In Part 1 of this “1894” post I suggested an asymmetrical effect like cross education might have a useful application when there is already an existing clinical asymmetry. Stroke (officially known as “cerebrovascular accident”) fits the bill for that. And, for what we’ll talk about later, so does having a limb immobilized in a cast.
This asymmetry after stroke is experienced as a more pronounced effect on one side of the body compared with other. This leads to a form of one sided weakness (hemiplegia). Sometimes this gets talked about as affected and unaffected sides or weak or strong sides (clinically referred to as paretic and non-paretic). Prior evidence (and some more recent work from my own laboratory) shows that generally a stroke affects both sides of the body but to lesser degrees. So there really isn’t an unaffected side. I like to refer to more and less affected sides because of this.
********Next, we interrupt this post to provide a public service announcement…
Signs and symptoms of a stroke include:
A key issue is the sudden onset of these symptoms. They can be very brief and temporary but indicate significant problems and must be followed up. Think of them as symptoms of “brain attack” and put it in the same category as “heart attack”. Seek medical assistance promptly. And if you see or experience this happening to someone, help get them to appropriate care.
*******Back to the post…
The actual issue in stroke relates to damage to neurons in the brain that results from lack of oxygen (ischemia) due to a blockage of blood flow. It’s this lack of blood flow and oxygen delivery to different parts of the brain that produces those signs and symptoms of stroke listed above.
Strokes can be classed into two broad categories, ischemic and hemorrhagic, which account for 70-80% and 20-30% of incidents, respectively. Stroke is a leading cause of death and disability in North America. In a 2007 report the World Health Organization indicated that ~15 million people succumb to stroke each year with roughly 1/3 of the incidents being fatal and 1/3 leading to permanent disability.
Both the ability and strength to move are reduced after stroke. Because of all the changes in coordination that happen after the injury, the effort to do things and the energy to do them is increased. In something like walking, the energy needed to cover the same distance can be almost twice as high as the person would have needed before the stroke.
Muscle weakness after stroke depends on the size and exact location of the lesion in the brain and can range between ~20 and 90% of the less affected side. This overall weakness can have a huge impact on daily activities and doing relatively simple things like rising from a chair and walking.
In my own research work I have conducted many studies that have looked at coordination of movement in walking with people who have no neurological damage and in those who have had strokes. My biggest clinical interest is in ways to improve walking ability after stroke. Part of this has to do with helping retrain walking ability (a kind of motor learning piece) and part has to do with helping strengthen the body so that walking can happen. For sure your nervous system likely doesn’t think of this as two parts but this is often how it’s addressed experimentally.
For many of our studies the focus has been on the first part, the bit about walking ability. Recently, though, I have gotten much more interested also in the strengthening part of this issue. And in particular the idea of strengthening a limb indirectly. Like by training the other side of the body. As in cross education.
Which is what brought us to study cross education in stroke recently. A senior PhD trainee of mine at the time, Katie Dragert, did a study where we had participants with “chronic stroke” do a 6 week strength training intervention. By the way, “chronic stroke” is somewhat arbitrarily defined as being at least 6 months after the incident. (I will have an entire post later that talks about the problem with making this distinction.)
Because of the focus on walking that I have in my lab, we did the study using the legs and particularly the ankle muscles. Our participants spent six weeks strength training their less affected ankle dorsiflexor muscles. Those are the ones that you use to pull up your foot when walking and standing. They did training in three sessions per week each of which took 25-minutes. This protocol used a design that Katie had previously tested out in people who didn’t have strokes.
No one had studied cross education in stroke so we weren’t exactly sure what we would get. Since cross education seems to have effects at multiple levels of the nervous system, we thought it likely we’d get some strength gains. But a typical outcome in neurological damage is that you can often find a bit of what you find in the undamaged nervous system, but the effects are smaller or weaker.
So I kind of imagined a best case outcome would be that the stroke group would maybe get half of what you’d get in a group without nervous system damage. Since the maximum cross education effect we could ever imagine seeing would be about half of the strength gain on the trained side, that would mean we could reasonably expect maybe to get half of that. Which would mean getting about a quarter of the strength gain in the untrained, more affected leg.
So when Katie and I found about the same effect in both legs—about a 30% strength gain from pre-study levels—I was pretty stunned. But in a good way. That’s the kind of outcome that I don’t mind being stunned by. And it was particularly stunning to realize that on average our participants had their strokes 80 months before the study! This is testament to the amazing amount of residual plasticity the nervous system has.
It’s still early days for this result but it was a nice proof of principle that cross education could work after stroke. We are now doing other studies to explore the effect with the eye to refining a protocol that could one day be used therapeutically.
A colleague at the University of Saskatchewan, Jon Farthing, has been doing studies on cross education of strength for many years. He and his collaborators have also been searching for a clinical application. And they came up with something that is very elegant.
When someone has a limb in a cast a major concern is muscle atrophy and weakness. Then when the cast is removed lots of therapy is often needed to help retrain and strengthen the limb that was casted. It can take an awful lot of work restore the limb to function after the broken bone or whatever has healed.
What if you did strength training with the other limb, the one not in a cast? First they did studies in people who didn’t have broken bones but voluntarily adopted wearing casts or slings. The simple answer is that the strength training on the other side seemed to help reduce the muscle weakness. This is a great clinical application. They are now conducting other studies in people who actually had been in accidents that led to broken bones and had to wear a cast therapeutically.
And all of what I’ve written about here was made possible because Edward Scripture had a searching mind and a great sense of curiousity. Who read the literature widely—and in multiple languages—and drew together influences from quite different experimental approaches. He used that to create an idea to test in his own lab using a very simple design, and very “primitive” equipment.
But it worked, did this study “On the education of muscular control and power” from 1894. And it laid the groundwork for a phenomenon of “cross education” that we continue to exploit in present day.
© E. Paul Zehr, 2013