So the peroneals/fibularis group and the tibialis posterior and long toe flexors effectively create a stirup around the ankle. If the fibularis group on the outside of the lower leg are more short and stiff than the  tibialis posterior and long toe flexors deep on the inner portion of the calf, the ankle will more easily rotate into what’s called eversion and the inner arch of the foot will tend to flatten. Now, the opposite is true if you have very high rigid arches and tend to walk around on the outside edge of your foot. The fibularis muscles will be long and stretched out while the tibialis posterior and long toe flexors are shorter and stiffer.

I used a lacrosse ball in this video but have been using the dollar-store-nubby-ball in classes and I really like the “tackyness” of the more foamy rubber nubby ball. Seems to grab onto the skin more allowing to snag the fascia more effectively.