Locking model positions

so i'm not seeing a lot of people talking about this, typically nothing in any 3D environment moves without you making it do so, however in my situation it's a bit more complicated. I'm using perception neuron for motion capture of a model, however i only what the upper body captured while the feet and butt stay put in a chair. is there a way to anchor the model to the chair without impeding capture ability? Any terms i should be looking into to come up with a custom solution if it proves more difficult?