The correct assessment of energy expenditure in very active individuals is important to ensure that dietary energy intake is sufficient. We aimed to validate a combined heart rate (HR) and motion sensor in estimating total (TEE) and activity energy expenditure (AEE) in males and females with high physical activity levels.Cross-sectional.Doubly-labelled water (DLW) was used to assess 7-day TEE in 12 male and female elite junior basketball players, aged 16–17 years. Resting energy expenditure (REE) was assessed with indirect calorimetry and AEE was calculated (AEE=TEE-RMR-0.1×TEE). Simultaneously, TEE and AEE were measured by combined HR and motion sensing. Individual HR calibration was performed with step-test. TEE and AEE were estimated from accelerometry and HR with individual (ACC+HR step ) and group calibration (ACC+HR group ).No mean differences were found between TEE and AEE from the ACC+HR step and ACC+HR group with DLW. TEE values (kJ/day) from ACC+HR group and ACC+HR step explained TEE from DLW by ∼60% and 53%, respectively whereas AEE (kJ/day) estimated by ACC+HR group and ACC+HR step explained 53% and 41% of the variability of AEE from the reference method. Concordance correlation coefficients for TEE and AEE using ACC+HR group were 0.74 and 0.69, correspondingly while for ACC+HR step values of 0.69 and 0.45 were found. Large limits of agreement were found for TEE and AEE using both ACC+HR group and ACC+HR step .ACC+HR models are a valid alternative to estimate TEE but not AEE in a group of highly active individuals however the considerable rate of equipment failure (∼50%) limits its usefulness.