The Thomas test is a clinical examination used to detect tightness or contracture in the hip flexor muscles, most commonly the iliopsoas. Named after the 19th-century Welsh surgeon Hugh Owen Thomas, the test was originally designed as a simple pass/fail screening for hip flexion contracture, a condition where the hip joint cannot fully straighten. Over time, clinicians developed a modified version that attempts to measure how much extension range the hip actually has, and the test has become one of the most widely used assessments in orthopedic and sports medicine settings. Its reliability is generally strong, but its validity has come under serious scrutiny in recent years due to the influence of pelvic position during testing.
What the Test Actually Detects
The Thomas test targets the group of muscles that cross the front of the hip and pull the thigh forward. The primary muscle in question is the iliopsoas, a deep hip flexor that connects the lumbar spine and pelvis to the thigh bone. When this muscle is abnormally short or tight, the hip cannot extend fully, meaning you cannot straighten the leg behind you or even flat on a table. In its original form, a patient lies on their back and pulls one knee to the chest. If the opposite thigh lifts off the table, the test is considered positive, indicating a flexion contracture on that side.1PeerJ. The modified Thomas test is not a valid measure of hip extension unless pelvic tilt is controlled
The modified Thomas test (MTT) takes this a step further. Instead of lying flat on the table, the patient sits at the edge and rolls back while pulling one knee to the chest, letting the opposite leg hang freely off the table edge. This hanging-leg position allows the examiner to observe not only whether the thigh stays below the table surface (indicating adequate hip extension), but also how much the knee bends and whether the thigh drifts outward. Those additional observations help distinguish which specific muscle is tight. A thigh that sits above horizontal with a relatively straight knee points to the iliopsoas. A thigh that drops below horizontal but with a sharply bent knee suggests the rectus femoris, the quadriceps muscle that also crosses the hip, is the culprit. A thigh that drifts outward may implicate the tensor fasciae latae (TFL) and the iliotibial band on the outside of the hip.
Reliability Between Testers
For a clinical test to be useful, different examiners should get roughly the same result on the same patient, and the same examiner should get consistent readings when retesting. The modified Thomas test fares well on both counts. One study using goniometric measurement found high intra-rater reliability (meaning the same tester gets consistent results across sessions) and high inter-rater reliability (meaning different testers agree with each other), with intraclass correlation coefficients above 0.85 for hip extension range of motion and an average measurement error of about 3 degrees.2PubMed Central. Reliability of Goniometric Techniques for Measuring Hip Flexor Length Using the Modified Thomas Test Another study comparing goniometers and inclinometers found similarly high correlations between instruments and between testers, suggesting clinicians can use either tool interchangeably without sacrificing consistency.3Physiotherapy Theory and Practice. Reliability of inclinometer and goniometric measurements of hip extension flexibility using the modified Thomas test
When the test is scored categorically rather than in degrees, reliability for the iliopsoas and rectus femoris components remains strong. One multi-rater study reported high agreement across raters and sessions for those two muscles, though agreement for TFL tightness was only moderate, likely because the side-to-side drift that indicates TFL involvement is harder to judge by eye.4PubMed. Intrarater and interrater reliability of the modified Thomas Test Adding a lumbo-pelvic stabilization technique to the setup can push reliability even higher and reduce the minimum detectable change to about 2.4 degrees, meaning smaller real differences become identifiable.5PubMed Central. Reliability of the modified Thomas test using a lumbo-plevic stabilization
The Pelvic Tilt Problem
Reliability and validity are different questions. Two examiners can consistently get the same number, but that number can still be wrong if what it claims to measure is not what it actually measures. This is where the Thomas test runs into trouble. The key issue is pelvic tilt. When a person pulls one knee toward their chest, the pelvis rotates backward. That rotation alone can make the opposite thigh appear to drop further below the table, inflating the apparent hip extension even if the hip flexor muscles have not changed at all.
