Formal Assessment of Arthrogenic Muscle Inhibition
What does gold standard assessment of muscle inhibition look like? There are three main techniques:
Hoffmann Reflex to assess motor neuron pool availability
Superimposed Burst Technique to assess volitional activation
Transcranial Magnetic Stimulation to assess cortical inhibition
Each of these techniques provides a unique piece of information so in an ideal world, we would assess all three in our patients.
Hoffmann Reflex
This technique involves electrical stimulation of the femoral nerve in the inguinal fold. It seeks to record the amount of muscle activity generating by a sensory stimulation (reflex volley) versus motor stimulation. The proportion of muscle activity generated by a reflex represents the motor neuron pool that is able to be recruited ("H wave"). Whereas, the more intense motor stimulation recruits 100% of the muscle capacity (i.e., all motor neurons) ("M wave").
After ACLR, this H:M wave ratio is severely diminished for about 3 months - indicating spinal-level inhibition.
Superimposed Burst Technique
This technique involves electrical stimulation of the quadriceps muscle during a maximal voluntary isometric contraction. By superimposing a strong (~150V) electrical stimulation to the quadriceps, we believe we artificially contract 100% of the muscle fibers. We then divide the torque generated volitionally (pre-stimulus) by the torque generated artificially (post-stimulus) to get a "Central Activation Ratio".
This number is an estimate of the % of individuals muscle capacity one can voluntarily activate (i.e., "volitional activation").
Transcranial Magnetic Stimulation
This technique involves magnetic stimulation of the primary motor cortex. This stimulus, if large enough can generate an impulse down the spinal cord and cause a muscle twitch. There are many outcomes we are able to measure with this technique, but two key ones are (1) motor threshold and (2) cortical inhibition.
Motor threshold is the lowest possible stimulator intensity that results in a muscle twitch. The thinking here is, if lower stimulator intensities cause muscle twitches, the cortex is more excitable (a good thing). If motor threshold is higher, then the patient must commit more brain resources to generate a muscle contraction.
Cortical inhibition is an extension of this technique. Essentially, we can measure the size of the signal that arrives at the muscle. Smaller signals between the brain and the muscle indicate weaker connection (i.e., more inhibition).
All 3 for complete picture
Unfortunately, all three outcomes are necessary to put together a full diagnostic picture of muscle inhibition. For example, patient 1 is 2 weeks post-op ACLR, has diminished H-reflex, low volitional activation, and normal motor thresholds. Patient 2 is >3 months post-ACLR, has normal H-Reflex, low volitional activation, and high motor thresholds. These patients will need different treatments!
But, do we need to know for sure? Our clinical decision tree is how we would tease these impairments out in the absence of advanced assessments. Let me know how they work for you!
