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.2 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.3 However, individuals with ACL reconstruction rarely reach 90% symmetry.4 Moreover, 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, since they may be more sensitive in predicting second ACL injuries.5 Therefore, to meet the concerns of the double leg problem I argue for that when LSI is used, it should be interpreted in combination with applicable normative data of controls or of preinjury capacity.

Underlying sensorimotor deficits that might have led to the initial injury can be found when studying initial risk factors for ACL injury. Indeed, risk factors for injury are multifactorial,6 but 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).7 Therefore, quantification of altered movement patterns should be included in test batteries. One such quantifiable measurement, reliable and valid for individuals with ACL injury, is the Test for Substitution Patterns. 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.8 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 these situations, symmetry measurements are of great value.

In focus when designing future test batteries and rehabilitation programmes: motor control recapitulated

To optimise test batteries, the included tests should reflect the complexity of movement and stabilisation—motor control.9–11 In short, in the non-injured situation, sensorimotor control in posture and movement comprise 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. To 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) contribute to the continuous corrections in functional/dynamic joint stabilisation (the ability to remain stable in single joints and in kinematic chains).10 11

After knee injury, side-to-side differences are described as deficits in biomechanical stability and in dynamic joint stabilisation.10 11 The sensorimotor deficits that can follow joint injury relates to alterations and adaptations demonstrated as: altered somatosensory input from the injured side to the spinal cord and the brain, altered central information processing in the brain resulting in erroneous efferent motor commands to the muscles, resulting in (altered) movements that in turn provide new afferent stimuli. Manifestations of sensorimotor deficits can, for example, be delayed or altered muscular activation, reduced muscle strength and/or altered movement patterns12; characteristics so complex that they have to be measured separately and with different tests. 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. For a more complete review of return-to-sport testing, see ref 2.

What to do before return to sports? Implications for rehabilitation to prevent further complications

Rehabilitation programmes integrating strength and sensorimotor control training have been found effective in ACL injury rehabilitation, but the exact content is under debate.13 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 side14 (figure 1A–D). Patients with non-reconstructed ACL injury and activity modification, trained with such exercises, were found to have good knee function, acceptable activity level and favourable outcome concerning osteoarthritis 15 years after injury.15

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Figure 1

Examples of exercises suggested to efficiently train the central control, with the aim to improve dynamic joint stabilisation. Voluntary movements of the contralateral leg, trunk or arms are used to generate compensating, postural, stabilising muscular reactions. Exercises in figure 1A–C have the aim to generate stabilising reactions mainly in the right leg, and the exercise in figure 1D in both legs. Arrows denote movements. Circles denote what is primarily in focus of the training. (A) Subject in supine position, right knee bent about 90°, right foot in line with knee and hip placed on a ball that is positioned in a corner. Seat is lifted from floor, and left leg is moved repeatedly up and down in rapidly hip flexion–extension. Arms aside body. (B) Subject standing on right leg with proper alignment in foot, knee, hip, trunk and neck. Left leg is repeatedly moving in hip flexion–extension with resistance from a pulley machine. The arms are moved in flexion–extension as in walking/running. (C) Subject standing on right leg with proper alignment in foot, knee, hip and trunk with support from hands. The subject is repeatedly flexing right knee and hip about 90°, moving left leg behind the body without support from floor and then moving left leg rapidly up towards abdomen on the same time as moving up on tiptoes on right leg. Left and right figures indicate end positions. (D) Subject standing on both legs with slight knee and hip flexion and proper alignment in foot, knee, hip, trunk and neck. A medicine ball is repeatedly and rapidly moved from right side to left side.

Suggestions in summary

• Use LSI measurements in combination with applicable normative or preinjury data of controls.

• Quantify movement patterns before, during and after rehabilitation.

• Use versatile test batteries reflecting also sensorimotor deficits.

• Rehabilitation aims: relearning of movements and dynamic joint stabilisation by, for example, using voluntary movements generating postural, stabilising, muscular activity in muscular synergies, performed without unfavourable alterations.