Repetitive strain injuries (RSI) affect tens of thousands of people and cost workers and the US economy more than $7 Billion each year. It is believed that improper postures or movements made during repetitive tasks increase the risks of developing RSI. Muscle fatigue may be an important intermediary factor in this process, since muscle fatigue can induce changes in coordination, which may in turn increase the risks of RSI. The purpose of this R21 application is to develop new methods to track the changes that occur in both muscle function and coordination during fatiguing repetitive movements and to test the feasibility of these approaches. First, we will construct a device to simulate a task (sawing) known to induce changes in coordination after fatigue. We will establish appropriate task parameters to induce fatigue over 30 to 45 minutes. Second, we will develop an appropriate set of analytical tools for tracking fatigue from observed changes in coordination. We will extend existing nonlinear dynamics algorithms developed for tracking damage accumulation in mechanical systems. Because our approach tracks distortions in appropriately reconstructed state spaces, it can provide valid measures of the underlying (hidden) damage dynamics without the need for detailed physics-based mathematical models of either the system or damage dynamics. We will need to modify these algorithms, however, to accommodate the most prominent differences between mechanical and biological systems: noise, multiple time scale dynamics, and non-monotonic damage dynamics (i.e. biological adaptability). Finally, we will apply these methods, along with more traditional measures, to explore the time courses of changes in muscle function and motor coordination that occur during the sawing task. Thirty healthy subjects will perform the continuous sawing task until voluntary exhaustion under two conditions: more restricted and less restricted. We will test three hypotheses: (1) changes in local muscle fatigue precede changes in muscle coordination, which in turn precede overt changes in kinematics, (2) this sequence of events will be delayed in the less restricted condition, and (3) the nonlinear tracking approaches will reveal subtle changes in coordination that reflect underlying (hidden) changes in muscle fatigue state. This project will generate new insights into the nature and time course of the biomechanical and neural adaptations that occur during repetitive tasks and will provide the necessary foundation from which we will begin to develop improved diagnostic techniques for identifying early-onset (pre-clinical) RSI. We hope these efforts will one day help reduce the tremendous monetary and personal costs associated with these injuries [unreadable] [unreadable]