Understanding The Impact of Muscle Weakness
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.