The use of digital human models has been increasing rapidly in various fields from medical to engineering applications. Most of the works on human models involving muscle activation have been concentrated on rigid body simulation so far, because the dynamics of human body motion has been primary concern regardless of the effects on human musculoskeletal body. Recently the need for flexible body simulation including muscle activation has been increasing for engineering applications. In this paper, a musculoskeletal model with muscle activation of an upper limb for the dynamic simulation of FE based flexible body is presented. In order to estimate the in vivo forces of muscles in motion, optimization technique is employed to solve multiple solutions problem. The simulated results were compared with the experimental data, EMG for validation. As a result it was found that muscle activation as part of musculoskeletal model can be employed for a FE based flexible body software.