"My body parts" is another way of saying "my flesh" ([7:18](./23.md)), which is a metaphor for the old nature, opposed to God, with which Paul was born. Here it is a metonym for the actions people do because of that old nature. Alternate translation: "I know that what I actually do is different from what my inner man rejoices in" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-metonymy]])