Williams Flexion Exercises
Williams flexion exercises (WFE) — also called Williams lumbar flexion exercises or simply Williams exercises — are a set or system of related physical exercises intended to enhance lumbar flexion, avoid lumbar extension, and strengthen the abdominal and gluteal musculature in an effort to manage low back pain non-surgically. The system was first devised in 1937 by Dr. Paul C. Williams (1900-1978), then a Dallas orthopedic surgeon.
WFEs have been a cornerstone in the management of lower back pain for many years for treating a wide variety of back problems, regardless of diagnosis or chief complaint. In many cases they are used when the disorder’s cause or characteristics were not fully understood by the physician, Athletic trainer or physical therapist. Also, physical therapists and athletic trainers often teach these exercises with their own modifications.
The WFEs were developed out of the Regen exercise (also called “squat exercise”), advocated in the 1930s by Eugene M. Regen (1900-1983), a Tennessee orthopedic surgeon, and which consist in squatting and emphasizing the convexity of the lumbar area. (The Regen exercise was originally publicized in a film by the Veterans Administration.) Williams first published his own modified exercise program in 1937 for patients with chronic low back pain in response to his clinical observation that the majority of patients who experienced low back pain had degenerative vertebrae secondary to degenerative disk disease. These exercises were initially developed for men under 50 and women under 40 who had exaggerated lumbar lordosis, whose x-ray films showed decreased disc space between lumbar spine segments (L1-S1), and whose symptoms were chronic, but low grade.
Williams suggested that humans, in evolving to stand erect, severely deformed the vertebral column, redistributing body weight to the posterior aspect of the intervertebral discs in the lumbar spine. At the 4th and 5th lumbar levels, great pressure is said to be exerted on the posterior aspect of each vertebra and transferred from the vertebra to the disc. Williams specified that in most cases the 5th lumbar disc ruptures and the nuclear material of the disc moves into the spinal canal causing pressure on the spinal nerves. In addition to the ruptured disc, irritation of the nerve at the intervertebral foramen where the nerve exits from the spinal canal may result. He believed that this rarely occurs except at the 5th lumbar level, and that the likelihood of the nerves being impinged was greatly increased by extending the lumbar spine. Williams emphasized the universality of this problem: "The fifth lumbar disc has ruptured in the majority of all persons by the age of twenty...." He went on to explain that although most people at this age have not experienced severe low back pain, they will, in all likelihood be subject to mild bouts of low back pain which can be attributed to the ruptured disc. The solution, Williams explained, was to have the patient perform exercises and adhere to postural principles which serve to decrease the lumbar lordosis to a minimum, thereby reducing the pressure on the posterior elements of the lumbar spine.
Procedures and mechanisms
The WFEs were for many years the standard for non-surgical low back pain treatment. These exercises were performed in the supine position on a floor or other flat surface. There were variations, but the primary maneuver is to grab the legs and pull the knees up to the chest and hold them there for several seconds. The patient then relaxes, drops the legs down and repeats the exercise again. The primary benefit is supposed to be the opening of the intervertebral foramen, the stretching of ligmentous structures, and the distraction of the apophyseal joints. The goals of performing these exercises were to reduce pain and provide lower trunk stability by actively developing the "abdominal, gluteus maximus, and hamstring muscles as well as..." passively stretching the hip flexors and lower back (sacrospinalis) muscles. Williams said: "The exercises outlined will accomplish a proper balance between the flexor and the extensor groups of postural muscles...". Williams suggested that a posterior pelvic-tilt position was necessary to obtain best results.
Both flexion and extension exercises have been shown to help mitigate back pain and has been demonstrated to accomplish the following: a) significantly increase the canal area, b) increase the midsagittal diameter, c) increase the subarticular sagittal diameter, and d) increase all the foraminal dimensions significantly 
Seven of the variations of the WFEs are outlined below (Ref):
- Pelvic tilt. Lie on your back with knees bent, feet flat on floor. Flatten the small of your back against the floor, without pushing down with the legs. Hold for 5 to 10 seconds.
- Single Knee to chest. Lie on your back with knees bent and feet flat on the floor. Slowly pull your right knee toward your shoulder and hold 5 to 10 seconds. Lower the knee and repeat with the other knee.
