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. 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). Q: What is the effect of posterior tibial inhibition? A: Dr. Dananberg notes posterior tibial dysfunction is a common result of inhibition to the posterior tibial tendon. He recently saw a 65-year-old woman who suffered for one year with this condition, which began following an inversion ankle sprain. He treated her with extensive physical therapy, casting and then an F/U orthotic, all to no avail. On examination, she exhibited –3 degrees of ankle joint dorsiflexion on the affected side and only one-quarter posterior tibial strength. “Following manipulation of the ankle, she had a dramatic response with complete reversal of the posterior tibial strength,” recalls Dr. Dananberg. “With an appropriate prescription orthotic, she returned to normal walking within several weeks.” As for posterior tibial inhibition, Dr. Guiliano assesses it via manual muscle testing, as she does for peroneal inhibition. Patients hold their feet in plantarflexed, inverted positions while Dr. Guiliano tries to evert/plantarflex the foot. An inability to resist force is considered inhibition. She warns practitioners to be careful the patient does not recruit other muscles while trying to resist the force because this will make the assessment inaccurate. For Mr. Prior, tibialis posterior function is important in early and latter stance. In the initial contact period, eccentric contraction helps to slow rearfoot pronation and thus control the loading process. Failure of this mechanism can result in rapid and increased rearfoot pronation, and an inability to withstand the pronatory forces from ground reaction. As the body progresses over the foot and the heel begins to unweight, elastic recoil within the muscle tendon unit (in conjunction with some concentric contraction) will help to initiate inversion, leading to supination of the foot. In turn, this will provide mechanical advantage for the calf muscle complex and facilitate ankle push-off power. Weakness and inhibition of this process will result in prolonged pronation and instability of the medial column with a failure of re-supination and an apropulsive gait pattern. “Once again, you should aim your treatment at facilitating function and providing appropriate levels of strength,” says Mr. Prior. Dr. Guiliano cites the example of the classic hyperpronated foot to illustrate the effect of posterior tibial inhibition on normal foot function. If the posterior tibial muscle is not functioning properly, the peroneal muscles can take over and cause a lateral imbalance, notes Dr. Guiliano. She says this lateral imbalance will prevent the medial longitudinal arch from becoming a rigid lever during propulsion, resulting in inadequate propulsion. This foot typically would abduct during gait and an abductory twist can be present during propulsion. “I feel it is important to note that posterior tibial inhibition is not the only cause of the classic hyperpronated foot but it is an important etiology to be considered,” she says. Q: Is there a similar effect to ankle manipulation on the posterior tibial muscle? A: Dr. Guiliano says you first should perform neurolymphatic stimulation to address the inhibition of these muscles. Neurolymphatic stimulation is based on Chapman points, which are correlated with specific muscles. She describes these points as “light switches” to her patients, explaining every muscle in the body has a light switch that can turn it on and off. When you properly stimulate the point associated with a dysfunctioning muscle, there will be an increase in strength of that muscle when you test it again. “This is a wonderful exercise for patients because they will be able to see and feel the lack of strength upon initial testing and the dramatic improvement following the neurolymphatic stimulation,” explains Dr. Guiliano. You would perform neurolymphatic stimulation with a light touch in a circular, very subtle motion. When there is a chronic dysfunction of the muscle, the point will be very sensitive and often will feel like a pea underneath the skin. Dr. Guiliano says deep palpation is not required to find the point. It is very superficial, as though it is in the subcutaneous tissue. You should also note these points are more like energy points because you will not find them along the nerve or lymph vessels that support the specific muscles. For example, you’ll locate the peroneal point on the anterior pubis bone lateral to the pubic symphysis on the affected side and you will find the posterior tibial point two inches up and one inch over from the umbilicus on the affected side. Dr. Guiliano notes the posterior tibial point is the hardest to find because it is not located over a specific bony landmark and it varies with torso length and other malalignments of the body. As she explains, this explanation is just a brief introduction to neurolymphatic stimulation and it takes time and practice to palpate and stimulate these points readily. Then perform manual muscle testing to ensure adequate stimulation occurred. “While we can address inhibition of muscles through neurolymphatic stimulation, often there is an underlying cause to the chronic inhibition and this must be addressed as well,” says Dr. Guiliano. “Lack of ankle dorsiflexion can cause this inhibition and I feel ankle manipulation can have a positive effect on the posterior tibial muscle.” While ankle joint and superior tibio-fibula joint mobilization can facilitate inhibited peroneal function, Mr. Prior says he is unaware of a similar mobilization that has the same result for tibialis posterior. However, many patients with inhibition and recurrent pronation will present with reduced talo-navicular, first ray and first MTPJ motion. Mobilizations of the affected joints can improve motion, dynamic function and complement the strengthening program. Muscle stimulation points have been described for both the peroneals and tibialis posterior muscles, and can be a useful adjunct in this treatment process. You should note mobilization and muscle facilitation are usually part of the treatment process and rarely replace the need for addressing the underlying biomechanical abnormalities. However, Mr. Prior emphasizes that clinicians who make the effort to learn and master these techniques will have an invaluable tool for enhancing patient care. Mr. Prior has found these techniques particularly beneficial in reducing the dysfunction caused by post-operative stiffness. Dr. Dananberg (pictured) practices in Bedford, NH. Dr. Guiliano has a private practice in Rutland, Vt. with an emphasis on holistic podiatric care. Mr. Prior has a private practice in London and is a National Health Service consultant in the U.K.