Muscle rigidity is one of the hallmarks of Parkinson's Disease. Having said this, you need to have a good stretching program that will not be too strenuous. That can trigger and cause tremors and fatigue the muscles.
If there is a balance issue, the exercises in the Unlock Your Hip Flexors program are still doable by using the "exercise swaps" which are modifications of the exercise positions, hence, you can do it either in sitting and lying down positions. For more details about the program, you can check this link: http://healingthroughmovement.com/get-DVD-now