Introduction

During the past decade, there was an exceptional development of microelectronics and computer systems, enabling sensors and mobile devices with unprecedented characteristics. Their high computational power, small size, and low cost allowed people to interact with these devices as part of their daily living. That was the genesis of Ubiquitous Sensing , an active research area with the main purpose of extracting knowledge from the data acquired by pervasive sensors [91]. Particularly, the recognition of human activities has become a task of high interest within the field, especially for medical, military, and security applications. For instance, patients with diabetes, obesity, or heart disease are often required to follow a well-defined exercise routine as part of their treatment [67]. Therefore, recognizing activities such as walking, running, or resting becomes quite useful to provide feedback to the caregiver about the patient’s behavior. Likewise, patients with dementia and other mental pathologies might be monitored to detect abnormal activities and thereby prevent undesirable consequences [114]. An interactive game or simulator might also require information about which activity the user is performing in order to respond accordingly. Finally, in tactical scenarios, precise information about the soldiers’ activities along with their locations and health conditions is highly beneficial for their performance and safety. Such information is also helpful to support decision making in both combat and training environments. Given the relevance of HAR and its applications, an overview of the most noticeable approaches is presented next.