When designing and evaluating new interactive systems, we are quick to discourage and reject new technical opportunities that would cause fatigue. This is especially constraining as we design evermore body-based devices and experiences, where, initially, users' arms can feel heavy and sensations of fatigue be more acute. We argue that our simplistic view and avoidance of fatigue discredits opportunities around adaptation, training, optimisation, and the resilience built through expertise and repeated use. Despite advances in wearable and physiological sensing and modelling, there is still much room for developing a more nuanced view of fatigue. From its effects on low-level action-perception loops, through to impacts on decision-making in social-technical systems, to modelling and predicting its onset, development and experiential properties, significant work remains for the Human-Computer Interaction (HCI) community and beyond. This Dagstuhl Seminar aims to explore the role of fatigue in body-based interactions, bringing together experts from HCI, biomechanics, ergonomics, and cognitive psychology.