PPG measurements of the user when the wearable device is attached to the user's arm or wrist by the arm strap. The wearable device also comprises control circuitry arranged to switch the wearable device between an ECG only mode and a PPG only mode. In the ECG only mode, the control circuitry is configured to control the ECG sensor to collect ECG measurements but prevent the PPG sensor from initiating PPG measurements. In the PPG only mode, the control circuitry is configured to control the PPG sensor to initiate PPG measurements but prevent the ECG sensor from collecting ECG measurements.

Abstract: A method, system and computer program for predicting a VO2max measurement for a subject, the method comprising: collecting a heart rate recovery value, measured over a first period starting at the end of a fitness test exercise completed by the subject, wherein the fitness test exercise comprises a plurality of phases, each phase having a predetermined duration and each phase being associated with a respective heart rate band, and during each phase, the subject adjusts their effort to maintain their heart rate in the respective heart rate band, and calculating a predicted VO2max measurement using the heart rate recovery value.