In self-directed chronic pain physical rehabilitation it is important that the individual can progress as physical capabilities and confidence grow. However, people with chronic pain often struggle to pass what they have identified as safe boundaries. At the same time, over-activity due to the desire to progress fast or function more normally, may lead to setbacks. We investigate how musically-informed movement sonification can be used as an implicit mechanism to both avoid overdoing and facilitate progress during stretching exercises. We sonify an end target-point in a stretch exercise, using a stable sound (i.e., where the sonification is musically resolved) to encourage movements ending and an unstable sound (i.e., musically unresolved) to encourage continuation. Results on healthy participants show that instability leads to progression further beyond the target-point while stability leads to a smoother stop beyond this point. We conclude discussing how these findings should generalize to the CP population.