Ankle weakness can contribute to flat feet in several ways. By weak ankles, I mean instability at the ankle joint or impaired strength and coordination of the muscles around the joint.
Ankle Sprains and Instability
The most common type of ankle sprain occurs when landing on the outside edge of the foot, causing the foot to roll inward. This stretches the ligaments on the outside of the ankle that hold the bones together. This injury is called a lateral ankle sprain.
After the ankle is sprained the ligaments may not completely regain the strength they had before the injury. This can make it easier to re-sprain the ankle. Having a previous ankle sprain is the biggest risk factor for having another one.
One possible cause of flat feet is that the body flattens the arch and rolls the ankle inward to keep the ankle stable and prevent it from rolling to the outside. The body may be protecting the ankle from another sprain by putting the foot into a position that would make it difficult to roll the ankle.
If the inside edge of the foot is making contact with the ground the ankle would have to travel a lot farther to roll again. A flattened arch also compresses the structures on the outside of the ankle which can give the ankle a greater feeling of stability.
The posterior tibialis muscle starts in the calf and travels down along the inside of the ankle and attaches to the underside of the foot near the arch. One of the main functions of this muscle is maintaining the height of the arch. We know that weakness in the posterior tibialis muscle can contribute to flat feet because when the muscle degenerates or ruptures (this commonly happens in older adults) the result is often a completely flat foot.
If you stand on one leg for a long period of time you'll notice that the muscles around the ankle start to feel fatigued. Balancing on one leg requires frequent activation of muscles on all sides of the joint. These muscles work together to keep the body centered.
Why is it harder to stand on one foot? Because the base of support under the body is much smaller compared to standing on both feet. A flat foot may by one strategy the body uses to compensate for poor balance or weakness of the muscles responsible for providing balance and stability.
Strengthening Weak Ankles
Ankle strengthening can be done by doing resistance exercises that target the muscles around the joint like calf raises and exercises that challenge balance.