Osteoarthritis is a prevalent and debilitating joint disease. The global prevalence of osteoarthritis is estimated to be 3.6-4.1% of the global population, making it the 11th highest contributor to global disability. Osteoarthritis most commonly occurs in the medial tibiofemoral compartment of the knee. Inappropriate knee loading, during daily activities, is believed to be a principle cause of knee osteoarthritis. Moreover, individuals who have sustained anterior cruciate ligament (ACL) rupture and reconstruction (ACLR) are at particularly high risk of onset of knee OA in the near future. Within 12 years following ACL injury approximately 50% of people will develop knee OA. Worse still, many individuals who have sustained an ACL injury show signs of early knee degeneration as little as 4-5 years post-surgery. The question arises, what is it about ACL injury that results in such high risk of future OA onset?

Individuals with ACLR have abnormal gait biomechanics and muscle activation patterns, which may influence the knee contact forces. As well, the autograft donor muscles are often impaired following harvesting. This muscle impairment also likely influences the knee contact forces. However, what links exist between the altered knee contact forces and degeneration of the articular tissues have not been established. To date, there are no literature reports of the knee contact forces during both daily gait tasks and sporting movements. Likewise, an investigation of the differences in the knee contact forces between ACLR and healthy individuals during different movement tasks has not been performed. Finally, the literature lacks an assessment of the relationships between the tibiofemoral contact forces that arise during daily activity and the structural health of the articular tissues in the knees of ACLR individuals.

The overall aim of this thesis was to explore the tibiofemoral contact forces in healthy individuals and those with ACLR during common everyday activities and sporting movements to better understand the nature of the contact loads sustained by the tibiofemoral articular tissues. Furthermore, we aimed to examine the relationships between the tibiofemoral contact forces and the structure of the articular tissues in these two populations.

In the first study of this thesis the tibiofemoral contact forces were explored, as well as the relative contributions made by muscle and external loads to those contact forces, in healthy individuals performing walking, running and running with diagonal sidestepping. Furthermore, the relationships between several external gait measures and the tibiofemoral contact forces during these gait tasks were examined. A calibrated electromyography-driven neuromusculoskeletal model was used to estimate the tibiofemoral contact forces, as well as the relative contributions of muscle and external loads to those contact forces during walking (1.44±0.22 m.s⁻¹), running (4.38±0.42 m.s⁻¹) and running with diagonal sidestepping (3.58±0.50 m.s⁻¹) in healthy adults (n=60, 27.3±5.4 years, 1.75±0.11 m, and 69.8±14.0 kg).

The tibiofemoral contact forces increased from walking (~1-2.8 BW) to running (~3-8 BW) and to sidestepping (~3-8.5 BW), peaking in running for the medial contact forces (~8 BW), and peaking in sidestepping for the total (~8.5 BW) and lateral contact forces (~4.5 BW). The mean relative contributions of muscle to the medial tibiofemoral contact forces increased across gait tasks, from ~50% during walking to >90% during sidestepping. While the medial compartment bore the majority of the load during walking and running (~65% of total), during sidestepping both medial and lateral compartments experienced similarly large contact forces (4.3–4.6 BW), generated primarily by muscle. For all the gait tasks, but particularly sidestepping, muscle was the primary stabilizer of the knee in the frontal plane.

The knee adduction moment (KAM) had weak relationships with tibiofemoral contact forces across gait tasks (all R²<​0.33), particularly during the more vigorous gait tasks of running and sidestepping. Gait task was found to be a significant factor in the relationships between the KAM and tibiofemoral contact forces, suggesting that the correlations between KAM and contact forces were specific to the gait task, and that other biomechanical and neuromuscular measures are influential over the tibiofemoral contact forces.

When the multiple external measure were combined into step-wise regression models the resulting linear relationships were strengthened (0.28<​ 𝑅²_adj<​0.59). The most important measures were the vertical ground reaction force (VGRF), knee flexion moment (KFM) and KAM. However, the relationships were only weak-to-moderate and highly variable across the tibiofemoral contact forces and gait tasks. When a step-wise regression equation from a particular gait task (e.g. walking) was applied to a different gait task (e.g. running or sidestepping) they produced large prediction errors, indicating that the relationships between multiple external measures and the tibiofemoral contact forces were specific to the gait task.

