There are several approaches to include physics: Firstly check MustardUI, if the Physics tab is available, some parts might already have physics available. The fastest, simplest, but maybe more inaccurate and inefficient way, is to use a bone addon. A great tool is Bone Dynamics Pro (there is also a free version). Prepare a mesh cage, i.e. create a mesh which enclose the part which you would like to bounce, enable cloth physics on this mesh (which should be parented to an armature bone) and then bind the mesh to the body with a Mesh Deform modifier. I also prepared a video to explain how to add physics easily with MustardUI. At this link you can also find a pre-prepared cage to use. The last method is to use soft body. This is maybe the more complicated approach, but if correctly implemented it should give the best results. I personally use the x-muscle addon to simplify the process, which is not available for free. However, some tutorials are available online on this (google it!).