Introduction: Jumping involves complex control processes of sensory input and feedback with coordination of multiple joints. Following anterior cruciate ligament reconstruction (ACLR), altered movement strategies identified by kinematics and/or kinetics unload the injured leg and increase demands on nearby joints. Dynamic knee stability (DKS; resistance to linear/angular accelerations) has not been evaluated for ACLR persons during sport-similar tasks. It is therefore unknown if there is true instability or altered task execution, e.g. due to fear. We hypothesized that DKS following rehabilitation after ACLR would be similar to that of knee-healthy controls, but with prevailing protective movement strategies.
Methods: A motion capture system synchronized with two force plates registered hip and knee 3D angles and moments during a one-leg standardized side hop for 30 ACLR persons (7-129 months post-reconstruction) and 30 controls. EMG provided mean knee extensor and flexor muscle activation patterns and co-contraction ratios and indexes before (50ms window) and during landing. DKS was evaluated using a helical axis rotation interval of 10˚ to describe how much knee kinematics diverges from flexion-extension. DKS and muscle activation variables were analysed with t-tests. Hip and knee movement strategies were analysed for angles at initial contact, peak angles during landing, and peak moments during landing with MANOVAs between and within groups (p<0.05).
Results: No significant differences in DKS was found between groups. A significant main effect of group was however observed for angles at initial contact (p=0.028) with ACLR showing greater hip and knee flexion than controls. A significant main effect of group was also found during landing for peak angles (p=0.001) and moments (p=0.017) with ACLR displaying greater hip flexion and knee internal rotation moment, and also greater mean knee flexor activation than controls (p=0.049). No group differences in muscle activation patterns were shown before landing. ACLR had between-leg asymmetries at initial contact (p=0.010) with greater hip flexion, and asymmetries for peak angles (p=0.008) and moments (p=0.030) during landing with greater hip flexion angle, greater hip flexion moment and less knee flexion moment in the injured leg.
Discussion: Despite similar DKS, ACLR displayed different movement strategies and asymmetries relative to controls, which indicates a task-coping strategy in preparation for and during landing. The greater hip and knee flexion angles at initial contact in relation to the greater hip flexion moment and knee flexor activation that followed may have increased knee joint stiffness, which could explain the similar DKS relative to controls. The between-leg asymmetry with greater reliance on the hip and simultaneous unloading of the knee further supports a coping strategy for ACLR. Further attention should be given to the hip during rehabilitation for ACLR persons to avoid future injuries due to movement compensation.