“In terms of improving flexibility and range of motion for patients experiencing rigidity, generalized relaxation techniques such as gentle rocking have been found to decrease excessive muscle tension. Other effective techniques to promote relaxation include slow rotational movements of the extremities and trunk, rhythmic initiation, diaphragmatic breathing, and meditation techniques.

Illustration of Parkinson's disease by William Richard Gowers, which was first published in A Manual of Diseases of the Nervous System (1886)

Illustration of Parkinson’s disease

Also, due to the forward flexed posture, and respiratory dysfunctions in advanced Parkinson’s disease, deep diaphragmatic breathing exercises are beneficial in improving chest wall mobility and vital capacity.”

Found in wikipedia, from the original source in:
O’Sullivan, Susan B, Schmitz, Thomas J (2007). Physical Rehabilitation (5th Ed.) “Parkinson’s Disease”. Philidephia: F.A. Davis. pp. 873 & 87