Recent advances in muscle biology have greatly expanded our knowledge of how muscle functions in health and disease and how muscle adapts to a variety of stimuli. The purpose of this review is to consider how this information can be applied to the practice of physical therapy. The opinions and ideas represent those of a clinician who is also involved in research on the functional performance of muscle and how performance relates to basic biological mechanisms. The review examines selected aspects of muscle physiology, muscle mutability, and problems that arise when basic research material must be generalized for clinical practice.