Response to the Position Paper of the European Interdisciplinary Society for Clinical and Sports Application (EIScsa): Muscle imbalances – fact or fiction?

The authors of Muscle Imbalances – Fact or Fiction, have correctly identified the complexities of quantifying muscle imbalances as related to predisposition to injury. It seems intuitive that significant muscle imbalance would predispose one to injury during high performance sport or strenuous activities common with laborers in industrial settings. However, scientific evidence in support of this impression requires prospective research paradigms involving pre-injury screening of large numbers of uninjured subjects. One good analogy is the clinical assumption that stretching and muscle extensibility reduces sports injury risk. Thacker et al. [1] conducted a systemic review and meta-analysis of the literature in an attempt to find evidence that stretching prevents or reduces injuries in sports. The authors found that stretching is not significantly associated with a reduction in injury risk, and concluded that the evidence for or against stretching is equivocal. The authors have also correctly pointed out that single-factor-related analysis (e.g., uniplanar strength ratios) of muscle function reflects an insufficient approach to identifying a higher risk of overuse or traumatic injury or re-injury. However, I continue to contend that open chain analysis of single muscle group performance is the best way to identify preor postinjury deficits in torque, power, and work, that can be overlooked with more functional closed chain analysis. For example, Kowalk et al. [2] evaluated bilateral joint angles, moments, powers, and work in uninjured and anterior cruciate ligament (ACL) deficient subjects during stair climbing preand 6 months post-operatively. Post-operative deficits were found for peak moment, power, and work in the injured knee, yet these reductions were compensated by increases in excursion, moment, and power in the contralateral ankle. Ernst et al. [3] conducted a video analysis of ACL reconstructed and matched uninjured subjects during vertical jump, lateral step-up, and hop activities. They found that the ACL-injured side had lower knee extension moments compared to the uninjured side and matched subjects. However, no differences in hip+knee+ankle summated moments were found, and the injured side hip was found to have compensated for the knee moment deficits. These studies provide evidence that while closed chain functional analysis is important, it alone is insufficient in identifying single muscle group deficits that can be masked with proximal or distal joint, or contralateral extremity compensations. I agree with the authors that a detailed analysis of muscle function that includes passive and active characteristics of muscle lengthening (flexibility, stiffness) and sensorimotor function parameters (e.g., stabilometry, proprioceptive aspects, reflex and electromyographic activity) would more comprehensively assess injury risk. However, this comprehensive analysis of muscle function is impractical for screening large num