The cortical inhibition myth: are we targeted the wrong thing?
“In patellar tendinopathy, the known deficit when it comes to force output is max strength. When we look at max force output, irrespective of the mode of contraction — isometric, isotonic, and so on — it’s reduced. For the better part of a decade, we put that deficit down to cortical inhibition.”
“We thought for a long time it was all happening at the brain. That stemmed the whole tendon neuroplastic training arc — using metronomes to produce a motor training effect so the brain would adapt and overcome cortical inhibition. That stacks up from a theory perspective. But what we didn’t do is look at the spinal level.”
“My PhD looked at the cortical level, the spinal level, and what was altered compared to healthy people. Cortical inhibition was fine — unaffected compared to healthy controls. But at the spinal level, that was where the problem was. We’ve been targeting the wrong thing.”
The garden hose analogy
“Think of motor drive through the pathway like a garden hose, with water pressure at the tap. Cortical inhibition is the tap. In patellar tendinopathy, the tap’s open — cortical inhibition isn’t disrupting drive. But there’s a kink further down the hose, at the spinal level, and that’s stopping water flow. You can have all the water pressure in the world, but it’s not going to get past that kink. We need to do something about the kink.”
Self-paced vs metronome training
“After three weeks of training, the people that did the metronome-based paced training had their cortical inhibition drop, with a bit of a boost in strength. Those that did the self-paced training had no change to cortical inhibition, but their reticulospinal pathway excitability went through the roof — their strength went up, and their power went up as well.”
“Self-paced training presents as a really nice way to access and target that spinal-level inhibition, which seems to be the deficit in patellar tendinopathy responsible for max force deficits.”
“With metronome-based training, control goes external — you have to focus on something and keep pace with it, so it becomes more of a control task. When it’s self-paced, you self-select a pace. It becomes less of a motor learning task and more of an automatic task, which seems to be really important for the reticulospinal pathway.”
Arthrogenic muscle inhibition in the patellar tendon
“We used to think patellar tendinopathy was really unique compared to other conditions — cortical inhibition, and the rest of the pathway seems fine. But it’s actually quite similar to a lot of other knee-based conditions. What’s happening appears to be much like arthrogenic muscle inhibition.”
“You have pain going up towards the brain, and as it passes the spinal level, it activates the nerves that make it harder for the motor signal coming back down. The pain system might be the clue — it’s potentially activating the nerves that block that signal at the spinal level.”
“We demonstrated that increased activity of the pain system was linked to greater spinal inhibition. In people that had enhanced pain signalling, they had greater spinal inhibition, and that was linked to force deficits.”
Pain: nerve ingrowth and a self-sustaining loop
“There’s increased nerve growth within the tendon — a greater volume of nociceptors to produce more pain. If you’ve got more nerves to produce sensation, you’re going to get more sensation. It makes sense that you’re going to have elevated pain, because you’ve got greater apparatus to produce that pain in the first place.”
“At a chemical level it’s a self-sustaining loop. Those pain nerves at the tendon, when they’re activated, release substance P and CGRP. That reactivates the same nociceptor, so it just keeps firing in a loop. It’s cruel.”
The isometrics-for-pain myth
“It just has stuck, hasn’t it? This idea that isometrics are awesome for pain relief, that you can bank on it. But it’s probably not.”
“The original work had seven individuals doing five by forty-five seconds of isometrics at around seventy-five percent of max isometric force, and they all got dramatic improvement — pain went from a seven out of ten to a zero. Naturally everyone jumped on board and used isometrics for pain in every condition under the sun.”
“Multiple studies have recreated that intervention, and the results are much more modest and variable. In Achilles tendinopathy there was actually an increase in pain on average. In patellar tendinopathy the response was much more modest and hugely variable — some people got better, some got no change, and some got worse. This idea that we could use isometrics as a cure-all for acute pain has been disproven pretty well at this point.”
“They also looked at an isotonic intervention and it was much the same — some better, some no change, some worse.”
Isometrics as an assessment tool
“Where that response to pain is useful is that it might be a really good assessment tool. It doesn’t need to be isometrics — the response to isotonics is comparable, and you can throw aerobic exercise in there as well. The acute response to exercise corresponds with the lab tests we use to work out if people have signs of sensitization.”
Nothing special about isometrics
“As soon as you engage in activity there’s going to be a response from the nervous system, along the lines of potentiation. You’re waking up the nervous system. The reticulospinal pathways get more excitable and you can produce more force.”
“I don’t think there’s anything special with isometrics versus general activity. The more specific the movement you warm up with is to the task that follows, the more likely you are to get the greatest warming effect in the nervous system.”
The calf is not the quad
“In the quads, all the muscles seem to fire together — common synaptic input — so you can produce really good max force output. The calf is different. We don’t have that same common synaptic input.”
“When we’re running, the soleus is the workhorse — it’s active throughout. Then there’s synchronization between the medial and lateral head of the gastroc: the medial head switches on first, and as it starts to fatigue, the lateral head kicks in and picks up the slack.”
“If we’ve got altered motor drive coming down, it’s probably going to be more isolated to one of the three muscles, because we don’t have that common activation route.”
Patellar vs Achilles: max strength vs endurance
“In Achilles tendinopathy, max strength output wasn’t consistently affected. What was affected was muscular endurance — submaximal force production.”
“It highlights that we should probably be looking at these conditions as their own entity — patellar tendinopathy, self-paced; Achilles, maybe metronome.”
The new trial: who responds, and why
“Now we’re going that next step — looking at what actually changes with rehab, who responds, who doesn’t, and why. If you look at the Breta trial, anywhere between ten and forty percent of people didn’t achieve the cutoff for a minimally important clinical difference. There’s this large portion of people that aren’t getting better despite it being our gold standard, and we don’t know why.”
Closing line
“The leg extension machine is worth its absolute weight in gold — but you can probably predict I’m suggesting self-paced leg extensions.”
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