My dad has terrible nausea, and I am wondering if it might be due to the vestibular portion of the brain possibly being affected in Parkinson's, which possibly might be why there is a forward posture (total speculation). Here is some of the research I found, plus a few articles on the use of galvanic vestibular stimulation in combatting nausea from virtual reality:

Hacking the Inner Ear for VR—And for Science
Hacking the Inner Ear for VR—And for Science | WIRED

New Non-Invasive Galvanic Vestibular Stimulator Helps Parkinson’s Patients Improve Balance
New Non-Invasive Galvanic Vestibular Stimulator Helps Parkinson's Patients Improve Balance | Medgadget

Mayo Clinic and vMocion Introduce Technology which Creates the Sensation of Motion, Transforming Virtual Reality
Mayo Clinic News Network

"We conclude that a system of vestibular sensory autoregulation exists and that this probably involves central and peripheral mechanisms, possibly through vestibular efferent regulation. We propose that failure of these regulatory mechanisms at different levels could lead to disorders of movement perception and balance control during standing."

Passive motion reduces vestibular balance and perceptual responses
Passive motion reduces vestibular balance and perceptual responses

Galvanic vestibular stimulation may improve anterior bending posture in Parkinson's disease.
Galvanic vestibular stimulation may improve anterior bending posture in Parkinson's disease. - PubMed - NCBI

Multifaceted effects of noisy galvanic vestibular stimulation on manual tracking behavior in Parkinson's disease.
https://www.ncbi.nlm.nih.gov/pubmed/25698944

Effects of Stochastic Vestibular Galvanic Stimulation and LDOPA on Balance and Motor Symptoms in Patients With Parkinson's Disease.
https://www.ncbi.nlm.nih.gov/pubmed/25573070

Noisy galvanic vestibular stimulation induces a sustained improvement in body balance in elderly adults.
https://www.ncbi.nlm.nih.gov/pubmed/27869225

Vestibular feedback maintains reaching accuracy during body movement.
https://www.ncbi.nlm.nih.gov/pubmed/27730646

Short-term galvanic vestibular stimulation promotes functional recovery and neurogenesis in unilaterally labyrinthectomized rats.
https://www.ncbi.nlm.nih.gov/pubmed/27444558

Noisy vestibular stimulation improves dynamic walking stability in bilateral vestibulopathy.
https://www.ncbi.nlm.nih.gov/pubmed/27164706

A central processing sensory deficit with Parkinson's disease.
https://www.ncbi.nlm.nih.gov/pubmed/27059036

Can Postural Instability Respond to Galvanic Vestibular Stimulation in Patients with Parkinson's Disease?
https://www.ncbi.nlm.nih.gov/pubmed/26648182

Disrupting Vestibular Activity Disrupts Body Ownership.
https://www.ncbi.nlm.nih.gov/pubmed/26595957

Noise-Enhanced Vestibular Input Improves Dynamic Walking Stability in Healthy Subjects.
https://www.ncbi.nlm.nih.gov/pubmed/26422129

The effect of optokinetic and galvanic vestibular stimulations in reducing post-stroke postural asymmetry.
https://www.ncbi.nlm.nih.gov/pubmed/26051751