What a question.......difficult for a medical student to answer and difficult to answer on a forum.
However.....touching your toes from the standing position mainly involves the leg muscles and the hip joint with only a "steadying " from the abdominal muscles.
However, touching your toes from the sitting position is almost all abdominal muscles.
So, in answer to your question....the abdominal group.........
Rectus abdominis.. External oblique. Internal oblique.
Called the anterior abdominal muscles.
For exercises, Google it.