Useful work, as defined for a person's activities, includes climbing stairs or accelerating to a run. Why does it exclude internal work such as the heart pumping blood?

1. Useful work is defined as work that involves a force exerted through a distance on the outside world, such as lifting weights or climbing stairs. 2. Pumping blood is work the heart does inside the body, not on any external object, so it does not count as useful work under this definition. 3. The heart does consume real energy to do this internal work, so the exclusion is not because the heart uses no energy. 4. Internal work is not always larger than useful work, and the definition of useful work has nothing to do with whether friction specifically is involved. _Source: OpenStax College Physics (CC BY 4.0), Ch 7 "Work, Energy, and Energy Resources", section 7.8 Work, Energy, and Power in Humans_