Several studies discuss about the difference in the amount of applied force from a simultaneous two homonymous limb contraction compared to the sum of individual one-limb contractions, this is known in the literature as bilateral deficit phenomenon. The purposes of our revision were 1) to examine and classify the main factors affecting to bilateral deficit, 2) to determine the influence of strength training on this, and, 3) to recommend strategies to improve performance of the neuromuscular system in different conditions (bilateral and unilateral). Cochrane, Ibecs, Lilacs, Medline, Pedro and Scielo databases were searched from 1991 to 2014. Studies must conform to the definition of bilateral deficit. A total of 45 studies were included. The underlying mechanisms of bilateral deficit are not fully clear, the physical fitness condition could be a high influence factor (Howard and Enoka, 1991, J Appl Physiol (1985), 70(1), 306–16). Moreover the two-limb unilateral training increases the bilateral deficit (Weir et al., 1997, J Orthop Sports Phys Ther,25(4), 264–70), but the two-limb bilateral training decreases this phenomenon (Janzen et al., 2006, Eur J Appl Physiol,97(3), 253–60). Therefore, the main mechanisms responsible for the bilateral deficit were classified into biological and mechanical factors. Additionally, the plasticity of the bilateral deficit is mainly influenced by the execution (bilateral or unilateral) performed during the training protocol. Understanding the bilateral deficit and bilateral facilitation as two situations in which a subject can trigger a loss of strength when performing different tasks, took us to hypothesise the bilateral optimisation. The aim of this would be to achieve through strength training an optimal situation between bilateral and unilateral force relationship. Because numerous studies have shown relationships between force loss and increased risk of injury, this hypothesis could help to reduce these potential risk situations.
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