Differences in Vertical and Lower-limb Joint Stiffness in RTS Assessments Between ACLR Patients and Non-injured Controls

The aim of this study was to establish alterations in vertical and lower-limb joint stiffness following anterior cruciate ligament reconstruction (ACLR). 127 male patients 8-10 months post-ACLR and 45 non-injured controls performed unilateral and bilateral drop jumps, and cutting, while ground reaction forces (GRFs) and 3D kinematics were recorded. Stiffness and changes in vertical GRF were lower in ACLR patients during bilateral drop jumps compared to non-injured controls. ACLR patients also displayed lower knee stiffness in the bilateral drop jumps (-0.91,  < 0.001 and  0.53,  < 0.001, respectively) and cutting (-0.85,  < 0.001 and  0.19, 0.040, respectively). In the unilateral drop jump, there were no differences in ankle, knee, or hip stiffness between groups, yet ACLR patients displayed smaller changes in knee moments (-0.63,  < 0.001) and decreased knee range of motion (0.44, 0.013). During the bilateral drop jump, ACLR patients displayed lower ankle stiffness (0.46, 0.003) and smaller ankle moment changes (0.48, 0.006), compared to controls. Hence, joint level analysis provides practitioners with a more detailed insight into an athlete's movement strategy following ACLR than whole body analysis. Range of motion, change in moment, and stiffness of the knee joint especially, can help practitioners to assess fitness for return-to-sport in ACLR patients.