This article presents how we use functional anatomy to develop a simplified model of the arm usable to estimate muscle forces. The model is developed as a motion analysis tool. The first application is the muscle forces estimation of the flexion/extension and pronation/supination of the forearm joints. This estimation is based on an inverse dynamics method, improved with additional constraints such as co-contraction factor between flexors and extensors of a joint. The article first presents the context of the study, and then presents the biomechanical model developed. Then we present the estimation of muscle forces step. Some results obtained for samples movements of the arm are presented and discussed. At last we conclude on the method and the perspectives that it presents.