AIM: Accurate estimation of energy expenditure (EE) is important in human and animal behavior analysis. Rate of oxygen consumption (VO2) reflects EE during aerobic metabolism but is not always convenient. Alternative methods include heart rate (HR) and overall dynamic body acceleration (ODBA). A favorable ODBA-VO2 relationship was recently reported but the strength of association between VO2, ODBA, HR and its variability (HRV) is less clear. METHOD: Fifteen young (23+/-4 years) healthy males of similar aerobic fitness (maximal oxygen uptake, VO2max=49.7+/-8.5 mL.kg(-1).min(-1)) carried out progressive maximal exercise. ODBA, HRV and V虈O2 were recorded continuously. Relationships between ODBA, HRV and V虈O2 were explored using regression methods. RESULTS: VO2 was strongly related to ODBA and RR during walking (R=0.45,0.30; P<5x10(-5)) and running (R=0.60,0.38; P<5x10(-5)). HRV was related to VO2 during walking only (R=0.11-0.26; 0.005<P<5x10(-5)). A strong ODBA-RR relationship during walking (R=0.45; P<5x10(-5)) was diminished during running (R=0.25; P<5x10(-5)). CONCLUSION: ODBA is a stronger proxy for EE than RR or HRV, especially during running gaits. HRV is weakly related to EE and cannot be recommended for its estimation. ODBA and RR are relatively easily measured but careful attention to gait is imperative as it changes these relationships markedly.