We explored the tibiofemoral contact forces and the relative contributions of muscles and external loads to those contact forces during various gait tasks. Second, we assessed the relationships between external gait measures and contact forces. A calibrated electromyography-driven neuromusculoskeletal model estimated the tibiofemoral contact forces during walking (1.44 ± 0.22 m s−1), running (4.38 ± 0.42 m s−1) and 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). Contact forces increased from walking (∼1–2.8 BW) to running (∼3–8 BW), sidestepping had largest maximum total (8.47 ± 1.57 BW) and lateral contact forces (4.3 ± 1.05 BW), while running had largest maximum medial contact forces (5.1 ± 0.95 BW). Relative muscle contributions increased across gait tasks (up to 80–90% of medial contact forces), and peaked during running for lateral contact forces (∼90%). Knee adduction moment (KAM) had weak relationships with tibiofemoral contact forces (all R2 < 0.36) and the relationships were gait task-specific. Step-wise regression of multiple external gait measures strengthened relationships (0.20 < Radj2 < 0.78), but were variable across gait tasks. Step-wise regression equations from a particular gait task (e.g. walking) produced large errors when applied to a different gait task (e.g. running or sidestepping). Muscles well stabilized the knee, increasing their role in stabilization from walking to running to sidestepping. KAM was a poor predictor of medial contact force and load distributions. Step-wise regression models results suggest the relationships between external gait measures and contact forces cannot be generalized across tasks. Neuromusculoskeletal modelling may be required to examine tibiofemoral contact forces and role of muscle in knee stabilization across gait tasks.