Picture this: You’ve finally established your workout routine, and it’s going great. You’re getting your 10,000 steps in a day, you’re at the gym three times a week, and you even go on a park run at the weekend. Then disaster strikes.

It’s a cold January evening, and the pavements are icy. You’re rushing to the corner shop to pick up a carton of milk because you’ve just run out, and it’s a necessity for tomorrow’s breakfast. You’re trying to avoid all the icy patches, but some of them are difficult to see, and one wrong step later, you feel yourself slipping. You stretch one arm out to break your fall… Crack!

After heading to the ER and getting treatment, your arm is in a cast for a few weeks. Workout regimen ruined. You can’t lift weights with one arm anymore, and running is no longer an option. Yes, you can use machines to train legs in the gym, and you can certainly use a recumbent bike until you recover, but what about your upper body? You can’t just keep training your uninjured arm, because that could result in imbalances in strength and muscle development. So, it’s better to stop training your upper body altogether, right?

Wrong.

Enter: Cross-education.

What is Cross-Education?

You’re probably wondering: what is cross-education?

This is a phenomenon where training one limb, typically the uninjured one, leads to strength gains in the opposite, injured limb (Scripture et al., 1894). This likely happens because of neural adaptations that occur when you work one side of your body (Lee & Carroll, 2007).

Let’s take a step back and look at how we increase our strength before delving into these neural adaptations.

The Science of Strength: Neural vs Muscular Adaptation

When we perform weight or resistance training against a load that’s greater than what we experience in daily living, our strength levels tend to increase. Research has shown that there are two main reasons for this (Lee & Carroll, 2007).

The first is muscular adaptation (Abernathy et al., 1994). When we engage in resistance training, we create small tears in our muscles, which increase muscle mass as they heal . This is known as hypertrophy. But it’s not the only adaptation. Resistance training also leads to changes in muscle pennation angle, i.e. the orientation of muscle fibers relative to the tendon (Abernathy et al., 1994). Muscles with a greater pennation angle can produce more force, but over shorter distances. Additionally, muscle enzymatic activity improves, which enhances the force we can generate (Abernathy et al., 1994).

The second is neural adaptation. In the early stages of training (e.g., when someone begins resistance training for the first time), force-producing capabilities increase rapidly due to improved agonist muscle activation and better selective activation and inactivation of synergist and antagonist muscles (Sale, 1988). You might have heard this phenomenon referred to as “newbie gains.” At this stage, you’re likely not building much muscle mass, but you’re teaching your nervous system to activate the right muscles effectively and efficiently. These adaptations occur much faster than hypertrophy-related changes (Sale, 1988).

Now, back to cross-education.

If you’re only training one limb, it’s unlikely that muscle morphology of the contralateral limb (the opposite limb) will adapt. While cross-education can help maintain strength in the untrained limb, it’s unlikely to lead to significant hypertrophy (muscle growth) because the uninjured limb isn’t being directly loaded (Lee & Carroll, 2007). Mechanical stress must be placed on a muscle for these changes to occur. Without loading a muscle, you won’t get microtears that increase muscle mass as they heal. Similarly, you won’t reorient muscle fibers without stressing them.

Instead, it seems that neural adaptation is what results in strength maintenance, improved coordination, and enhanced motor unit recruitment in the contralateral limb when unilateral resistance training is performed.

How Cross-Education Helps with Recovery

So, how does this happen?

There are a couple of theories that may explain what happens in cross-education.

The first is that training one limb alters pathways in the motor cortex of the brain, which may then extend to the contralateral limb. This would mean that changes occur in the motor pathways of the contralateral limb directly, leading to more efficient neural drive in the untrained limb, and a resultant increase in strength (Lee & Carroll, 2007).

The second hypothesis is that unilateral training causes adaptations in the cortical regions of the trained side only. However, the opposite hemisphere of the brain may access these modified circuits during contralateral limb voluntary contraction, boosting force output (Lee & Carroll, 2007).

Regardless of which hypothesis is correct (or whether it is in fact a combination of both), it’s clear that we see better preservation of muscle, maintenance of strength, and improved recovery outcomes with cross-education. Losing strength during a period of inactivity can make recovery longer and more difficult, so as a strategy that can slow down muscle loss, cross-education is incredibly helpful.

Therefore, even if the injured arm can’t be trained directly due to pain or immobility, the neural signals from the uninjured arm can help maintain strength and muscle mass (Andrushko et al., 2018). Maintaining some strength in the injured limb and minimising atrophy can help speed up recovery. Once the injury heals and training of the previously-injured limb can resume, the uninjured limb’s training may help re-establish muscle mass and strength on the formerly-injured side more quickly (Hendy et al., 2012).

