Assessment and training of the endurance capacity of back muscles are reported to be more important than strength in the prevention and rehabilitation of low- back pain. However, back extensor muscles'' endurance has been less investigated in health and disease. The understanding of the determinants for the performance of isometric back extension endurance test may allow accurate interpretation of the back function test. This book, therefore provides an insight into the determinants of back extenor muscles'' endurance in healthy adults. The set of reference values for isometric back extensor muscles'' endurance and the influencing factors established in this book can be used to compare a patient''s score at intake and also serve as clinical targets in rehabilitation.