Understanding The Impact of Muscle Weakness
Why Peroneal Inhibition Is Difficult To Detect
Michelle Guiliano, DPM, assesses this muscle inhibition through manual testing. She asks the patient to hold the foot in a plantarflexed, everted position while she tries to invert/plantarflex the foot. There is inhibition if the patient can’t resist her force, according to Dr. Guiliano. She says you should make sure the patient does not recruit other muscles while trying to resist the force because this will make the assessment inaccurate.
There are other factors that make it difficult to detect peroneal inhibition. “The peroneal and tibialis posterior muscles function together—not to move the foot, but to stabilize the foot,” says Dr. Guiliano. “The posterior tibial muscle promotes supination of the subtalar joint and external rotation of the leg while the peroneals provide motion in the pronatory direction and through internal rotation of the leg. This is the reason why you may not recognize peroneal or tibialis posterior inhibition if there is not an obvious sign such as the collapse of the medial longitudinal arch.”
There is not a distinct lack of function when these muscles are inhibited as there would be with a muscle like the tibialis anterior, according to Dr. Guiliano. When the tibialis anterior is not functioning properly, you would see foot slap. When the peroneal muscles are not functioning, you have a more difficult task of assessing instability.
While you may detect direct mechanical dysfunction during your gait analysis, Prior cautions that ineffective peroneal activity may also predispose to lateral ankle instability, explaining why this condition can occur in the pronated foot.
As an example of the effect of peroneal inhibition on normal foot function, Dr. Guiliano speaks of a patient who has chronic ankle sprains. The patient often describes frequent, inconsequential inversion ankle sprains or an ankle that “gives out for no reason.” Manual muscle testing of the peroneal muscle on the affected side as described above often reveals little or no resistance to the directed force.
The reason for the “sprains” is not necessarily “weak” or “loose” lateral ankle ligaments but rather the dominance of the tibialis posterior muscle, according to Dr. Guiliano. If the peroneal muscle is not functioning properly, it cannot balance out the tibialis posterior muscle, thereby allowing the tibialis posterior to be dominant. When it is uninhibited by the peroneal muscle, the tibialis posterior muscle plantarflexes and inverts the foot. As a result, this may lead to frequent inversion sprains of the affected ankle.
What are the best treatment techniques? Dr. Dananberg recommends ankle manipulation and follow-up exercises to maintain peroneal strength, noting they “can do wonders” to resolve symptoms of peroneal inhibition. Whether there is complete inhibition or a functional weakness, Mr. Prior seeks to restore normal activity by facilitating function (via mobilization/stimulation) and providing sufficient power, strength and endurance (via exercises).