Truss Analysis

I have truss (an inverted V) which is internaly unstable but restrained enough to prevent rigid body modes. Its a 2D structure which means m<2j-3. I am giving density to this structure and analysing it in dynamic implicit and explicit analysis both are running and giving same results. There is also stress. I know why numerically I am getting it. No inversion in explicit and full mass matrix in implicit. My question is what does that result mean? Is the truss deforming because of mass? I expect it to just fall down without any strain energy. There is strain energy stored in the element too.

 why there is strain?  

what does it physically mean? 

B.C are both roller