The Thomas test is a physical examination technique used to check whether your hip flexor muscles are tight or shortened. Named after the 19th-century Welsh surgeon Hugh Owen Thomas, it was originally designed to detect hip flexion contracture, a condition where the muscles at the front of your hip become so tight that your leg can no longer fully straighten. Today, physical therapists, athletic trainers, and orthopedic specialists use it routinely to assess hip flexibility in anyone from desk workers with low back pain to athletes with recurring knee problems.
What the Test Is Looking For
The hip flexors are a group of muscles that run across the front of your hip joint and pull your thigh toward your torso. When these muscles become chronically shortened from sitting, repetitive training, or injury, they limit how far your leg can extend behind you. That restriction doesn’t just affect the hip. Tight hip flexors can tilt your pelvis forward, increase the curve in your lower back, and change how forces travel through your knee. Conditions linked to restricted hip flexor length include patellofemoral pain syndrome (pain around the kneecap), psoas syndrome, lower back pain, and certain forms of arthritis.
The Thomas test specifically targets two layers of muscle. The deeper layer is the iliopsoas, a powerful one-joint hip flexor that connects your lower spine and pelvis to your thigh bone. The more superficial layer includes the rectus femoris, the only quadriceps muscle that crosses both the hip and the knee. Because these muscles have different attachment points, the test can help distinguish which one is contributing to tightness, a detail that matters when choosing the right stretches or treatment approach.
How the Test Is Performed
For the original Thomas test, you lie flat on your back on a firm surface or examination table. The examiner checks for an exaggerated arch in your lower back, which often signals tight hip flexors before the test even begins. You then pull one knee firmly toward your chest with both hands, which flattens your lower back against the table and locks your pelvis in a neutral position. While you hold that knee, the examiner watches what happens to your opposite leg, the one resting on the table.
In a person with normal hip flexor length, the resting thigh stays flat against the table. If the hip flexors are tight, that resting thigh lifts off the surface. The gap between the back of your thigh and the table is the key observation: any visible lift is considered a positive test, indicating a hip extension deficit on that side. Both legs are tested in turn so the examiner can compare sides.
The Modified Thomas Test
The modified version, sometimes called the Kendall test or rectus femoris contraction test, adds information about the knee. Instead of lying fully on the table, you sit at the very edge so your tailbone is right at the end. You then lie back and pull one knee to your chest while the other leg hangs freely off the edge. This setup lets the examiner observe two things at once: the position of your thigh (which reflects iliopsoas length) and the angle of your knee (which reflects rectus femoris length).
If your hanging thigh rises above the horizontal plane of the table, the iliopsoas is likely tight. If your knee straightens out rather than bending to roughly 80 or 90 degrees, the rectus femoris is likely restricted. Sports medicine practitioners commonly prefer this modified version because it gives a more detailed picture of which muscles need attention.
What the Results Mean
A negative Thomas test is the normal finding. It means your hip flexors are flexible enough to allow your thigh to rest flat (or slightly below horizontal) while the opposite hip is fully flexed. Your knee on the hanging leg should bend comfortably without effort.
A positive test can present in several patterns, and each pattern points to a different source of tightness:
- Thigh lifts off table, knee bends normally: This suggests the iliopsoas is the primary tight muscle, since it crosses only the hip joint.
- Thigh lifts off table and knee straightens: Both the iliopsoas and rectus femoris are likely shortened. This is common in people who spend long hours sitting with their knees bent.
- Thigh drifts outward (abducts) as the back flattens: This pattern, sometimes called the J sign, suggests tightness in the iliotibial band, the thick band of tissue running along the outside of your thigh.
Why Pelvic Position Matters
The biggest source of error in the Thomas test is what your lower back does during the maneuver. Tight hip flexors naturally pull the pelvis into a forward tilt, which creates an exaggerated curve in the lumbar spine. That arch effectively hides the tightness because it lets the resting leg drop toward the table even when the muscles are shortened. The whole point of pulling the opposite knee to your chest is to flatten that curve and lock the pelvis into a neutral position, unmasking the true restriction.
If the pelvis isn’t adequately controlled, the test can produce a false negative result, meaning it appears normal when tightness is actually present. Research from PeerJ has noted that the modified Thomas test is not a valid measure of hip extension unless pelvic tilt is controlled. This is why examiners typically place a hand under your lower back or watch closely to confirm that the lumbar spine is flat before interpreting the resting leg position.
A standing variation has also been described for situations where lying on a table isn’t practical. In this version, you stand on the unaffected leg and flex the opposite knee. The examiner watches from the side for an increase in your lower back curve. You then gradually lean backward to correct the pelvic tilt, which unmasks the true amount of flexion deformity on the affected side. This dynamic version is less commonly used but can be helpful when the standard test is difficult to perform.
Reliability and Limitations
The Thomas test is widely used in clinical and sports medicine settings, but it is a screening tool rather than a precise measurement. Its strength is simplicity: no equipment is needed, it takes under a minute, and it provides immediate, visual feedback about hip flexor length. For the modified version, studies in the Journal of Athletic Training have examined its reliability specifically for assessing rectus femoris flexibility at the knee, confirming that it is a reasonable clinical tool when performed consistently.
Its main limitation is examiner dependence. Results can vary based on how firmly the pelvis is stabilized, how the patient is positioned on the table, and whether subtle compensations are caught. Two different clinicians testing the same person might disagree on borderline cases. For that reason, many practitioners use the Thomas test as one piece of a larger assessment rather than relying on it alone, combining it with hands-on muscle length measurements, movement screens, or imaging when needed.
Who Gets This Test and Why It Matters
You might encounter the Thomas test in a physical therapy evaluation for low back pain, a sports medicine screening before a season, or a post-surgical assessment after hip or knee procedures. Runners, cyclists, and anyone training through hip or knee pain are frequently tested this way. Office workers being evaluated for postural issues related to prolonged sitting are another common group.
The practical value of the test is that it connects symptoms to a specific, correctable cause. If your Thomas test reveals a tight iliopsoas, targeted hip flexor stretching and strengthening of the opposing muscles (the glutes) becomes a clear treatment path. If the rectus femoris is the culprit, stretches that address both the hip and the knee, like a kneeling lunge with the back foot elevated, take priority. The test turns a vague complaint like “my hip feels stiff” into a concrete finding that guides what you do next.