My model isn't very complex, but I decided to add a simple blend shape to the body beneath the armour so that when he moves his arms, there is the suggestion of muscles tensing and relaxing.
To do this, I duplicated this part of his body, placing the geometry on a new layer.
On the duplicated geometry I used the soft select tool to move the vertices roughly where the muscle would be.
I then, in object mode, selected the altered footage and then the base model. Under Window > Animation Editors > Blend Shape, I clicked Create BlendShape under the Edit menu.
A slider appears in the window which can be increased or decreased depending on how obvious the blend shape will be.
You can then delete the duplicated geometry or simply keep the layer hidden.