On the outside of the leg we have two very interesting muscles with a couple of different names. Called either peroneus brevis and peroneus longus or fibularis brevis and longus, both of these muscles (we’ll go with peroneus) originate on the fibula, the smaller bone on the outside of the lower leg.
Peroneus brevis starts on the middle of the fibula and travels down around the ankle bone to connect on the fifth metatarsal on the pinky side of the foot. You can see the bone where it attaches; there is usually a small bony projection in the middle of the outer foot. This is the bone peroneus brevis attaches to.
Preoneus longus attaches higher up the fibula just below the knee. It follows the same path as Peroneus brevis until it turns under the foot while peroneus brevis ends. Peroneus longus goes all the way under the sole of the foot to connect to the first metatarsal, or the mound of the big toe, and the medial cuneiform bone directly behind it.
They have a couple of different functions. Both peroneus brevis and peroneus longus are both plantar flexors—they step on the gas pedal of a car. And they are both involved in eversion or pronation of the foot—they roll from the outside to the inside. Peroneus brevis aids the successful roll from the outer foot to the inner foot in a good walking step and peroneus longus supports the transverse arch and helps to stabilize the first metatarsal bone against the ground when you push off to walk or run. If these muscles are weak or don’t function correctly the midfoot and the transverse arch will be unstable and unable to provide support to the inner arch of the foot..
The stepping on the gas image is key here. We should always have a sense that we are grounded evenly through the inner and outer foot and ready to accelerate at any moment. Instead most of are living on the outside of the foot in walking, standing, running, you name it. When this happens peroneus brevis and peroneus longus have little to do and from my perspective often lie dormant and unused when they have such an important job to do.