Additionally, cross-education has been shown to work not only in strength but also in muscle endurance and motor control (Lee & Carroll, 2007).

Furthermore, continuing to train and make improvements when injured can be great for morale and boost motivation throughout your recovery process. Dealing with an injury can be mentally tough, and it’s incredibly frustrating when your progress comes to a sudden halt. Just keep in mind that staying mentally engaged with your training is half the battle, and that even when injured, staying active can help you feel more in control of the situation.

It’s important to keep in mind, however, that cross-education should not replace limb-specific rehabilitation exercises. If you become injured, always consult a healthcare professional or physiotherapist to create a recovery and rehabilitation plan, which will likely include training your injured limb again at some point.

How to Maximize Cross Education for Your Injury

Now, you might be wondering: what sort of training should I be doing to reap the benefits of cross-education?

It appears that unilateral training 2-3 times a week at a high intensity is enough to attenuate strength losses (Voskuil et al., 2023), with eccentric training providing added benefits (Voskuil et al., 2023; Hendy & Lamon, 2017). Interestingly, when training at an externally-controlled pace (like following the rhythm of a metronome), large magnitudes of cross-education are observed (Hendy & Lamon, 2017). This suggests that in cases of increased cognitive demand and movement control during training, there may be an increase in neuroplasticity, thus enhancing adaptation (Hendy & Lamon, 2017).

So, what might this look like in practice? It could involve 3-4 sets of unilateral exercises, like bicep curls, at an intensity of 80% 1RM (i.e., your one-rep max, or the maximum load you can lift for one repetition). Focusing on controlled movements and deliberate motions, especially with a slow eccentric phase, can maximize neural adaptations. You can also use external pacing (like a metronome or timed intervals) to increase the challenge and boost neuroplasticity.

In addition to your resistance training, consider maintaining low-impact cardio. For upper body injuries, the recumbent bike could be a good option, while for lower body injuries, an upper body ergometer (arm bike) might help you stay active.

Final Thoughts and Summary: Cross Education as a Complement to Rehabilitation

While cross-education can help maintain strength and muscle mass during recovery, it shouldn’t replace rehabilitation of the injured limb. Always consult a healthcare professional or physiotherapist to create a personalized recovery plan if you experience an injury. Staying engaged with your training during tough periods of injury can be a powerful motivator and can help speed up recovery, but make sure to give your body the time and care it needs to heal properly.

So, keep training smart, stay patient, and remember that every step you take – whether with your injured limb or your healthy one – will bring you closer to recovery.

References

Abernethy, P. J., Jürimäe, J., Logan, P. A., Taylor, A. W., & Thayer, R. E. (1994). Acute and Chronic Response of Skeletal Muscle to Resistance Exercise. Sports Medicine, 17(1), 22–38. https://doi.org/10.2165/00007256-199417010-00003

Andrushko, J. W., Gould, L. A., & Farthing, J. P. (2018). Contralateral effects of unilateral training: sparing of muscle strength and size after immobilization. Applied Physiology, Nutrition, and Metabolism, 43(11), 1131–1139. https://doi.org/10.1139/apnm-2018-0073

Hendy, A. M., & Lamon, S. (2017). The Cross-Education Phenomenon: Brain and Beyond. Frontiers in Physiology, 8. https://doi.org/10.3389/fphys.2017.00297

Hendy, A. M., Spittle, M., & Kidgell, D. J. (2012). Cross education and immobilisation: Mechanisms and implications for injury rehabilitation. Journal of Science and Medicine in Sport, 15(2), 94–101. https://doi.org/10.1016/j.jsams.2011.07.007

Lee, M., & Carroll, T. J. (2007). Cross Education. Sports Medicine, 37(1), 1–14. https://doi.org/10.2165/00007256-200737010-00001

Sale, D. G. (1988). Neural adaptation to resistance training. Medicine & Science in Sports & Exercise, 20(5), S135. https://journals.lww.com/acsm-msse/Abstract/1988/10001/Neural_adaptation_to_resistance_training.9.aspx

Scripture, E. W., Smith, T. L., & Brown, E. M. (1894). On the education of muscular control and power. Studies from the Yale Psychological Laboratory, 2(5).

Voskuil, C. C., Andrushko, J. W., Huddleston, B. S., Farthing, J. P., & Carr, J. C. (2023). Exercise prescription and strategies to promote the cross-education of strength: a scoping review. Applied Physiology, Nutrition, and Metabolism, 48(8), 569–582. https://doi.org/10.1139/apnm-2023-0041