- Double knee to chest. Begin as in the previous exercise. After pulling right knee to chest, pull left knee to chest and hold both knees for 5 to 10 seconds. Slowly lower one leg at a time.
- Partial sit-up. Do the pelvic tilt (exercise 1) and, while holding this position, slowly curl your head and shoulders off the floor. Hold briefly. Return slowly to the starting position.
- Hamstring stretch. Start in long sitting with toes directed toward the ceiling and knees fully extended. Slowly lower the trunk forward over the legs, keeping knees extended, arms outstretched over the legs, and eyes focus ahead.
- Hip Flexor stretch. Place one foot in front of the other with the left (front) knee flexed and the right (back) knee held rigidly straight. Flex forward through the trunk until the left knee contacts the axillary fold (arm pit region). Repeat with right leg forward and left leg back.
- Squat. Stand with both feet parallel, about shoulder's width apart. Attempting to maintain the trunk as perpendicular as possible to the floor, eyes focused ahead, and feet flat on the floor, the subject slowly lowers his body by flexing his knees.
McKenzie extension exercises
The WFEs stand in some opposition to another type of back exercises, devised by Robin McKenzie (b. 1931) and known as the “McKenzie extension exercises”, which involve the opposite motion of extending the spine backwards. (One review stated that “Perhaps no two methods of physical therapy treatment for low back pain are so contradictory in both theory and practice”) In contrast to Williams, McKenzie suggested that all spinal pain can be attributed to alteration of the position of the disc's nucleus pulposus, in relationship to the surrounding annulus; mechanical deformation of the soft tissue about the spine which has undergone adaptive shortening; or mechanical deformation of soft tissue caused by postural stress. McKenzie concluded that a continually flexed lifestyle may cause the nucleus to migrate more posteriorly, resulting in low back pain. Moreover, these anomalies of the lumbar spine are largely due to our modern lifestyle and “an almost universal loss of extension”. As treatment, McKenzie recommended exercises and postural instructions which restore or maintain the lumbar lordosis. Although exercises involving lumbar spine extension are emphasized in this treatment protocol, particularly in the early stages, lumbar flexion exercises are usually added at a later time in order that the patient has full range of spinal flexion and extension. Although the Williams and McKenzie treatment protocols differ markedly, both continue to be widely prescribed despite the paucity of clinical evidence measuring their efficacy (at least in 1984, at the time of the publication cited). 
- Williams, Paul C. (1965), The Lumbosacral Spine: Emphasizing Conservative Management; 202 pp, 87 illus, New York: Blakiston Division, McGraw-Hill Book Co.
- Ponte, David Joseph, et al (1984), “A Preliminary Report on the Use of the McKenzie Protocol versus Williams Protocol in the Treatment of Low Back Pain”, JOSPT, Vol. 6, No. 2 (Sept-Oct), 6:130-9.
- Williams P.C. (1937), “Lesions of the Lumbosacral Spine: 2. Chronic Traumatic (postural) Destruction of the Lumbosacral Intervertebral Disc”, J Bone Joint Surg; 29:690-703.
- Williams (1965), Op. cit., pp 80-98.
- Blackburn SE and Portney LG (1981), “Electromyographic Activity of Back Musculature during Williams' Flexion Exercises”, Phys Ther; 61:878-885.
- Ponte (1984), Op. cit.
- Williams (1937), Op. cit.
- Elnagger, I.M., et al (1991), "Effects of Spinal Flexion and Extension Exercises on Low Back Pain", Spine, Aug, 1.
- Infusa, A., et al (1996), “Anatomic Changes of the Spinal Canal and Intervertebral Foramen Associated with Flexion-extension Movement”, Spine; Nov 1;21(21):2412-20.
- Ponte, et al (1984), Op. cit.
- Ponte, et al (1984), Op. cit.
- Williams, Paul C. (1974), Low Back and Neck Pain: Causes and Conservative Treatment, Ed 3; Springfield:Charles C Thomas; 78 pages.
- Williams P, (1955), “Examination and conservative treatment for disc lesions of the lower spine”, Clinical Orthopaedics and Related Research 528-40.