Locomotor Control of Limb Force Switches from 1 Minimal Intervention Principle in Early Adaptation to Noise Reduction in Late Adaptation

27 During movement, errors are typically corrected only if they hinder performance. Preferential correction 28 of task-relevant deviations is described by the minimal intervention principle, but has not been 29 demonstrated in the joints during locomotor adaptation. We studied hopping as a tractable model of 30 locomotor adaptation of the joints within the context of a limb-force-specific task space. Subjects hopped 31 while adapting to shifted visual feedback that induced them to increase peak ground reaction force (GRF). 32 We hypothesized subjects would preferentially reduce task-relevant joint torque deviations over task33 irrelevant deviations to increase peak GRF. We employed a modified uncontrolled manifold analysis to 34 quantify task-relevant and task-irrelevant joint torque deviations for each individual hop cycle. As would 35 be expected by the explicit goal of the task, peak GRF errors decreased in early adaptation before 36 reaching steady state during late adaptation. Interestingly, during the early adaptation performance 37 improvement phase, subjects reduced GRF errors by decreasing only the task-relevant joint torque 38 deviations. In contrast, during the late adaption performance maintenance phase, all torque deviations 39 decreased in unison regardless of task relevance. In de-adaptation, when the shift in visual feedback was 40 removed, all torque deviations decreased in unison, possibly because performance improvement was too 41 rapid to detect changes in only the task-relevant dimension. We conclude that limb force adaptation in 42 hopping switches from a minimal intervention strategy during performance improvement to a noise 43 reduction strategy during performance maintenance, which may represent a general control strategy for 44 locomotor adaptation of limb force in other bouncing gaits, such as running. 45