Does Vertical GRF Asymmetry Reflect Knee Extensor Moment Asymmetry in Squat in Individuals following ACLr?

Abstract

Individuals following ACL reconstruction (ACLr) adopt a strategy that shifts mechanical demands away from the knee of the surgical limb. Vertical ground reaction forces (vGRF) asymmetry has been used to characterize a shift in demands to the non-surgical limb in bilateral tasks. However, given the potential for both inter- and intra-limb compensations it is possible that vGRF asymmetry does not accurately reflect knee extensor moment (kEXT) asymmetry. PURPOSE: To determine how well between limb symmetry in vGRF predicts between limb symmetry in kEXT during a squat in individuals following ACLr at 2 time points; 3 and 5 months. METHODS: 11 subjects (7 females, 25.3 ± 10.7 yrs) performed bilateral squats to self-selected depths 3 (91± 11.1 days) and 5 months (148.8 ± 9.2 days) post-surgery. 3-D kinematics (250Hz), ground reaction forces (1500 Hz), anthropometrics were used to calculate kEXT moments (inverse dynamics). Symmetry indices (SI) were calculated (surgical/non-surgical limb) for peak vGRFs and kEXT during deceleration; SI<1 indicates smaller kEXT and vGRF in surgical compared to non-surgical limb. Separate linear regression analyses were used to determine how well vRGF symmetry (SIvGRF) predicts knee extensor moment symmetry (SIkext) at 3 and 5 months. RESULTS: On average, SIvGRF and SIkext were .86 ± .11 and .63 ± .18 at 3 months, respectively. At 3 month, SIvGRF predicted of SIkext (r2=.62, p=.004). On average, SIvGRF and SIkext were were .89 ± .070 and .70 ± .12 at 5 months, respectively. At 5 months, SIvGRF did not predict SIkext (p=.28). CONCLUSIONS: The larger asymmetries noted in kEXT versus vGRF indicates that SIvGRF underestimates SIkext. SIvGRF explained 62.4% of the variance in SIkext at 3 months suggesting that the reduction in knee loading is accomplished largely by inter-limb compensations. However, by 5 months post ACLr, reductions kEXT appear to be accomplished through adjustments made within the limb. Together these data suggest that caution should be taken when relating vGRF symmetry to knee loading during bilateral task.