Oscillating behavior in coupled muscles

[Lukuas31415]

When coupling two (cuboid) muscles with a spring with constant $k=500$, we get unrealistic behavior in the muscles. The connection between the muscles is done by adding neumann bc. If they are applied at the faces 2+ and 2- as it would make sense, we get negative determinants which indicates unrealistic deformations of the mesh. If we apply the force at the faces 2+ and 2+, we do not get negative determinants, but the muscles oscillate unrealistically. This is the same effect that happens in issue #44 and is probably closely related to this issue. For small spring constants this does not happen.