A user-specific model of muscular activity can be used to control an external device based on muscular activity within a limb of a user. The user-specific model of muscular activity can include single movements and corresponding one or more primary muscle patterns. New single movements can be added to the user-specific model of muscular activity can be by a system that includes a processor by receiving user-specific EMG signals (including one or more EMG patterns that indicate a single movement); decomposing the user-specific EMG signals into the one or more EMG patterns in EMG feature space that indicate the single movement; and updating the user-specific model of muscular activity to include the single movement and corresponding one or more primary muscle patterns based on the one or more EMG patterns in EMG feature space.