Samantha has always been a very active person and exercises regularly. During pregnancy she was concerned that maybe she was doing too much and it could be harmful for the fetus. Her physician told her that her exercise routine, which consisted of meditation, active stretching, yoga, and aerobic exercise, was fine. He even suggested that one of the activities positively stimulated her circulatory and muscular systems. According to the text, which of the following is Samantha's physician referring to?