Injury Prevention: From Symmetry to Asymmetry, to Critical Thresholds

Many postural and therapeutic protocols rely on reestablishing idealized levels of symmetry, specifically, left–right symmetry. For example, the ninth edition of the American College of Sports Medicine’s Guidelines for Exercise Testing and Prescription states that a “training program should induce symmetrical and balanced muscular development” [1]. There is even a clinic called Symmetry Physical Therapy (https://www.symmetry-physicaltherapy.com/). Contrariwise, the rationale developed so far suggests that this perspective may be flawed. In this context, the systematic review of Bishop et al. [2] highlights the fact that we don’t actually know the practical effects of attempting to reduce existing asymmetries. That is to say that even if such a reduction would be desirable, it may not be possible to achieve in the long run. In fact, motor asymmetries may be functional for performance and not necessarily associated with a greater injury risk [3]; remember that this feature has also been reported in polo horses [4]. Indeed, because symmetry is less efficient than asymmetry, most systems will spontaneously break symmetry [5], and human bodies that are forced toward more symmetric states might simply break symmetry at the earliest opportunity. In addition, most activities performed during sports performance will likely increase certain asymmetries [6].