Standard CPR is performed with the person in supine position. Prone CPR or reverse CPR is CPR performed on a person lying on their chest, by turning the head to the side and compressing the back.
Non-spinal structural causes of weak legs may include sacroiliac joint pain syndromes and piriformis syndrome. Non-sciatica related sources of weakness in the legs may include diabetes, peripheral neuropathy and various circulatory and neurological conditions.
It works the other way around too: muscle weakness puts extra stress on the knee joint, causing arthritis-like symptoms or making existing arthritis worse. Weakness and stiffness in the legs can quickly cause a downward spiral of immobility. Movement hurts, but lack of movement makes things worse.