Knee-to-chest stretch

Movement

Where studies name it

Three studies cited on this page name the stretch, each as one of several exercises^1,2,3.

Study	Where the stretch sat	What the study compared
Randomised trial, 56 patients with axial low back pain lasting more than six months1	One of four flexion exercises, with pelvic tilt, trunk flexion, and forward bending with a hip flexor stretch1	A flexion-based programme against an extension-based one1
Randomised trial, 89 people aged 18 to 55 with acute (recent) low back pain2	One of three standardised exercises, with abdominal bracing and lumbar rotation, done by both groups2	An added manual method, not the exercises2
Surface electromyography study, eight men and eight women3	One of six Williams’ flexion exercises3	The same exercises done with an anterior and with a posterior pelvic tilt3

Electromyography measures a muscle’s activity by picking up its electrical signals. In the two trials cited here that name it, the stretch was not tested alone^1,2.

In the two trials cited here that name it, the stretch was not tested alone.

The hip and the pelvis

One study filmed hip flexion in 17 young subjects lying supine (face-up)⁴. Pelvic rotation supplied between a quarter and a third of the hip flexion movement⁴. The rest was the thigh moving on the pelvis⁴. That held for one leg and for both legs, moved actively and passively⁴. The pelvis began rotating within the first 8 degrees of the movement⁴.

What it loads

The lumbar spine

Two studies measured the gaps between the lumbar spinous processes with the hips flexed^5,6. A spinous process is the bony point at the back of a vertebra. One study measured people sitting, and the other compared sitting with lying on the side^5,6.

• In volunteers X-rayed sitting, flexing the hips widened the gaps by an average of 7%, 11% and 21% at three lumbar levels⁵.
• In 16 healthy adults measured by ultrasound, the gap was significantly greater sitting chest to knees than lying on the side, knees to chest⁶.

Bending forward compresses the front of an intervertebral disc and expands the back⁷. In cadaver (donated body) lumbar joints loaded to simulate flexion, the facet joint capsules and the disc resisted considerably more than three named ligaments did⁸. Facet joints are the small joints between neighbouring vertebrae.

Flexion and extension compared

Three studies compared lumbar flexion with extension, each in a different group^9,10,11:

• In 25 cadaver lumbar segments, flexion enlarged the spinal canal and the intervertebral foramen (the side opening between two vertebrae), and extension shrank them⁹.
• In six cadaver spines, a device was pushed through a lumbar disc against a nerve root to represent a herniated disc¹⁰. Flexion raised the compressive force and tension on the root, and extension lowered them¹⁰. A nerve root is the part of a nerve where it leaves the spinal cord.
• Ten patients had lumbar spinal stenosis (a narrowed spinal canal)¹¹. Pressure in the epidural space (a space inside that canal) at the narrowed level rose with extension and fell with flexion¹¹.

The back muscles

The electromyography study recorded the back muscles during the six exercises³. Activity over the lumbar and sacral areas was greater with an anterior pelvic tilt than with a posterior tilt³. An anterior tilt rocks the top of the pelvis forward, and a posterior tilt rocks it back. The sacral area is at the base of the spine.

The authors concluded that four of the exercises, knees to chest among them, minimised that activity when done with a posterior tilt³. They described the muscles as lengthening at the same time³. The authors of a 2025 review of yoga protocols describe forward-bending poses as lengthening the back muscles¹². That is those authors’ description, not a measurement¹².

How the body adapts

Held lumbar flexion

The studies below held the lower back in flexion or used a trunk-flexion stretch^13,14,15. Their findings, as recorded here, do not name the knee-to-chest stretch.

Study	What was held	What was found
22 healthy men13	A 3-minute static (held) trunk-flexion stretch13	Better scores on two forward-bending flexibility tests, with no significant group-level change in lumbar multifidus stiffness13
Ten healthy participants15	Full lumbar flexion for 10 minutes15	A larger full-flexion angle, and more back extensor activity in slow lifts done during the hold15
27 men and 20 women14	Full lumbar flexion for 20 minutes14	Peak flexion 5.5° greater by the end of the 20 minutes14

The multifidus is a deep muscle running along the spine. Creep is the slow stretch of a material under a steady load. In the 10-minute study, a 30-second rest break in the middle of the hold moderated the creep and the rise in muscle activity¹⁵.

Trials of flexion programmes

The year-long trial followed 56 patients with axial low back pain¹. Its flexion programme included the stretch¹.

• Average back pain was similar between the flexion and extension groups at one month¹.
• In the flexion group, average pain was 6.00 out of 10 at baseline and 3.78 at one year¹.
• In the extension group it was 2.26 at one year, a difference that favoured extension exercise¹.
• The two groups did not differ at one year in worst pain or on functional scales¹.
• Adverse events were mild and transient¹. Transient sciatica occurred in the flexion group alone, in 3 of 27 patients¹.

Sciatica is pain felt along the path of the sciatic nerve, in the buttock and down the leg. The trial’s authors wrote that the mechanism is unknown¹. They offered one possible explanation: repetitive flexion movements can irritate nerve roots¹.

Another study compared spinal flexion exercises with spinal extension exercises in people with chronic (long-lasting) mechanical low back pain¹⁶. Both groups had less pain afterwards, with no significant difference between them¹⁶. The flexion group gained more forward-and-back spinal mobility¹⁶.

