Neuromuscular disorders such as stroke or multiple sclerosis have consequences that affect the control of muscular force. Even though the origin of many disorders may be centrally determined, the fundamental unit of force control, the motor unit, will reflect the consequences. There is abundant information on control strategies such as the rules governing the recruitment of motor units. What we do not understand, however, is the nature and meaning of variability that exists within motor unit discharge behavior. The fluctuations in motor unit discharge during sustained muscular contractions have received very little attention compared to the study of variability in settings such as cardiovascular research. The mere appearance of random noise in a signal has often resulted in premature conclusions. However, it may be the case that conditions such as pathological tremor result from structure that exists within motor unit discharge fluctuations. Indeed, pilot data indicate that non-random structure is inherent in these fluctuations and I wish to further explore the characteristics and meaning of motor unit discharge variability. Motor unit discharge variability will be compared among muscles that are designed for precision control or gross force production. The effects of muscular force and sensory manipulations will be investigated in both muscle types. Each of these experimental conditions will be performed by older and young individuals to evaluate the impact of known age-related changes in motor unit morphology on motor unit discharge variability. The proposed research will systematically investigate analytical techniques that are new to motor unit research. The information content of such techniques will be compared to that provided by established analytical methods. Like the research on heart rate variability, I hope to provide effective diagnostic tools for evaluating neuromuscular health.