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Extravehicular Activity (EVA) is a highly demanding activity during space missions. The current NASA spacesuit, the Extravehicular Mobility Unit (EMU), might be thought of as the 'world's smallest spacecraft' and is quite an engineering achievement. However, the EMU has also led to discomfort and musculoskeletal injuries, mainly due to the lack of mobility in the pressurized suit that makes moving and operating within the suit challenging. A new musculoskeletal modeling framework is developed in OpenSim to analyze human-spacesuit interaction and musculoskeletal performance during EVA. Two spacesuits are considered: the current EMU and NASA's Mark III spacesuit technology demonstrator. In the model, the effect of the spacesuits is represented as external torques applied to the human body, based on experimental data. Muscle forces during knee flexion/extension are calculated and compared in "suited" and "unsuited" conditions. Results suggest that the maximum peak force exerted during knee flexion significantly increases from unsuited conditions to Mark III-suited conditions to EMU suited conditions. In particular, the peak forces exerted by the biceps femoris long head (BFL), the gastrocnemius (GM), the gracilis (GR), and the sartorius (SR) knee-flexor muscles are significantly higher in "suited" conditions. Conversely, the knee-extensor muscles do not show significant differences between the unsuited and suited conditions. The musculoskeletal analysis provides new insights into human spacesuit interaction and musculoskeletal performance in "suited" conditions, and contributes to the assessment of astronaut health and safety during EVA, informing flight surgeons, EVA operation teams, researchers and spacesuit designers.