Introduction
Many people are now familiar with the concept of when you have a limb immobilised through injury, you can benefit it by training the unaffected side. This week’s article takes a deeper look at the best way to maximise this benefit by conducting a meta analyses of various studies conducted on this topic. A total of 10 studies were included, comprising 409 subjects (187 females and 222 males), and yielding 24 effect sizes. The studies had a mean duration of 6.3 ± 2.31 weeks, and subjects trained a mean of 3.5 ± 0.7 times per week. Of the 10 studies, 2 included groups using a varying number of sets (including single sets to muscular failure) while in the remaining 8 studies the groups performed the same numbers of sets and reps. Across these 8 studies, a mean of 4.4 ± 1.7 sets and 10 ± 3.5 reps were performed.

Article Title
Cirer-Sastre, R., Beltrán-Garrido, J. V., & Corbi, F. (2017). Contralateral Effects After Unilateral Strength Training: A Meta-Analysis Comparing Training Loads. Journal of Sports Science and Medicine, 16, 180-186.

Background
The cross-education effect (also known as the cross transfer effect or the cross-over effect) is the observation that long-term resistance training of a limb can lead to gains in voluntary strength in the contralateral limb. This effect has well-established, with a meta-analysis of 13 randomized controlled studies of voluntary unilateral resistance training (using >50% of 1RM and lasting >2 weeks) showing contralateral effects of resistance training of around an approximately 8% (range -3% – +22%) increase from the initial strength levels, or approximately 35% of the change in the ipsilateral limb. In order to explain the cross-education effect, it has been suggested that there exist central factors that are altered during strength training that affect both ipsilateral and contralateral sides similarly, although the ipsilateral limb also benefits from peripheral (muscular) adaptations. Currently, the underlying mechanisms for the cross education effect are unknown but may include changes at cortical, subcortical or spinal levels. The possibility of changes in many (but not all) peripheral factors has been explored and rejected. Related to the cross-education effect is the finding that imagined contractions can lead to meaningful gains in strength. In a remarkable study, it was found that left hand fifth digit metacarpophalangeal abduction torque increased after a 4-week period of strength training in both the conventionally-trained and imagined contractions groups (by 30% and 22%, respectively) and that both of these groups also displayed cross-education effects to the right hand (by 14% and 10%, respectively. In contrast, a control group displayed minimal changes in measurements for both left and right hands. Other researchers have reported marked gains in strength after imagined contractions. In addition, there appears to be an additive effect of imagined contractions and conventional resistance training, particularly in respect of the lower body.

Conclusion
The researchers concluded that training programs using single sets to failure produce the lowest cross-over effects to the contralateral limb, while those with multiple sets and/or eccentric-only training involve the highest cross-over effects.
So the take home message if you are re-training an immobilised limb is to train your good side with multiple sets with a focus on controlling the “down” phase of the movement.

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