Cross-education: A super tool for training injured athletes?

OBJECTIVE
Cross-education is the phenomenon whereby the exercise of one limb can induce strength gains in the contralateral untrained limb. The aim of this study was to review the current research findings regarding cross-education (CE) following unilateral strength training and determine the overall magnitude of the contralateral strength gains.

WHAT THEY DID
The databases MEDLINE, the Cochrane Library, Scopus, Web of Science, and PubMed were searched for randomised controlled trials focusing on unilateral resistance training from inception to December 2016. Inclusion criteria included: (1) participants randomly assigned to unilateral training and to a control group receiving no intervention; and (2) at least one group of the study undergoing a unilateral resistance training with a minimum duration of 2 weeks against at least 50% of maximal voluntary strength. Thirty-one studies were included in the meta-analysis, and the risk of bias of these studies was assessed using the Cochrane Risk-of-Bias tool.

WHAT THEY FOUND
The CE effect resulted in a significant 11.9% contralateral strength increase on average. Results differed between body region. The upper-body saw a 9.4% contralateral increase, whereas the lower-body saw a 16.4% contralateral increase. Results also varied according to contraction type used – isometric (8.2%), concentric
(11.3%), eccentric (17.7%), and isotonic-dynamic training (15.9%). Finally, a high risk of bias was detected across all studies.

Practical Takeaways
This review proposes an evidence-based estimate of the CE effect and indicates that unilateral resistance training induces moderate to large contralateral gains in strength in healthy subjects. This has significant implications for injured athletes who are unable to train properly due to an injury on one side of the body. Using the CE effect, strength gains are better able to be maintained on the injured side without training it directly.
It is important to note that the size of the CE effect varies depending on the body region and the contraction type utilised. This study shows that the CE effect is far more pronounced for the lower-body, suggesting that lower-limb strength can be better maintained around injury. Strength and conditioning coaches should also use both eccentric and dynamic (concentric + eccentric) contractions over isometric and concentric contractions, as these contraction types provide a greater CE effect.
For example, if an athlete is in rehabilitation for a knee ligament injury (e.g. ACL) and is unable to strength train bilaterally, heavy single-leg leg press or weighted single-leg squats should help maintain strength on the injured side.

Tim’s Comments
“It is surprising how many strength and conditioning coaches are unaware of the effects of cross-education, it is a powerful tool to help maintain an athlete’s strength around an injury.
Interestingly, the magnitude of the contralateral strength gains appears to largely depend on the strength gains obtained ipsilaterally (i.e. with the uninjured side). Therefore, it is important that the athlete is in the correct mindset to train hard and really push to try to improve strength on the uninjured side, as this will increase the crossover effect.
This recent review fills an important gap in the research, as the last systematic review on this topic, was published a decade ago, and since then, numerous studies have been published on CE. As the authors of this study suggest, more high-quality studies are needed on this topic, due to the high risk of bias in the studies available to date.”