Recently, Dr. Erson Religisio III over at Modern Manual Therapy discussed differentiating weakness versus inhibition in this post. If you take a look at the video, he finds that an inhibited muscle will test much stronger if we gradually increase resistance while applying force and the patient can then match, or at least improve. A truly weak muscle doesn't exhibit this improvement.
When completing my residency, we found neurogenic weakness through repetition testing of the muscle. That is, we would quickly test the muscle strength and re-test the strength rapidly, looking for a loss of strength. While I think both Dr. E's method and the one I learned in my residency can play a role, personally, I look at any loss of muscle activity as potential neural involvement. What is key to consider is correlating with testing other muscles and other findings as well. For example, if the gluteus medius tests weak, as does EHL and peroneal longus/brevis, we should think about L5 being involved. The difficulty comes in differentiating peripheral nerve lesions from local muscular weakness, specifically if there are few muscles innervated via that peripheral nerve.
In reality, with the power of the nervous system, I find that we can frequently alter "strength" through treating the spine or nerves. When in comes to practice, I recommend tackling it from all ends. If a patient with hip pain and weak glutes also has a deficit in lumbar mobility, I would treat both the weak glutes and limited lumbar motion. These are impairments and may or may not be contributing to the patient's dysfunction. As I always recommend, treat the impairments, but it may be fun to start playing around with testing for true weakness vs inhibition in your patients. See how that impacts your examination and treatment!
-Dr. Chris Fox, PT, DPT, OCS