A study using 3D motion capture found that the modified Thomas test had a sensitivity of only about 32% and a specificity of about 57% for detecting actual hip extension deficits. In practical terms, the test missed roughly two out of three people who truly lacked hip extension, and mislabeled nearly half of those who had normal range. The 95% limits of agreement between the MTT reading and true hip extension were almost 20 degrees in either direction, a spread wide enough to make individual readings unreliable for clinical decision-making. However, the same study showed that when pelvic tilt was controlled for, the correlation between the MTT and true hip extension jumped to 0.98, an almost perfect relationship.6PeerJ. The modified Thomas test is not a valid measure of hip extension unless pelvic tilt is controlled
A radiographic comparison of pelvic position during the standard Thomas test versus a “simple modified” version found that lumbar curvature was lower and pelvic tilt was higher in the modified position, suggesting the modified setup does a somewhat better job of flattening the low back against the table.7The Journal of Physical Fitness and Sports Medicine. Lumbar lordotic angle and pelvic tilt angle in the simple modified Thomas test position are easier to assess than those in the Thomas test position: A radiographic study But “somewhat better” is not the same as “solved.” The takeaway for clinicians and patients alike is that the test is most trustworthy when the examiner actively ensures the pelvis is in a neutral or controlled position, typically by pressing the low back flat against the table or using a stabilization strap. Without that control, the angle you see may have more to do with how the pelvis moved than how tight the hip flexor is.
When researchers compared the MTT against three-dimensional motion capture as a gold standard, the correlation between the goniometric reading and the true joint angle was only moderate. That gap reinforces what the pelvic tilt data suggest: the number on a goniometer during a Thomas test should be interpreted cautiously, especially if no steps were taken to lock down pelvic motion.8eCommons. A Criterion Reference Validity Study for the Modified Thomas Test for Hip Extension Mobility Using Three-Dimensional Motion Capture
Special Considerations for Body Size and Specific Populations
Body composition affects the test in ways that are easy to overlook. In people with higher body mass, subcutaneous tissue around the thigh and abdomen can physically prevent the leg from hanging freely or may push the thigh upward, mimicking a positive result that has nothing to do with muscle tightness. The curvature of a larger thigh can also shift where the goniometer or inclinometer rests, introducing further measurement error.9PLOS ONE. Intra- and inter-rater reliability of joint range of motion tests using tape measure, digital inclinometer and inertial motion capturing This is worth knowing if you have been told you have tight hip flexors based on a Thomas test and you carry extra weight around the midsection: the test result might be partly an artifact of body shape rather than true muscle shortness.
In children and adults with cerebral palsy, the Thomas test is commonly used to gauge hip flexion contracture before and after interventions. A study comparing three different physical examination methods for hip flexion contracture in this population found that the Thomas test had the highest reliability of the three, though even its best intraclass correlation was 0.50 in patients with cerebral palsy, substantially lower than what is seen in healthy adults. The smallest typical measurement difference was about 6 degrees in patients versus just over 1 degree in controls, meaning the test is notably less precise in people with spasticity and limited motor control.10Journal of Bone and Joint Surgery. Reliability of Physical Examination in the Measurement of Hip Flexion Contracture and Correlation with Gait Parameters in Cerebral Palsy Clinicians working with this population know to pair it with gait analysis and imaging rather than relying on the Thomas test alone.
Hip Flexor Tightness and Low Back Pain
One of the most commonly cited reasons to care about hip flexor length is its possible relationship with low back pain. The logic is biomechanical: if the iliopsoas is chronically short, it pulls the pelvis into an anterior tilt, which deepens the lumbar curve and loads the posterior structures of the spine. Musculoskeletal modeling work supports the idea that iliopsoas contracture produces a pronounced anterior pelvic tilt and increased hip flexion during walking, a pattern distinct from contractures in other muscles like the hamstrings or gastrocnemius.11PubMed. Kinematics can help to discriminate the implication of iliopsoas, hamstring and gastrocnemius contractures to a knee flexion gait pattern
A large prospective study of ice hockey players found that players identified as having hip flexor tightness via the Thomas test were roughly three times more likely to develop low back pain over a season compared to those with normal flexibility. Players with tight hip flexors also missed far more games and practices due to back issues.12PubMed Central. The Association Between Hip Flexibility and Low Back Pain in Ice Hockey Players That is a striking association in an athletic population, where spinal loading from skating, shooting, and body contact is high.
