Joop-

I don't know if you were around when Anne Frobert and I were experimenting with the effects a simple cloth band around a leg or arm or even forehead on tremor and control. This led to the realization that there is another component to the action of a muscle group beyond simply "activate-release", namely sensory feedback informing the brain that the indicated action has taken place.

Perhaps there is a clue there. Maybe a process such as-
1) Signal muscle "A" to contract;
2) Receive confirmation that action is taking place;
3) Signal muscle "B" to relax;
4) Receive confirmation that action etc.

Blockage at step 2 would result in both muscles being contracted, thus freezing.

There is some interesting stuff at http://scienceweek.com/2004/sb040813-2.htm