What is an outcome of using a warm-water bag for treatment?

Using a warm-water bag for treatment can significantly increase blood circulation in the targeted area. The application of warmth causes blood vessels to dilate, leading to enhanced blood flow. This improved circulation facilitates the delivery of oxygen and nutrients to tissues and aids in the removal of metabolic wastes, resulting in a soothing effect on muscles and promoting healing.

While the other options suggest effects such as cooling down the body, promoting muscle spasms, or reducing metabolic rate, these are not associated with the application of heat via a warm-water bag. Instead, warmth is used to relax muscles and alleviate discomfort, making increased blood circulation the correct outcome of this treatment.