The second study of this thesis explored the tibiofemoral contact forces, as well as the relative muscle and external load contributions to those contact forces, in ACL reconstructed (ACLR) and healthy individuals during walking, running and running with diagonal sidestepping. A computational electromyography-driven neuromusculoskeletal model was used to estimate muscle and tibiofemoral contact forces in ACLR individuals with combined semitendinosus and gracilis tendon autograft (n=104, 29.7±6.5 years, 78.1±14.4 kg) and healthy controls (n=60, 27.5±5.4 years, 67.8±14.0 kg) during walking (1.4±0.2 m.s⁻¹), running (4.5±0.5 m.s⁻¹) and running with sidestepping (3.7±0.6 m.s⁻¹). In the electromyography-driven neuromusculoskeletal model, the strength deficits and morphological changes subsequent to autograft harvesting in the ACLR semitendinosus were accounted for by adjusting the maximum isometric strength and optimal fibre length, and then optimizing the tendon slack length to preserve the normalized muscle fibre and tendon operating curves.

The ACLR individuals had smaller maximum total and medial tibiofemoral contact forces than controls for the three gait tasks, despite no significant differences in gait spatiotemporal parameters or ground reaction forces and despite the ACLRs being significantly heavier than controls. The ACLR individuals were also found to have a smaller maximum external knee flexion moment, which likely drove their smaller tibiofemoral contact forces. The mean relative contributions of muscle and external loads to the tibiofemoral contact forces were statistically equivalent between the ACLR individuals and controls. This meant that the role of muscle in distributing the contact loads between the medial and lateral tibiofemoral compartments was not statistically different between the two groups. However, like in the first study, the relative contributions of muscle and external loads to contact forces in the ACLRs differed substantially across gait tasks, increasing from walking to the more vigorous running and sidestepping movements.

The third study of this thesis assessed the relationships between the walking tibiofemoral contact forces and the tibiofemoral articular tissue health in the ACLR and healthy individuals. The study involved 100 individuals 2-3 years following combined semitendinosus and gracilis tendon autograft ACLR and 30 healthy individuals who served as controls. Knee MRIs were acquired and processed to measure the tibial articular cartilage volumes, mean thicknesses and subchondral bone plate areas, as well as tibiofemoral articular cartilage defects and bone marrow lesions. The walking tibiofemoral contact forces were estimated using the aforementioned calibrated electromyography-driven neuromusculoskeletal model.

Significant positive relationships between walking tibiofemoral contact forces and articular cartilage volumes (R²=0.33) and thicknesses (R²=0.20) were found for the healthy individuals in the medial compartment, but not for ACLRs. Both ACLR and healthy individuals showed significant positive relationships between walking tibiofemoral contact forces and subchondral bone plate areas (0.1<​R²<​0.7). ACLRs had more, and more severe, articular cartilage defects and bone marrow lesions. No significant relationships were found between walking tibiofemoral contact forces and articular cartilage defects, but larger maximum walking medial tibiofemoral contact forces significantly decreased odds of severe bone marrow lesions in the medial tibiofemoral compartment of the ACLR individuals (odds=0.86, Wald χ²(1)=4.9, p=0.02, 95% CI, 0.79-1).

Within 2-3 years following ACLR, the articular tissue health of ACLR individuals was significantly worse than healthy individuals. ACLR individuals had less and thinner tibial articular cartilages and more articular cartilage defects than the healthy controls. Increased medial tibiofemoral contact forces in the ACLRs may have protected the medial tibiofemoral compartment from severe BMLs. Although, due to the cross-sectional study design we cannot infer cause and effect from these relationships, thus they should be interpreted with caution. The articular cartilages and subchondral bone responses to tibiofemoral contact forces were significantly different between the ACLR and healthy controls. Controls had stronger relationships between load and morphology, and this may have indicated that relationships between articular tissue structural health and applied loading in the ACLR individuals were disturbed. This may potentially expose the joint to further degeneration.