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Anterior cruciate ligament reconstruction (ACLR) has been reported to be effective at restoring function and stability to the knee and improving patient-reported outcomes. However, it does not guarantee an athlete will return to play (RTP) and that re-injury will not occur. This may be due, in part, to the absence of clear criteria identifying the completion of physical rehabilitation. Current RTP testing batteries often utilize performance outcomes during jumping and hopping tests and focus on the ability to achieve the same height/distance between limbs (i.e. symmetry of performance outcome). Rarely do these clinical tests assess the biomechanics (joint angles and forces) used by the athlete to achieve this performance outcome.

The analysis of movements through the entire stance phase may identify differences between limbs that can be targeted during rehabilitation to improve outcomes, such as re-injury and performance on return to play. In addition, it has been suggested that a battery of tests may provide a more robust assessment of rehabilitation status than single-test assessment. 

The aim of this study was to identify biomechanical and performance variable differences between ACLR and non-ACLR limbs 9 months after surgery across a number of jump tests. 

One hundred and fifty-six consecutive eligible subjects (males aged 24.8±4.8 years, height 180±8 cm, mass 84±15.2 kg) were recruited prior to ACLR. All subjects were multidirectional field sport athletes who intended to return to the same level of participation post-surgery. The average time of testing was 8.8±0.7 months post-surgery. 

All testing took place in the 3D biomechanics laboratory. An eight-camera motion analysis system (200 Hz), synchronized with two force platforms (1000 Hz), captured the position of 24 reflective markers and ground reaction forces. The reflective markers were secured at bony landmarks on the lower limbs, pelvis, and trunk per the plug-in-gait marker set. 

Four jump tests (double-leg drop jump (DLDJ), single-leg drop jump (SLDJ), single-leg hop for distance (SLHD) and hurdle hop (HH)) were carried out. 

In the DLDJ, the strongest differences (based on effect size) were in knee valgus moment (−0.92), knee external rotation moment (−0.81), and ankle external rotation moment (−0.80) with lower values on the ACLR side. Medium-size differences were found for less knee internal rotation angle (−0.73), hip abduction angle (0.68), knee extension moment (0.51), and hip abduction moment (−0.50 to −0.52) on the ACLR side. There were also lower vertical (0.61) and posterior ground reaction forces (0.54) on the ACLR side. 

In the SLDJ, the largest differences were reduced knee valgus moment (−0.74) and reduced posterior COM distance to the knee (0.74) on the ACLR side throughout the stance phase. There were also medium effect size differences for less knee extension angle (0.67), less hip extension angle (0.55) during end of the stance phase and greater ankle dorsiflexion angle (−0.51) for most of the stance phase on the ACLR side. There was less external ankle rotation moment (−0.67), knee internal rotation angle (−0.64), hip internal rotation moment (0.56), and knee external rotation moment (−0.54) on the ACLR side throughout most of the stance phase.

In the SLHD, the biomechanical differences with large effect size were less posterior COM to knee (0.82) and less knee valgus moment on the ACLR side (−0.8). Medium effect size differences included less knee internal rotation (−0.61), less hip abduction angle (0.6), hip internal rotation moment (0.59), ankle external rotation moment (−0.59), and ankle eversion moment (−0.51) on the ACLR side. There was also less dorsiflexion angle on the ACLR side (−0.5) and less knee flexion (−0.55) throughout landing.

The HH had the fewest number of variables with between-limb differences, but these variables were common with all the other tests. These include less knee valgus moment (−0.74), less ankle external rotation moment (−0.59), and less knee internal rotation angle (−0.59) on the ACLR side. In addition, the COM was more anterior to the knee (0.61) on the ACLR side.

There were significant differences between limbs (< 0.001) for height jumped and reactive strength index in the SLDJ with large effect sizes and distance jumped for the SLHD with a small effect size. Limb symmetry index for the SLDJ was 79% (±21%) and 78% (±34%) for the jump height and RSI, respectively, and 94% (±19%) for the SLHD.

This study demonstrated biomechanical differences throughout the kinetic chain and performance differences between limbs 9 months post-ACLR. The results suggest that the SLDJ may identify greater jump height/length deficits between limbs than SLHD, which may over-estimate rehabilitation status. The findings point to the importance of including biomechanical analysis through the stance phase during assessment of jump tests after ACLR.

Source: King E et al. (2018) Whole-body biomechanical differences between limbs exist 9 months after ACL reconstruction across jump/landing tasks. Scand J Med Sci Sports 28:2567–2578.