Maintaining appropriate lower-limb joint stiffness is critical for walking performance, as it facilitates tasks such as absorbing impact loading, maintaining balance, and providing body support and propulsion. Quasi-stiffness, an indirect measure describing the joint moment–angle relationship, is often used to assess joint stiffness during walking as it accounts for passive soft tissue stiffness and active muscle force generation. Thus, identifying the primary muscle contributors to joint moments and angles can elucidate how muscles are coordinated to maintain quasi-stiffness. However, determining individual muscle contributions experimentally is challenging. Therefore, the objective of this study was to use musculoskeletal modeling and simulation to identify individual muscle contributions to sagittal-plane quasi-stiffness during walking. Simulations of 15 healthy young adults were developed and individual muscle contributions to joint moments and angles were determined within discrete phases of the gait cycle. As expected, contributors to ankle, knee and hip moments were the primary dorsiflexors/plantarflexors, knee flexors/extensors, and hip flexors/extensors, respectively, as these muscles cross the joint and directly contribute to their respective joint moments. However, major contributors to the joint angles also included distant and contralateral muscles. Specifically, the hip extensors and ankle dorsiflexors were found to contribute to the knee angle (8.4–19.7% and 9.0–17.1% of total muscle contributions, respectively), while contralateral hip extensors were found to contribute (16.6–27.2%) to the hip angle. These results highlight the role of distant muscles in maintaining quasi-stiffness, and provide a foundation for developing rehabilitation strategies and assistive devices to target stiffness impairments in clinical populations.