Final answer:
Underactive muscles in pes planus syndrome include the tibialis posterior and the muscles in the plantar group. The plantar aponeurosis can also contribute to flatfoot if weakened.
Step-by-step explanation:
With pes planus syndrome, also known as flatfoot, certain muscles in the foot are often found to be underactive. One of the main ones is the tibialis posterior muscle, which is a significant dynamic stabilizer of the medial longitudinal arch of the foot. As a result, the arch can flatten when this muscle is weak or not functioning properly. Another group of muscles that may be underactive are the muscles in the plantar group, which form a four-layered group of intrinsic foot muscles responsible for various actions of the foot. Likewise, the plantar aponeurosis (or fascia), while not a muscle, is a critical supporting structure for the arch of the foot and can contribute to flatfoot if it becomes lax or weakened.