Final answer:
The fibularis longus muscle plantar flexes and everts the foot, whereas the fibularis tertius is involved in dorsiflexing the foot. However, the fibularis tertius is not found in every individual.
Step-by-step explanation:
The action of the fibularis longus differs from that of the fibularis tertius in that the fibularis longus muscle plantar flexes the foot at the ankle and everts it at the intertarsal joints. On the other hand, the fibularis tertius, which is associated with the extensor digitorum longus and sometimes fused to it, helps in dorsiflexion of the foot. However, it's important to note that the fibularis tertius is not present in all people.
The fibularis longus, present in the lateral compartment of the leg, also everts the foot. In contrast, the muscles in the anterior compartment, which include the fibularis tertius, contribute to raising the front of the foot by dorsiflexion. Specifically, the fibularis tertius assists an agonist muscle, usually the extensor digitorum longus, by stabilizing the origin of the agonist to prevent or reduce movement at another joint during dorsiflexion.