Strength After Stroke Begins in the Brain

Muscle weakness is a common outcome after stroke and is usually especially large on one (more affected) side. Rehabilitation to improve strength requires attempts to get stronger, but strength can improve in more than just the target that is trained. Over 100 years ago, Edward Scripture discovered this “cross-education,” a phenomenon where training one limb improves the strength in similar muscles on the untrained side.

Cross-education strength training has been confirmed in many muscles and with many protocols. The strength gain on the other, untrained side is less than the trained side, though. Why not just train both sides the same as you might do in the gym? After a stroke, that’s actually difficult due to strength asymmetries.

Following a stroke, damaged neurons in the brain cause one side of the body (the opposite to the lesion) to be more affected than the other (less affected) side with reduced strength and function. Typical rehabilitation training methods focus on training the weaker, more affected side directly. This makes a lot of sense and it’s been shown to be useful in many studies. However, training the more affected side may be hard to start if the weakness is so great that the training can’t be done well and take a long time to induce functionally significant changes for some people.

Some years ago, Katie Dragert and I piloted 19 participants with chronic stroke who trained their less affected legs for 6 weeks by performing maximal dorsiflexion (lifting the toes up and towards the shin) contractions. After a total of 18 sessions of training, maximal ankle dorsiflexion strength improved by 34 percent and 31 percent in the trained and untrained legs, respectively. Since dorsiflexion plays an important role during walking, participants also showed better walking performance.

Recently, Yao Sun, Noah Ledwell, Lara Boyd and I wanted to see if this approach could work in the arms, too. In our study, 24 participants performed wrist extension training on their less affected arm for 5 weeks. Maximal wrist extension force was increased by 42 percent and 35 percent on their trained and untrained side. Within the 17 responders who showed improvement in their trained arm, 8 participants showed significant strength gain on their more affected, untrained arm.

These two studies confirm that when one side of the body cannot be trained after stroke, cross-education training provides a complementary training option to boost the strength on the untrained side. The idea is then to follow up by training that side directly. It’s also notable that the percentage increase in cross-education strength after stroke is actually higher than in folks without injury. This underscores, even more, the need and benefit for ongoing rehabilitation interventions after neurological damage.

What causes strength improvement in the muscle that was not trained? Why does this exist as a thing? One component that plays the mediating role in strength training but which we, ironically, don’t think about is the role of the nervous system. Muscle contraction is triggered by the activity of the motor neurons in the spinal cord. When we train a muscle, we are also activating the neural pathways involved because that’s how the muscle activates! Strength training induces neurophysiological changes in the brain and spinal cord which affects the force development in the muscle.

Neural adaptation occurs bilaterally and affects the same muscle on both trained and untrained sides. In the two studies in stroke, altered spinal and corticospinal excitability were found in the untrained limb along with the strength gains. The mechanisms of these interlimb adaptations need some more study, but the presence of cross-education can be best understood from an evolutionary perspective linked to our quadrupedal ancestry.

In habitually quadrupedal animals like your pet cat or dog, injury to one limb usually compromises but does not prevent locomotion. A quick look at social media reveals a wealth of videos of two- and three-legged cats and dogs moving around with grace and power as a testament to adaptive neuroplasticity. In the wild, strong neural and biomechanical interlimb coupling enables quadrupedal animals to continue behaviors such as hunting for food or running away from predators while healing the injured limb. In fact, the use of the other limbs and related ongoing neural activation serves as the training stimulus leading to an enhanced restoration of function after injury.

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In humans who are, from a neural perspective, basically upright walking cats, such interlimb neural connection has been extensively studied during locomotor activities, such as walking or cycling. The current findings of cross-education training suggest that training one side of the muscle may also tap into the same interlimb neural circuitry.

It is worth noting that training-induced adaptation is not only widespread in the nervous system but also persists for many years after neurological injury. In our two studies, the average post-stroke duration for all the participants is 109 months, which is way beyond the six months recovery window that has been falsely believed by many. Efforts in neurorehabilitation are always worth pursuing regardless of time after injury.

We should credit the brain when our muscles get stronger, especially when training after neurological injury. When training can only be done on one side, exercises that stimulate interlimb neural networks can benefit the whole body. After a stroke, the stronger side can be used to give the weaker one a helping hand.

Note: This post was co-authored by Dr. Yao Sun, post-doctoral fellow at the University of Alberta.

© E. Paul Zehr (2020)