Surface electromyography (sEMG) represents the electrical activity generated in the muscle fibres in response to the activation provided by the innervation of motor neurons. An sEMG device amplifies, decomposes, and filters the electrical signals that occur during muscle contraction. In this book, Chapter One provides an overview on two decades of sEMG research in swimming and water polo conducted by the Faculty of Sport and Porto Biomechanics Laboratory, both from the University of Porto, Portugal. Chapter Two reviews the clinical use of sEMG in the fields of kinesiology and rehabilitation. Chapter Three examines the use of sEMG signals as a natural control interface. Chapter Four explains the use of SEMG to examine changes in muscular activation that are provoked by orthopedic insoles.