A seamless model-based methodology for requirements specification becomes increasingly the key to successful software systems engineering. The enhanced activity diagrams of UML 2 offer a promising intuitive modeling approach for requirements capturing. On this basis, we propose a formal syntax and operational semantics and extend activity diagrams to LADs (Live Activity Diagrams) by liveness-enriched constructs inspired by LSCs (Live Sequence Charts) for stating precise execution modalities. We aim at a tool-supported execution of LAD specifications allowing for early simulation and validation of requirements. We present an implementation of an LAD execution engine and describe the there upon suggested requirements engineering approach by means of an automotive case study.