In strength training, many people intuitively assume that only the muscles actually being trained get stronger. So if you train your right arm, only that arm gets stronger – or so the classic notion goes.
Training science, however, reveals a fascinating phenomenon: the so-called cross-education effect. It describes the fact that one-sided training not only strengthens the trained side of the body, but can also lead to measurable adaptations on the opposite side.
This effect has been known for over a hundred years and is today used especially in sports training and in rehabilitation.
What does cross-education mean?
By cross-education we mean the effect whereby strength training of one side of the body can lead to performance improvements on the opposite, untrained side.
If, for example, you train only your right arm, your left arm can become stronger too – even if it was not loaded at all during training.
The strength gains on the untrained side are, as a rule, smaller than on the trained side, but they are clearly measurable. Studies often show improvements of around 7–20 % of strength performance on the untrained side.
The reason for this lies not in the muscles themselves, but above all in the nervous system.
Why does the cross-education effect occur?
Most adaptations from strength training initially arise not primarily from muscle growth, but from changes in the nervous system. The brain learns, so to speak, to activate muscles more efficiently.
During one-sided training, motor programs are activated in the brain that are not strictly limited to one side of the body. Many of these control mechanisms are organized bilaterally..
So when the nervous system learns to perform a movement more efficiently, this “knowledge” can partly be transferred to the other side of the body as well.
You can picture it much like learning a new movement: when you practice a complex exercise with your right hand, it often becomes somewhat easier with your left hand too – even without specific training.
The significance of cross-education in training
For normal training, cross-education means that one-sided exercises can achieve more than one might first think.
Especially with unilateral exercises such as:
- one-arm dumbbell training, e.g. in pushing and pulling exercises
- single-leg squat variations and lunges
such neural adaptations are activated.
This does not mean that bilateral training becomes superfluous. Rather, the effect shows how strongly the nervous system is involved in training and how complex the body's adaptation processes are.
One-sided exercises have many benefits anyway: they improve coordination and stability and help balance out strength differences between the sides of the body.
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Cross-education in the case of injuries
Cross-education becomes especially interesting in connection with injuries.
When, for example, an arm or a leg cannot be trained due to an injury or surgery, a significant loss of strength normally occurs.
This is where the cross-education effect can help.
If a person continues to train the healthy side, at least part of the neural adaptations can be preserved. As a result, the loss of strength on the injured side is often smaller, and returning to training later becomes easier.
This principle is today taken into account in many areas of rehabilitation and sports physiotherapy.
Of course, cross-education replaces neither medical treatment nor targeted rehabilitation, but it can be a sensible complement.
How can you use cross-education in training?
To benefit from the cross-education effect, there is no need to create special programs. Many classic training formats already contain one-sided exercises.
Some practical approaches are:
- regularly integrating unilateral exercises into training programs
- testing strength differences between the sides of the body and training them in a targeted way
- in the case of injuries, continuing to train the unaffected side, provided this is medically permitted
- incorporating coordination and stability exercises
Especially in “functional” training or athletic training, such exercise formats are widespread anyway.
Conclusion
The cross-education effect shows how strongly our nervous system is involved in training adaptations. When you train one side of the body, the opposite side benefits to a certain extent too.
This effect plays a role both in normal strength training and in rehabilitation after injuries.
It also makes clear that training is not merely a question of muscles – but always an adaptation of the entire neuromuscular system as well.
