Symmetry Of Single-leg Vertical Jump Height Hides Abnormal Lower Limb Biomechanics In Athletes After Anterior Cruciate Ligament Reconstruction

Background: After anterior cruciate ligament reconstruction(ACLR),single-leg horizontal hop distance limb symmetry index(LSI)>90% is recommended as a cut-off point for safe return to sports.However,despite achieving the 90% LSI threshold in single-leg horizontal hop distance,abnormal lower limb biomechanics continue to persist in athletes after ACLR.This implies that athletes after ACLR were at high risk of reinjury.Symmetry of single-leg vertical jump height is more difficult to achieve and can be a better representation of knee function than single-leg horizontal hop distance.Objective: To assess the kinematic and kinetic characteristics of lower limbs in athletes after ACLR with a single-leg vertical jump height LSI>90%,and compared with the athletes after ACLR with a single-leg vertical jump height LSI<90% and healthy athletes.To explore whether a single-leg vertical jump height LSI>90% can represent normal lower limb biomechanics of athletes after ACLR.Methods: Forty-six athletes after ACLR were divided into low symmetry group(jump height LSI<90%,n=23)and high symmetry group(jump height LSI>90%,n=23)according to single-leg vertical jump height LSI,while 24 healthy athletes as control group.The kinematic,kinetic characteristics of the low symmetry group,the high symmetry group and the control group during bilateral vertical jump were compared.A 9-camera infrared high-speed motion capture system was applied to collect kinematic parameters of the lower limb during jumping(Vicon T40,200 Hz).The kinematic parameters were collected synchronously with marker trajectories using four ground-embedded force plates(Kistler,1000 Hz).All data is extracted during the propulsion phase and landing phase.Kinematic variables of interest include peak hip,knee,and ankle angles.The kinetic variables of interest included hip,knee and ankle average moment,hip,knee and ankle average power,mean total power of the lower limb in the sagittal plane,and mean vertical ground reaction force.Statistical analyses were performed using SPSS 25.0.For kinematic and kinetic variables,paired-sample t tests were used for within-group comparisons,and one-way ANOVA was used for between-group comparisons.P<0.05 was considered statistically significant.Results:(1)Both sides of the low symmetry group showed a higher peak hip flexion angle(P<0.05),peak ankle valgus angle(P<0.05)compared with the control group during propulsion,and the operated side of the low symmetry group showed a lower knee extension moment(P=0.002),knee power(P=0.01),ankle power(P=0.045),vertical ground reaction force(P=0.002).(2)Both sides of the high symmetry group showed a peak ankle valgus angle(P<0.05)compared with the control group during propulsion.Compared with the healthy side,the operated side in the high symmetry group demonstrated lower knee extension moment(P=0.001),knee power in the sagittal plane(P=0.002),total power of lower limb in the sagittal plane(P=0.001),hip abduction moment(P=0.002),and knee valgus moment(P=0.036).(3)Both sides of the low symmetry group showed a higher peak hip flexion angle(P<0.05)and a lower peak flexion knee angle(P<0.05)compared with the control group during landing,the operated side demonstrated lower peak ankle plantarflexion angle(P=0.018),knee extension moment(P=0.005),knee power in the sagittal plane(P=0.037),ankle power in the sagittal plane(P=0.002),total power of lower limb in the sagittal plane(P=0.042),higher ankle external rotation moment.(4)Both sides of the high symmetry group showed a higher peak ankle external rotation angle(P<0.05),lower ankle external rotation moment(P<0.05)compared with the control group during landing.The operated side of the high symmetry group showed a lower knee extension moment compared to the healthy side(P=0.002),ankle plantar flexion moment(P=0.003),knee power in the sagittal plane(P<0.001),total power of lower limb in the sagittal plane(P=0.001),and vertical ground reaction force(P=0.027).Conclusion:(1)Athletes after ACLR with jump height LSI>90% exhibited more favorable lower limb biomechanics compared to athletes with single-leg vertical jump height LSI<90%.(2)The operated side of athletes after ACLR with single-leg vertical jump height LSI>90% still had abnormal lower limb biomechanical characteristics compared to healthy athletes and the healthy side.(3)Symmetry in single-leg vertical jump height does not represent normal lower extremity biomechanics in athletes after ACLR.