Understanding The Impact of Muscle Weakness

Muscle strength testing is not always part of a standard podiatric biomechanical evaluation. Unfortunately, muscular weakness can often influence function and, if undetected, can lead to chronic pain in the joints which the weakened muscles support. There are several reasons for muscular weakness, but the most common cause is chronic inhibition signaling from the CNS. Since motor signals to muscles normally cycle between facilitation (excitation) and inhibition, an alteration in this signaling can often cause chronic inhibition and subsequent pain. With this in mind, our expert panelists offer their opinions on how you can use this effect to manage a variety of common podiatric conditions. Q: What is the effect of peroneal inhibition on normal foot function? A: Hallux limitus/rigidus, hallux valgus and sesamoiditis can all be related to suppressed function of the peroneal muscles, according to Howard Dananberg, DPM. Often, this can occur following a subtle inversion ankle sprain-type injury and lead to pain in and around the first MTPJ. The peroneals have both an eccentric and concentric function during gait, notes Trevor Prior, FCPod(S). In latter stance, as the heel starts to lift from the ground, first MTPJ dorsiflexion and ankle plantarflexion should occur. This motion initiates the windlass mechanism as the fascia begins to tighten. In turn, this will cause plantarflexion of the first metatarsal, which is stabilized against the ground by peroneus longus activity, according to Mr. Prior. As the tendon of peroneus longus pivots around the cuboid, it will cause some rotation and ultimately close packing of the calcaneocuboid joint as described by Bojsen-Moller. “Inhibition will affect the ability of peroneus longus to stabilize the first ray and predispose to instability,” notes Mr. Prior. “It may be difficult to determine whether first MTPJ dysfunction (due to functional hallux limitus, rearfoot pronation, etc.) prevents normal peroneal activity or vice versa. The net result is dysfunction.” 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.

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