Benjaminse A, Holden S, Myer GD. ACL rupture is a single leg injury but a double leg problem: too much focus on ’symmetry’ alone and that’s not enough!. Br J Sports Med 2018: British Journal of Sports Medicine (BJSM) -2017-098502

Criteria-based return to sport has focused on symmetry during hop and strength tests. However, leg symmetry index (LSI) measures underestimate the magnitude of performance and functional deficits. In one study where 11 out of 70 athletes sustained a second ACL injury, up to 3 out of 4 of those athletes who go onto second ACL injury pass 90% LSI return-to-sport criteria in quadriceps strength and single-leg hop tests 6 months after initial ACLR.
Most athletes did not achieve required preinjury knee function, which may have been the underlying determinant of their second ACL injury risk.
Future perspectives
The contralateral (uninjured) limb may not provide an appropriate ‘gold standard’ benchmark for rehabilitation, particularly considering the neurological changes that occur after injury.
As an example, during rehabilitation using a drop vertical jump technique to restore asymmetry, it would be suboptimal to instruct the athlete to control the knee of the injured leg with a unilateral cueing such as ‘don’t let your knee roll inward when landing’ (internal focus). Rather, the emphasis should be on cueing on an external goal; for example, ‘reach your knees towards the cones when landing’ (external focus) (figure 1). The latter is a more central approach, aimed at reducing the increased reliance on conscious (internal) control during movement seen after initial ACL injury.
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Additional perspectives on ‘ACL rupture is a single leg injury but a double leg problem…’ Anna Trulsson, april 2018 British Journal of Sports Medicine (BJSM) as a clinical commentary on the first article.

 

Limb Symmetry Index (LSI) is not perfect but can be relevant: when and how?
  • No consensus exists on how to decide when to return to sports.
  • Often, strength and hop tests are used, calculating an LSI (the ratio of injured/non-injured sides), and LSI >90% is frequently suggested as a cut-off criteria.
  • impaired capacity on both sides after ACL injury is well known, underestimating deficits measured as LSI. Therefore, measurements of preinjury capacity or normative data of non-injured controls have been suggested.
  • several authors conclude that one important sensorimotor risk factor is altered movement patterns (in specific increased frontal plane knee motion, poor trunk positioning or landing techniques). Therefore, quantification of altered movement patterns should be included in test batteries: This is an observation test measuring predefined, unfavourable movements/alignment in ankle–knee–hip and trunk regions and can be used before, during and after rehabilitation. It should be kept in mind that when measuring movement patterns, considerable interindividual variations exist, and the individual has to be his or her own control.
In focus when designing future test batteries and rehabilitation programmes: motor control recapitulated 
  • Use versatile test batteries reflecting also sensorimotor deficits.
    • a complex integration of neural and muscular mechanisms coordinated by the central nervous system, CNS, and takes into account soft tissue restraints, articular mechanics and joint loads to create appropriate movements and stabilisation.
    • maintain and modify posture during movements, the CNS coordinates visual, vestibular and proprioceptive information into automatic, continuous muscular activation in muscular synergies. Feed-forward control (anticipatory actions) and feedback control (corrective response to tasks/perturbation, also involved in motor learning) the continuous corrections in functional/ dynamic joint stabilisation (the ability to remain stable in single joints and in kinematic chains).
  • This implicates the use of more versatile test batteries to measure sensorimotor deficits that include strength- , hop- and movement-quality tests and measurements of, for example, muscle activation, joint stabilisation and motor learning aspects.
What to do before return to sports? Implications for rehabilitation to prevent further complications
Importantly, commonly used strength training alone does not efficiently train the central control responsible for timely muscular responses resulting in dynamic joint stabilisation. I therefore suggest that training programmes are studied aiming to optimise dynamic joint stabilisation by training muscular synergies, balanced in time and magnitude through the relearning of motor control to create appropriate, automatic and generalised movements, without unfavourable altered movements. Examples of this are exercises where voluntary movements of the contralateral leg, trunk or arms generate compensating, postural and stabilising reactions on injured side.
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My clinical interpretation of the articles. Do not relay on LSI if the non-injured leg also have been out 8 months