Progressive overload

The doses recorded for the two trials that name the stretch are doses of whole programmes^1,2.

• In the year-long trial, participants did the four exercises of their programme in each session from the first week¹. Sessions lasted at least 30 minutes, and home exercise was daily for at least 30 minutes¹.
• In the two-week trial, both groups did the three standardised exercises over four sessions².

A systematic review covered 13 randomised trials of exercise for neurogenic claudication, the main symptom of lumbar spinal stenosis¹⁷. Of 23 exercise programmes, 17 included some form of lordosis-reducing or flexion-based exercise¹⁷. Its authors said nothing could be concluded about dose¹⁷.

Connections

The stretch appears in flexion programmes for the lumbar part of the spine, trialled in people with low back pain¹. Lying supine, flexion at the hip is partly rotation of the pelvis⁴. The erector spinae group makes up most of the muscle mass of the back⁷.

Transient sciatica was reported by 3 of 27 patients in one flexion programme¹. The child’s pose page draws on the same studies of held lumbar flexion^13,15.

Sources

1. Park CH, Beom J, Chung CK, Kim CH, Lee MY, Park MW, Kim K, Chung SG (2024). Long-term effects of lumbar flexion versus extension exercises for chronic axial low back pain: a randomized controlled trial.. Scientific reports. doi:10.1038/s41598-024-51769-2 PMID 38302483
2. Lewis C, Souvlis T, Sterling M (2011). Strain-Counterstrain therapy combined with exercise is not more effective than exercise alone on pain and disability in people with acute low back pain: a randomised trial.. Journal of physiotherapy. doi:10.1016/S1836-9553(11)70019-4 PMID 21684490
3. Blackburn SE, Portney LG (1981). Electromyographic activity of back musculature during Williams' flexion exercises.. Physical therapy. doi:10.1093/ptj/61.6.878 PMID 6454146
4. Bohannon RW, Gajdosik RL, LeVeau BF (1985). Relationship of pelvic and thigh motions during unilateral and bilateral hip flexion.. Physical therapy. doi:10.1093/ptj/65.10.1501 PMID 4048287
5. Fisher A, Lupu L, Gurevitz B, Brill S, Margolin E, Hertzanu Y (2001). Hip flexion and lumbar puncture: a radiological study.. Anaesthesia. doi:10.1046/j.1365-2044.2001.01717-4.x PMID 11251435
6. Sandoval M, Shestak W, Stürmann K, Hsu C (2004). Optimal patient position for lumbar puncture, measured by ultrasonography.. Emergency radiology. doi:10.1007/s10140-003-0286-3 PMID 15290485
7. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, Korol O, Johnson JE, Womble M, DeSaix P (2022). Anatomy and Physiology 2e. OpenStax. Link
8. Adams MA, Hutton WC, Stott JR (1980). The resistance to flexion of the lumbar intervertebral joint.. Spine. doi:10.1097/00007632-198005000-00007 PMID 7394664
9. Inufusa A, An HS, Lim TH, Hasegawa T, Haughton VM, Nowicki BH (1996). Anatomic changes of the spinal canal and intervertebral foramen associated with flexion-extension movement.. Spine. doi:10.1097/00007632-199611010-00002 PMID 8923625
10. Schnebel BE, Watkins RG, Dillin W (1989). The role of spinal flexion and extension in changing nerve root compression in disc herniations.. Spine. doi:10.1097/00007632-198908000-00010 PMID 2781396
11. Takahashi K, Miyazaki T, Takino T, Matsui T, Tomita K (1995). Epidural pressure measurements. Relationship between epidural pressure and posture in patients with lumbar spinal stenosis.. Spine. PMID 7604339
12. Gandolfi MG, Zamparini F, Spinelli A, Saper RB, Prati C (2025). Yoga For Musculoskeletal Disorders: A Review of Prospective Clinical Studies.. Global advances in integrative medicine and health. doi:10.1177/27536130251388385 PMID 41282368
13. Albuquerque Brandão MC, Palmieri DC, Oliveira Meireles VB, de Oliveira LF (2026). Lumbar multifidus layers stiffness during static stretching measured by shear wave elastography.. Journal of biomechanics. doi:10.1016/j.jbiomech.2026.113523 PMID 42617536
14. McGill SM, Brown S (1992). Creep response of the lumbar spine to prolonged full flexion.. Clinical biomechanics (Bristol, Avon). doi:10.1016/0268-0033(92)90007-Q PMID 23915616
15. Shin G, Mirka GA (2007). An in vivo assessment of the low back response to prolonged flexion: Interplay between active and passive tissues.. Clinical biomechanics (Bristol, Avon). doi:10.1016/j.clinbiomech.2007.06.003 PMID 17709161
16. Elnaggar IM, Nordin M, Sheikhzadeh A, Parnianpour M, Kahanovitz N (1991). Effects of spinal flexion and extension exercises on low-back pain and spinal mobility in chronic mechanical low-back pain patients.. Spine. doi:10.1097/00007632-199108000-00018 PMID 1835157
17. Comer C, Williamson E, McIlroy S, Srikesavan C, Dalton S, Melendez-Torres GJ, Lamb SE (2024). Exercise treatments for lumbar spinal stenosis: A systematic review and intervention component analysis of randomised controlled trials.. Clinical rehabilitation. doi:10.1177/02692155231201048 PMID 37715644

Every source the library cites

Changes

• October 9, 2026 First published.