Tehran University of Medical Sciences Journal of Modern Rehabilitation 2538-385X 0 - 2025 12 14 1424 1424 EN Wajid Khalil University of Shanghai for Science and Technology, Shanghai, China 2025 09 13 2025 12 14 This study introduces a human-machine model utilizing OpenSim. It examines the impact of a passive (unactuated) lower-limb rehabilitation exoskeleton on biomechanics during ambulation. The model assesses how well the joints are aligned, how the muscles are used, and how well the design performs. The exoskeleton is made of T6061 aluminum alloy, which makes it light and easy to move. Each leg has three active joints and two passive joints that work in the sagittal plane. Musculoskeletal and exoskeleton models are simulated together in MATLAB and OpenSim. MATLAB scripts set their dynamic properties. A six-degree-of-freedom bushing models the interaction between the human and the exoskeleton at contact points. Joint angles come from experimental gait measurements. A residual-reduction algorithm reduces dynamic errors while keeping the resulting residual forces and moments within acceptable limits. Muscle activations and forces are estimated using computed muscle control, which follows joint paths. Simulations show that even in passive mode, the exoskeleton increases overall lower-limb muscle activation by more than 50% compared to walking without assistance. Significant increases occur in the rectus femoris, gluteus maximus, semimembranosus, and vastus lateralis. Joint torques also change: swing-leg hip and knee torques decrease by about 50%, support-leg torques increase because of the load, and ankle torque adjusts for compensation. These non-invasive simulations show reduced torque variability and support better design updates. This leads to improved exoskeleton alignment and evaluation before physical prototyping. https://jmr.tums.ac.ir/index.php/jmr/article/view/1424