In this paper a novel activity monitoring device based on the measurement of the body accelerations is introduced. The device consists of a smart sensor platform integrating a tri-axial acceleration sensor, a GPS module, a barometric pressure sensor as well as the communication to a commercial heart rate monitor via the ANT protocol. According to the different sensors the device has the capabilities to perform an activity classification and quantification of the energy expenditure in outdoor as well as in indoor environments. Based on the acceleration measurement it is possible to estimate the locomotion speed. By means of several experimental setups the accuracy of the speed estimation is evaluated. Further a method for estimation of the energy expenditure based on the estimated locomotion speed is introduced, investigated and compared to a heart-rate based method. It turns out that the locomotion speed can be estimated quite accurately, but for the estimation of the energy expenditure the acceleration based method tends to overestimate the energy expenditure compared to the heart-rate based method.