Keep constant stress over the elements of the structure

Hi,



I am working on mechanical modeling of the contraction of the muscle. I
am using ABAQUS software and I need to keep a constant initial stress
over the elements of the structure during simulation. I have used "*initial conditions, type=stress" to apply stress to the structure and then I used "*prestress hold" to keep the stress constant, but "*prestress hold" does not work properly and stress changes over the elements during the simulation. I am wondering if it is normal, I meant the stress should change during the simulation because of the mechanical equilibrium equations, or there is something wrong with my simulation. I would appreciate it if anybody guides me through this. I should add that my simulation is standard, the analysis is static, and the material
model is 2-dimentional (2D) linear elastic isotropic material with a
definite Poisson's ratio and Young's Modulus. Actually I am performing
2D plane stress analysis and then I am going to consider a more complex
3D analysis.



Thank you