Information about muscle forces helps us to understand human movements more completely. Recently, studies on estimating muscle forces in real-time have been directed forward; however, the previous studies have some limitations in terms of using a three-dimensional (3D) motion capture system to obtain human movements. In the present study, an electromyography (EMG)-based real-time muscle force estimation system, which is available for a variety of potential applications, was introduced with electrogoniometers. A pilot study was conducted by performing 3D motion analysis on ten subjects during sit-to-stand movement. EMG measurements were simultaneously performed on gastrocnemius medialis and tibialis anterior. To evaluate the developed system, the results from the developed system were compared with those from widely used commercially available off-line simulation software including a musculoskeletal model. Results showed that good correlation coefficients between muscle forces from the developed system and the off-line simulation were observed in gastrocnemius medialis (r = 0.718, p < 0.01) and tibialis anterior (r = 0.821, p < 0.01). However, muscle lengths and muscle forces were obtained with a maximum delay of about 100 ms. Further studies would be required to solve the delay limitation. The developed system yielded promising results, suggesting that it can be potentially used for the real-time diagnosis of muscle or interpretation of movements.