Patterns of load-sharing between the shoulder muscles during isometric flexion tasks were studied by using both a biomechanical shoulder model and electromyographic (EMG) recordings of ten subjects. The effect of changes in several model parameters and shoulder stiffness constraints on the predicted load-sharing patterns were studied, while the arm position, hand load and precision requirements of the tasks were varied. The results calculated using the model were, when compared to the EMG recordings, plausible predicting a high level of synergistic contraction of muscles of the shoulder muscles during flexion tasks. The trends of the model-predicted muscle forces corresponded well to the EMG recordings. At low hand load levels the increasing of the shoulder stiffness strongly increased the muscle force levels, thus increasing also the level of synergistic contraction of muscles. At higher load levels the increase in the muscle forces was not so high, because the model predicted a high level of simultaneous contraction of muscles already at a low level of shoulder stiffness. Cluster analysis of the EMG recordings revealed large inter-individual differences in the load distribution patterns during flexion tasks. The constraint angle of the glenohumeral joint contact force direction was found to be an important model parameter affecting both the predicted forces and the maximum force production ability of the shoulder.