Final answer:
Overly pronated feet tend to cause the muscles that support the arch and plantar flex the foot, such as the gastrocnemius, soleus, and especially the tibialis posterior, to become shortened or stiff.
Step-by-step explanation:
Overly pronated feet can cause certain muscles in the lower leg to become shortened or stiff. Among the choices provided, gastrocnemius and soleus are muscles located in the superficial posterior compartment of the leg and are responsible for plantar flexion of the foot. Pronation tends to cause increased tension in these muscles, which may lead to them becoming shortened or stiff. However, excessive pronation is often associated with a flattening of the arch, and therefore tensions the muscles that support the arch, such as the tibialis posterior, leading to stiffness or shortening. In contrast, muscles in the anterior compartment of the leg, like the tibialis anterior, are generally responsible for dorsiflexion and would not typically become shortened or stiff due to pronation. Thus, the muscles most likely to become shortened or stiff due to overly pronated feet would be those involved in supporting the arch and in plantar flexion, such as the gastrocnemius and soleus, and especially the tibialis posterior.