There is a large nerve called the sciatic nerve which branches each side of the body from the lower waist down. This nerve is the main roadway for everything that happens to and from the lower half of the body. When the nerve gets pinched, the immeasurable and unrelenting pain can and does travel this entire length down to the toes and often causes the thigh and calf muscles to ache.
The pinching can be caused by arthritis of the spine, a slipped disc or severe deep muscle spasm of the low back. If the searing pain happens fast and out of the blue, getting X-Rays or an MRI of the low back can help you figure out if it is arthritis or a slipped disc.
A symptom set of slipped disc often includes loss of bladder or bowel control. Sometimes, the symptoms mimic kidney troubles, so that has to be ruled out. If the X-Rays and MRI are negative, then you know it is deep muscle spasm. Sometimes, arthritis, slipped disc and muscle spasm happen all at once. No matter the cause, the pain is more than severe.
The immediate care is bed rest, anti-inflammatories, pain reducers/killers and muscle relaxants. Ice packs and heating pads are central, but different professional groups have different ideas on which is best to use and when. Some say add heat to relax the muscles and bring more blood flow, while some say ice the source area down for 20 minutes every hour to reduce inflammation. Others say rotate the heat and the ice every other hour.
From what I know and have seen, I'd ice immediately to reduce the inflammation. Later, after that has been effective, add heat according to comfort levels.
Chiropractic (non-bone crunching style), massage therapy, physical therapy and medical care all have a place and may all be included. Natural products such as high dose MSM, Ascorbyl Palmitate (a special vitamin C) as anti-inflammatories, Valerian root and Passion Flower as relaxants, electrolytes/minerals, fluids for muscle care, certain homeopathics and finally aromatics along with MSM creme applied topically can be of enormous help.
I wouldn't wish this ailment on even the most despicable person.