Last year I wrote an article about neurogenic inhibition testing. Neurogenic Inhibition is the concept that the muscle's ability to produce resistance and contractile force is limited by the neural input, not the true muscle strength. It is indicated when either a muscle became weaker with repetitive resistance testing or if the strength improved when resistance was gradually increased. In the past, I addressed these conditions with focusing on improving mobility along the path of the nerve and with strengthening the affected muscles.
About Neurogenic Inhibition
One of my favorite aspects of my fellowship mentoring hours is that my mentor has a different treatment style and background compared to me. My training is more in line with Optim Manual Therapy's Fellowship, while my mentor went through NAIOMT's Fellowship. His coursework put a greater emphasis on testing and treating Neurogenic Inhibition. To evaluate a patient for Neurogenic Inhibition, test a patient's muscle strength with a relaxed lumbar spine, then repeat the same testing with a PPT and APT. If the strength completely normalizes with a bias of the lumbar spine, it would be positive for Neurogenic Inhibition. NAIOMT's theory is that the affected segment is "unstable" in a certain direction (decreasing the signal from the nervous system) and the lumbar spine bias provides stability that improves the neural input. An example would be supine ankle DF strength testing that was 4/5, but with the extension bias to the lumbar spine immediately becomes 5/5. The opposite may apply as well. In that same example, the 4/5 ankle DF strength may become 3+/5 with a lumbar flexion bias. It is worth testing and re-testing.
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I'm not saying that I agree with the theory, or that there is even any research to support this assessment method; however, I have noted regularly that strength changes can occur with changes in spinal positioning. The method that my mentor treats these cases is that he works on increasing strength and activation of the spine in the deficient region and then supplementing that activity with exercises that work the affected myotomal levels. For example, I evaluated a patient this past week that had 4-/5 strength of his R glutes, ankle DF and ankle eversion, all of which became 5/5 with lumbar bias into extension (complaints of drop foot for 4 years after cervical spine surgery). Some exercises we went over included ones that bias the lumbar spine into extension (APT) and work the glutes, toe extensors, and ankle eversion.
It may be that the patients improve because of increased "stability" in the dysfunctional direction, it may instead be due to improving mobility in the dysfunctional direction, or it may be something else altogether. However, because there is so little research in the area, we don't even know how effective the method is in the long-term; however, it is worth exploring due to the immediate changes that can occur. I like to implement Neurogenic Inhibition Testing to help direct my treatment direction. I have found that this same assessment method tells me which direction a patient may respond to repeated motions. Using the same previous example, if the strength improves with lumbar extension bias, I would have the patient perform repeated lumbar extensions (or a variation of it) and recheck the strength. In most cases, the strength is improved afterwards without doing the same biasing. In fact, the patient I described came back from the evaluation with a HEP of press-ups with a R bias and his ankle DF/eversion and hip abduction strength were all 4+/5 without any lumbar spine bias. It is far too early to tell if any long-term or practical changes will occur however the testing may still play a role. It can be useful when a patient is so acute that they may not be ready for a full repeated motions assessment. In general, my treatment method is going to stay the same as discussed in the previous article: improve mobility of the nervous system along the entire path, wherever restricted, and strengthen the affected muscles. I may get there differently with this alternative testing method and I may incorporate some of the treatment theories as well.
-Dr. Chris Fox, PT, DPT, OCS
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