A biomechanical model of the human shoulder complex is proposed for quasi-static and dynamic estimation of the muscle forces and joint reaction forces in the glenohumeral joint. The model bases itself on the Garner & Pandy model but adds a dynamic layer. The muscle forces are estimated using a two stage approach. First, a set of joint motions are estimated using a minimal set of coordinates. Second, inverse dynamics combined with static optimization are used to estimate the associated muscle forces. To improve the solution, the muscle force estimation problem is decomposed into two parts solved sequentially: i) the forces in the muscles spanning the glenohumeral joint are found and ii) the rest of the forces are found so as to satisfy the equilibrium condition. The minimal set of coordinates are also found using the same decomposition of the system. The muscle-force and joint reaction force results are compared to values found in the literature.