@Override
    public void prePhysicsTick(PhysicsSpace space, float timeStep) {
        assert space == getPhysicsSpace();

        torsoLink.preTick(timeStep);
        for (BoneLink boneLink : boneLinkList) {
            boneLink.preTick(timeStep);
        }
    }
Callback from Bullet, invoked just before the physics is stepped. A good
time to clear/apply forces.

@param space the space that is about to be stepped (not null)
@param timeStep the time per physics step (in seconds, &ge;0)
jme3-jbullet/src/main/java/com/jme3/bullet/animation/DacLinks.java