Being a person suffering from Parkinson's Disease (PD), I notice other PD patients, including myself, suffer from a stooped posture, particularly while walking.
I would like to share with you my experience in the causation of the posture suffered by most, if not all PD sufferers.
I notice that my muscles surrounding my shoulders right down to my arms, are generally very stiff. Because of the stiffness in the said muscles, I have difficulty in swinging my arms when I walk. The swinging of the arms on opposite sides of your limbs, i.e. you swing your right hand when you walk with your left foot and vice versa, during walking gives us the counterbalance required to maintain a correct posture while walking.
Due to the lack of arm motion due to the stiffness of the muscles in the shoulder region, the body will naturally be off balance. To compensate for the lack of balance, I find myself exagerating the arm swinging movement, in that I need to push hard with my shoulder to swing the hands forward and backward.
This in turn lead to the muscles in the shoulders and upper back to tense up. This result in the body having to stoop forward to maintain the balance of the body during walking.