Halo, you're mistaken. His code already does that.
http://www.ioi.dk/Homepages/thomasj/publications/AdvancedPhysics_files/v3_document.htm
See:
Articulated bodies
Angular constraints
All he needs to do is add one distance constraint (spring) to each leg, and set the legs up so that they use hinge joints instead of the pin joints they use right now.
He just constructed his model in a simpler manner than he could have.
http://www.ioi.dk/Homepages/thomasj/publications/AdvancedPhysics_files/v3_document.htm
See:
Articulated bodies
Angular constraints
All he needs to do is add one distance constraint (spring) to each leg, and set the legs up so that they use hinge joints instead of the pin joints they use right now.
He just constructed his model in a simpler manner than he could have.