The picture is less clear-cut in general populations. A study of people who were not seeking medical care found that those reporting low back pain did have measurably less hip extension range (about 4 degrees on average versus 8 degrees), but once researchers adjusted for body mass index, the low back pain itself was no longer a significant predictor of hip flexor tightness. BMI turned out to be the dominant factor.13PubMed. Relationship between hip flexor tightness and low back pain in non-care-seeking individuals Another study in people with chronic non-specific low back pain found only weak to moderate correlations between lower-extremity muscle tightness and pain intensity or disability.14PubMed. The relationship between lower extremity muscle tightness and pain and disability in individuals with non-specific chronic low back pain
What this means in practice is that hip flexor tightness probably contributes to low back pain in some people, particularly those who are very active and place high demands on their spine, but it is far from the whole story. If a clinician finds a positive Thomas test and tells you your tight hip flexors are causing your back pain, the evidence warrants a bit more nuance than that.
Hip Flexor Tightness in Athletes
Hip flexor tightness measured by the Thomas test is remarkably common in athletic populations. In a study of male football (soccer) players, over 60% tested positive on the Thomas test, and the rate was highest among defenders, approaching 90%.15Journal of Health, Wellness and Community Research. Association of Hamstring Strain Injury With Hip Flexor Tightness Among Male Football Players: A Cross-Sectional Study The same study found that hip flexor tightness was associated with hamstring strain indicators in attackers specifically, but not in other positions, a reminder that the relationship between muscle tightness at one joint and injury risk elsewhere depends heavily on the movement demands of a given role.
The ice hockey data mentioned earlier reinforce this theme. The skating posture keeps the hips in a flexed position for extended periods, essentially training the hip flexors to stay shortened. When those athletes then try to extend their hips during a slap shot, a sprint, or a collision, the mismatch between the shortened resting length and the required range of motion may overload the lumbar spine. That biomechanical argument is well supported by the threefold increase in low back pain risk found prospectively.16PubMed Central. The Association Between Hip Flexibility and Low Back Pain in Ice Hockey Players
Sitting, Inactivity, and Hip Extension
You do not need to be an athlete for hip flexor tightness to be relevant. Prolonged sitting keeps the hip flexors in a shortened position for hours at a time, and there is direct evidence that this affects what the Thomas test measures. A cross-sectional study compared passive hip extension across groups defined by their sitting habits and physical activity levels. People in the most sedentary, least active group had about 6 degrees less passive hip extension than those in the most active, least sedentary group.17PubMed. Prolonged sitting and physical inactivity are associated with limited hip extension: A cross-sectional study Six degrees might not sound dramatic, but given that the total arc of hip extension is only about 10 to 15 degrees in most people, losing more than a third of that range is significant for everyday activities like walking uphill, climbing stairs, or transitioning from sitting to standing.
This is also where the BMI finding from the low back pain research makes more sense in context. People who sit more tend to have higher BMI, and higher BMI independently limits hip extension both through tissue bulk and reduced activity. Disentangling the effects of sitting, inactivity, body composition, and actual muscle shortening is difficult, and the Thomas test by itself cannot tell you which of these factors is driving the result. A positive test in someone who sits at a desk all day does not necessarily mean the iliopsoas is structurally shortened; it might mean the test is picking up a combination of deconditioning and body composition.
Does Stretching Actually Help?
If the Thomas test finds a restriction, the typical prescription is stretching. But how well does stretching work, and how long does the effect last? A study that used the modified Thomas test position itself as a stretching intervention (essentially having participants hold the test position as a stretch) found a significant acute increase in hip extension range of motion immediately after the stretch. However, the improvement did not persist. By 48 hours later, hip extension had returned nearly to baseline.18Biomechanics. Use of the Modified Thomas Test for Hip Flexor Stretching: What Are the Acute and Prolonged Effects?
