Did you know there is research indicating that unilateral resistance training can cause contralateral effects? In other words, if you strengthen one side of the body you will see strength gains on the other side.
A meta-analysis performed by Munn et al. (1) in 2004 analyzed 13 RCT’s of voluntary unilateral resistance training using training intensities of at least 50% of maximal voluntary strength for a minimum of 2 weeks. The data showed that unilateral strength training produced moderate increases in strength of the contralateral limb in young, healthy individuals. The recorded effect of unilateral strength training on the contralateral limb was an increase in strength by 35% of the ipsilateral side.
A study performed by Manca et al. (2) in 2015 investigated the contralateral effects of unilateral strength training on the ankle dorsiflexors. Similar to the previously mentioned meta-analysis, a cross training effect was found. This study reported that net peak torque and muscle work increased in both the trained and untrained dorsiflexors. In fact, there was actually a greater improvement in the untrained limb, which raises a interesting clinical implication in asymmetric conditions.
Carroll et al. (3) published an article trying to explain the possible mechanisms as to why this cross training effect occurs. One of their possible explanations was that the process of measuring the initial strength of the untrained limb could increase performance on subsequent tests. In addition, through the “trained” limb, patients could learn how to best position themselves or how to exert force against a dynamometer in the optimal direction. So, the increase in strength on the contralateral side may be due to familiarization with the testing protocol. Another thought is that strength training could cause a “spillover” of neural drive to the untrained side that leads to adaptation in the control system. Or perhaps unilateral strength training could cause neuromuscular adaptations in the control system of the trained limb that can be accessed by the opposite limb. Another possible explanation is that muscular adaptations occur due to anabolic hormonal changes that accompany resistance training. Systemic increases in anabolic hormone have ready access to the muscles of the untrained limb, so this may lead to some muscular adaptations.
Regardless of the underlying mechanism as to why the contralateral strength training effect occurs, through research it is hard to deny that some adaptation occurs. What is your take on the possible underlying mechanisms and how this cross training effect can be used? Due to most of the available research being performed on healthy individuals, it would be interesting to see if adaptations occur on an injured limb. Is it possible that contralateral strength training could lead to decreased atrophy of an injured extremity? Let us know what you think!
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