Hi Elven
Apart from the wall thickness and fill density there are two other properties which play an important role in determining the strength of the models such as Leonardo’s Vitruvian man. One reason could be the vulnerable points of load, in this case the area near the shoulders and the elbows, both of which are susceptible to breakage. You can slightly tweak the shoulders to include fillets at the shoulders and thicken the elbow so that the model gains physical strength at vulnerable points. Secondly, the strength also depends on the orientation of layers of the print. The layers should not be perpendicular to the direction of force. The layers are strongest in the direction of X and Y axes. The strength in the Z-axis largely depends on number of layers, thickness and material properties.
I am hoping that tweaking both these aspects should help you solve the problem.
