Thigh muscle co-activation during a single leg hop for distance one year after ACL-reconstruction and its relationship with a 2D knee flexion score: a cross-sectional study

Abstract Background Landing alterations during single leg hop to distance (SLHD) testing persist after anterior cruciate ligament (ACL) reconstruction and may contribute to a high risk of re-injury. Increased thigh muscle co-activation is recognised as a common neuromuscular adaptation after ACL injury, but conflicting results have been reported for jump landings. Therefore, this study aimed to assess changes in co-activation at one year post-reconstruction during the landing of a SLHD and to evaluate whether a 2D knee flexion scoring system could identify landings with heightened co-activation. Method This cross-sectional study assessed 18 sportingly active individuals one year after ACL-reconstruction alongside 18 matched control participants. Co-activation of the quadriceps and hamstring musculature during landing was recorded via electromyography. Additionally, peak knee flexion was assessed and scored as < 60°, between 60° and 80 ° and > 80° with a clinically practical 2D analysis. Co-activation was compared between the ACL-reconstructed leg, the contralateral leg and the matched control leg as well as between 2D-scored trials with statistical parameter mapping (SPM). Results SPM analysis revealed greater thigh muscle co-activation in ACL-reconstructed leg compared to the matched control leg from 229 ms to 250 ms after initial contact ( p = 0.007). No significant differences were observed between the ACL-reconstructed and contralateral leg. 2D knee flexion scoring did not reveal differences in co-activation during landing. Conclusion Neuromuscular control is not fully restored at one year post-ACL-reconstruction, and consideration of thigh muscle co-activation during single leg hop testing could be important. However, a 2D knee flexion scoring system is not suitable for revealing co-activation differences among recreational sportingly active people, and alternative methods are needed to assess these neuromuscular adaptations in the clinic. Trial registration The study was pre-registered at the German Clinical Trials Register (DRKS-ID: 00023002).