The topic of this book is the modeling of data uncertainty and knowledge for a health engineering problem such as the biomechanics of the musculoskeletal system. This is the first book on this subject. It begins with the state of the art in related topics such as data uncertainty, knowledge modeling, and the biomechanics of the musculoskeletal system, followed by fundamental and theoretical aspects of this field. Clinically relevant applications of musculoskeletal system modeling are then introduced. The book finishes with a chapter on practical software and tools for knowledge modeling and reasoning purposes.