Influence of hamstring activity on knee moments during loading response of gait in individuals following ACL reconstruction

Abstract

Purpose: Clinically, restoration of gait mechanics following anterior cruciate ligament reconstruction (ACLr) is expected by 3 months post-surgery. However, biomechanical studies at this time consistently report the presence of reduced knee extensor moments in the surgical limb during loading response (LR). This reduction in knee loading is quantified by net joint moments (NJM) estimated using inverse dynamics and represents the minimum moment required for the given kinematics. As a result, the NJM does not consider the contribution from individual muscle groups including quadriceps and hamstrings. Thus, alterations in quadriceps or hamstrings activity could confound our interpretation of the NJM and characterization of knee loading in the surgical limb. In the presence of increased knee flexor activity, the true contribution from the knee extensors may not be reduced at all. Moreover, increased hamstring and consequential quadriceps activity could result in increased tibiofemoral compressive forces and lead to the progression of knee osteoarthritis. Thus, the purpose of this study is to compare the influence of hamstring activity on knee moments during LR between limbs in individuals post-ACLr. Methods: 17 individuals (11F, 24 ± 11 yrs) 114 ±1 7 days post-ACLr walked at 1.4 m/s. 3D kinematics (250 Hz), ground reaction forces (GRF; 1000 Hz) and anthropometrics were used to quantify sagittal plane joint kinematics and kinetics (inverse dynamics). Medial and lateral hamstring surface electromyography (EMG; 1000 Hz) was collected. Maximum voluntary isometric contractions (MVIC) were performed bilaterally. Raw EMG signals were band-pass filtered (10–500 Hz) and rectified. A 6 Hz low-pass filter was used to create a linear envelope. All data were normalized to 101 points of stance. A standard musculoskeletal SIMM (Software for Interactive Musculoskeletal Modeling) model was used to estimate knee flexor moments. Sagittal plane hip, knee, and ankle kinematics and hamstring EMG (biceps femoris long head, BFLH, and short head, BFSH, semimembranosus, SM, and semitendinosus, ST) normalized to MVIC were used as inputs. SM and BFSH activation were assumed to be the same as ST and BFLH, respectively. To account for differences in hamstring strength, the knee flexor moment from SIMM was scaled using a ratio of the subject’s maximum knee flexor torque during MVIC divided by the maximum knee flexor torque of the SIMM model in the testing position. The sum of the scaled estimated knee flexor moment and net knee moment (Visual3D) were used to estimate knee extensor torque. Moment impulses were calculated as the area under the curve from initial contact to peak knee flexion. Four successful trials were collected for the surgical (ACLr) and non-surgical (NS) limbs. Paired sample t-tests were used to compare net knee and estimated flexor and extensor moment impulses between limbs (mean ± SE); α = 0.05. Results: Net knee (P = 0.02; ACLr: −0.028 ± 0.003; NS: −0.041 ± 0.004 Nm*s/kg) and estimated extensor (P = 0.02; ACLr: −0.035 ± 0.004; NS: −0.048 ± 0.005 Nm*s/kg) moment impulses during LR were reduced in ACLr compared to NS limb. However, estimated knee flexor moment impulse (P = 0.99; ACLr:0.008 ± 0.001; NS:0.008 ± 0.001 Nm*s/kg) was not different. Conclusions: These data are consistent with the literature reporting reduced net knee loading in the surgical limb post-ACLr. When accounting for the torque-producing capabilities of the knee flexors, the estimated knee extensor moment was greater than the net moment in both limbs. The estimated knee extensor moment remained reduced in the ACLr limb compared to the NS limb in the absence of between limbs differences in SIMM estimated knee flexor moment. These data indicate that individuals do not disproportionately increase hamstring contribution to the net knee moment in the surgical knee during gait and the reduced net knee moment reflects a pattern in which the contribution of knee extensors is reduced during LR of gait.