That rapid reversal is consistent with what the broader flexibility literature shows for single bouts of static stretching: the immediate gains are real but transient, driven more by changes in stretch tolerance (your nervous system’s willingness to let the muscle lengthen) than by actual tissue remodeling. Sustained improvements in hip flexor length typically require weeks or months of consistent daily stretching, often combined with strengthening the opposing muscles (the glutes and deep abdominals) that help hold the pelvis in a more neutral position. The Thomas test can be useful as a progress marker over that longer time frame, but checking your hip extension five minutes after a single stretch session does not tell you much about lasting change.
The Thomas Test in Surgical Planning
Outside the rehab clinic and the gym, the Thomas test plays a role in preoperative assessment for hip replacement surgery. Surgeons evaluating patients with hip osteoarthritis routinely use it to document the degree of fixed flexion contracture before total hip arthroplasty.19Acta Ortopédica Brasileira. Spinopelvic mobility in patients with hip osteoarthritis and total hip arthroplasty indication In this context, the test helps quantify how much the arthritis has restricted the joint’s ability to straighten, which informs both the surgical approach and the expected rehabilitation trajectory. A patient with a 20-degree contracture going into surgery has a different recovery path than someone with only 5 degrees of limitation. Postoperatively, serial Thomas testing tracks whether the contracture has been fully corrected or whether residual tightness persists, guiding decisions about the aggressiveness of physical therapy.
The pelvic tilt validity concerns described earlier apply here as well, perhaps even more so. Patients with end-stage hip arthritis often have significant compensatory postural changes, including exaggerated lumbar lordosis and altered pelvic alignment, which can inflate or mask the true contracture on a Thomas test. Combining the test with imaging and gait analysis gives a more complete picture in these complex cases.
How to Get the Most Accurate Result
Whether you are a clinician administering the test or a patient curious about what makes for a reliable result, a few practical points stand out from the research:
- Flatten the low back: The single most important step is controlling pelvic tilt. The examiner should ensure the lumbar spine is as flat against the table as possible before assessing the hanging leg. A strap or manual pressure on the opposite thigh can help.
- Use a measurement tool: Eyeballing whether the thigh is above or below horizontal is far less informative than taking a goniometer or inclinometer reading. Both tools produce equivalent results and are easy to use.
- Interpret with context: A positive test in someone with a high BMI or significant abdominal girth should be interpreted cautiously. Tissue bulk alone can prevent the thigh from dropping below horizontal.
- Watch the knee and the thigh angle separately: The hip angle tells you about the iliopsoas; the knee angle tells you about the rectus femoris; lateral drift tells you about the TFL. Collapsing all three into a single “positive” or “negative” loses clinically useful information.
- Retest over time: Given the transient nature of single-session stretching effects, meaningful change shows up across weeks, not minutes. Tracking Thomas test results at regular intervals provides a more useful picture than a one-off snapshot.
Newer Measurement Technologies
Digital inclinometers and inertial motion capture (IMC) sensors are increasingly showing up in clinics and research labs as alternatives to the traditional goniometer. A study comparing all three tools during the Thomas test found that digital inclinometers and IMC sensors produced tighter coefficients of repeatability than a standard goniometer, with IMC sensors achieving repeatability margins as narrow as about 1.4 to 3.1 degrees.20PLOS ONE. Intra- and inter-rater reliability of joint range of motion tests using tape measure, digital inclinometer and inertial motion capturing Smartphone-based inclinometer apps are another option that several clinics have adopted, though their validation data vary depending on the specific app and phone model.
None of these tools solve the underlying pelvic tilt validity issue. A digital sensor strapped to the thigh will give you a very precise number, but if the pelvis has tilted 10 degrees during the test, that precise number is still 10 degrees off from true hip extension. The technology improves precision (how tightly clustered repeated readings are) without necessarily improving accuracy (how close the reading is to the real joint angle). The distinction matters, and it is one reason why some researchers advocate pairing a thigh sensor with a second sensor on the pelvis to calculate the true hip-to-pelvis angle rather than just the thigh-to-horizontal angle.