It is well established that patients who have an ACL rupture demonstrate physical impairments and disability related to the injury [1–3]. Despite conservative treatment most patients will undergo ACL reconstruction. Traditionally, clinicians have utilized various outcomes as criteria to assess impairment and disability following ACL injury [2, 3].
Impairments following ACL injury are functional (anterior displacement of the tibial relative to the femur) and physiologic (range of motion, muscle performance and pain). These can be measured by the KT-1000 knee arthrometer, goniometer, isokinetic muscle tests, and visual analogue scale of pain .
Disability following ACL injury is related to performance of daily activities, leisure time activities, or sports activities and has traditionally been measured with valid questionnaires, such as the Lysholm and Cincinnati knee scores, and functional knee tests [3, 4].
Quadriceps muscle dysfunction- weakness or reduced accelerated reaction time- is recognized as significant complication following ACL reconstruction [5–8]. Quadriceps muscle activity causes an anterior translation of the tibia approximately in the range from 20° to 60° or 75° of knee flexion with maximal tibia displacement occurring at 45° (quadriceps mechanical disadvantage) and less at 90° of flexion (maximum strength produced) [9–14].
Despite, the plethora of the progressive and accelerated exercise programs for ACL reconstructed patients, long term impairments and quadriceps deficiency often persists [5, 15, 16].
Therefore, identifying an exercise protocol such as cross exercise (CE), as an adjunct to traditional ACL rehabilitation program may facilitate rehabilitation strategies and thereby maximize functional outcomes.
CE is referred to the contralateral limb, by increasing strength in the homologous muscle of the untrained limb, without directly involving the latter in the motor activity . Several neural mechanisms have been proposed for CE including diffusion of impulses between hemispheres, coactivation via bilateral corticospinal pathways, postural stabilization and the cerebral cortex theory which has been referred to as the most dominant mechanism [17, 18]. It is explained by the theory that during the voluntary contraction of a muscle on the trained side is produced a facilitation effect on the same motor point in the opposite side of the cerebral cortex [17, 18]. This is also explained anatomically by the fact that 10% of the corticospinal fibers enter in the lateral and anterior corticospinal tract of the trained side, whereas the remaining fibers cross to the opposite side of it through diffusion of impulses [17–19].
Benefits of CE have been well established on quadriceps strength improvement in healthy subjects [19–23]. As far as the type of CE is concerned, eccentric exercise has been found to be superior to isometric and concentric exercise [19, 24, 25]; and has had the greatest effect on quadriceps strength improvement accounting for the greater increases in eccentric and isometric forces [19, 24–26]. However, an intraspinal mechanism is probably more likely to mediate CE especially in studies that observed remarkably greater CE using eccentric contractions [17, 20, 22, 26].
In addition, it would be suggested that eccentric contractions are associated with unique motor unique activation strategies by the nervous system and that the process of inducing CE may be different for training with concentric and isometric contractions [19, 24–27].
Thereby, eccentric exercise benefits have been well established in the literature as the muscle forces which are produced during muscle lengthening are extremely high, despite the requisite low energetic cost .
As far as the frequency of CE is concerned, there is no consensus across the literature which supports an association between the training frequency and the benefits of CE . However, most studies have used as the most appropriate frequency three days per week [17, 19, 20, 22, 28].
Limited studies have reported the effect of cross exercise in patients following knee reconstruction [28, 29]. Papandreou et al  have shown that cross eccentric exercise has been proved to be a useful mechanism in strengthening the quadriceps muscle on the ACL reconstructed knee by training the uninjured knee, at knee angles at 45° and 90° of knee motion at a sequence of 3 d/w, in the early rehabilitation phase of ACL reconstruction.
Based on the above, it is not unreasonable to assume that the use of cross eccentric exercise more than three days per week-such as five days per week- as an adjunct to traditional ACL rehabilitation program might have an enhanced effect on CE, assist rehabilitation methods and thereby maximize quadriceps functional outcomes, in the early ACL postoperative period.
Thus, the primary purpose of this study was to investigate the effect of cross eccentric exercise (CEE), applied three and five days per week, on the quadriceps accelerated reaction time at the angles 45°, 60° and 90° of knee flexion, in the early rehabilitation phase of ACL reconstructed knee.
The secondary purpose was to investigate the effect of CEE, applied three and five days per week, on the subjective scores of disability questionnaire, in the early rehabilitation of ACL reconstructed patients.