The Nachlas test is a hands-on clinical examination performed with a patient lying face down while an examiner passively bends the knee, drawing the heel toward the buttock. It is used to help identify problems in the lumbar spine, hip joint, sacroiliac joint, and the rectus femoris muscle of the anterior thigh. Despite being a staple in orthopedic and chiropractic assessment textbooks, the test occupies an interesting gray zone: widely taught, routinely performed, but rarely studied in isolation for its diagnostic accuracy.
How the Test Is Performed
The patient lies prone on an examination table, face down and relaxed. The examiner stands at the side and grasps the ankle or lower leg of the limb being tested. From a fully extended starting position, the examiner passively flexes the knee, slowly bringing the patient’s heel as close to the ipsilateral buttock as possible. The movement should be smooth and controlled. The examiner watches for two things: the range of motion the knee achieves before the patient reports discomfort, and the location where pain or restriction is felt.
Because the patient is prone, gravity keeps the hip in a neutral or slightly extended position throughout. This is a deliberate feature of the test. By combining hip extension with progressive knee flexion, the maneuver places a stretch on structures that run along the front of the thigh and into the lumbar spine, including the rectus femoris, the iliopsoas, the femoral nerve, and the upper lumbar nerve roots. The location and character of any pain provoked during the maneuver guide the examiner toward one of several possible diagnoses.
Reading the Results Based on Pain Location
A positive Nachlas test does not point to a single condition. Where the patient feels discomfort tells the examiner which structure is likely involved, and the clinical reasoning branches from there.
- Anterior thigh pain: Discomfort or tightness felt along the front of the thigh suggests shortening or contracture of the rectus femoris, the long quadriceps muscle that crosses both the hip and knee joints. This is the most common finding in athletic populations and often indicates flexibility deficits rather than a pathological condition.
- Low back pain: If the maneuver reproduces or worsens pain in the lumbar spine, the examiner suspects involvement of the upper lumbar discs, facet joints, or nerve roots. A case report involving a patient with sciatica found the Nachlas test positive on the affected side alongside other findings suggesting a disc lesion or nerve root compression.1Frontiers in Integrative Neuroscience. Spinal manipulation and mobilization forces delivered treating sciatica: a case report
- Sacroiliac region pain: Pain felt at the back of the pelvis, near the sacroiliac joint, suggests stress being transmitted through the hip and pelvis to the SI joint. This can indicate joint dysfunction or inflammation in that area.
- Hip pain: Deep pain in or around the hip joint itself during the test may implicate intra-articular hip pathology, such as labral issues or early degenerative changes, since the maneuver places the hip in extension while loading it through the lever of the bent knee.
Because the test can provoke symptoms from so many different sources, it is rarely used as a standalone diagnostic tool. Clinicians treat it as one piece in a larger puzzle, combining its findings with those of other provocation tests, imaging, and patient history.
Why Knee Flexion in a Prone Position Stresses the Lumbar Spine
The connection between bending someone’s knee while they are lying face down and producing pain in the low back is not immediately intuitive. The link runs through the femoral nerve and the upper lumbar nerve roots. The femoral nerve originates from the L2, L3, and L4 spinal nerve roots. It travels through the pelvis and down the front of the thigh, giving off branches that control the quadriceps and provide sensation to the anterior thigh and inner leg. The rectus femoris, the muscle most obviously stretched by the Nachlas maneuver, is innervated by the femoral nerve.
When the knee is flexed while the hip remains extended, the femoral nerve is placed under tension. That tension is transmitted proximally, pulling on the upper lumbar nerve roots where they exit the spinal canal. If a disc herniation, osteophyte, or other space-occupying lesion is compressing one of these roots, the added tension reproduces or worsens the patient’s symptoms. A cadaveric study examining nerve root behavior during femoral nerve stretch maneuvers found that the L4 nerve root experiences measurable extension as the knee is progressively flexed, with tension increasing at higher degrees of knee flexion and during hip extension.2PubMed. Does the L4 nerve root extend during femoral nerve stretch test? A cadaveric study of four cases analyzing the extension rates of the lumbar nerve root This helps explain why the Nachlas test, which combines exactly those two joint positions, can provoke lumbar radicular symptoms.
This mechanism mirrors what the straight leg raise does for the lower lumbar and sacral nerve roots (L5, S1). The straight leg raise stretches the sciatic nerve from below; the Nachlas test does something analogous for the femoral nerve and the roots above it. The two tests together give the examiner a way to screen both the upper and lower lumbar spine for nerve root irritation without advanced imaging.
Nachlas Test Versus Ely’s Test
Anyone who reads about the Nachlas test will almost immediately encounter the Ely test, and the confusion between them is understandable. Both tests begin with the patient prone and both involve the examiner passively flexing the knee. The setups look identical. The distinction lies in what the examiner is watching for and how the result is interpreted.
The Ely test focuses specifically on rectus femoris tightness. Its hallmark positive sign is that the patient’s hip on the tested side spontaneously lifts off the table as the knee is flexed. Because the rectus femoris crosses both joints, a tight muscle resists knee flexion; the body compensates by flexing the hip instead, and the pelvis tilts anteriorly. The examiner’s eye is on the hip and pelvis, not on the patient’s reported pain.
The Nachlas test, by contrast, asks the patient to report where they feel symptoms. A positive Nachlas test might involve the same hip-lifting compensation seen in the Ely test, but it also captures lumbar, sacroiliac, and hip joint pain that the Ely test is not designed to evaluate. In practice, many clinicians perform the maneuver once and interpret it through both lenses simultaneously: watching for hip flexion compensation while also asking the patient about pain location. Some textbooks treat them as interchangeable names for the same maneuver, while others insist on the distinction. When reading clinical literature, it helps to look at how the authors define their positive finding rather than assuming consistency across studies.
Use in Sports Medicine and Injury Prediction
Outside the spinal examination room, the Nachlas test has found a second life as a flexibility screening tool in sports medicine. Because it places a direct stretch on the rectus femoris and hip flexors, it serves as a quick measure of anterior thigh range of motion. In athletic populations, restricted range on the Nachlas test often correlates with tightness that predisposes athletes to muscle strains, particularly in sports involving sprinting and kicking.
A study of amateur soccer players used six range-of-motion tests, including the Nachlas test and the Ridge test, to evaluate whether hip and knee flexibility could predict hamstring injuries. Both male and female players who went on to suffer a hamstring strain had lower range of motion on the Nachlas test compared to players who stayed injury-free.3Research Quarterly for Exercise and Sport. Predicting Hamstring Strains in Soccer Players Based on ROM: An Analysis From a Gender Perspective This finding aligns with a broader understanding in sports medicine that anterior muscle tightness can alter pelvic mechanics and increase vulnerability on the opposing side. When the rectus femoris and hip flexors are short and stiff, the hamstrings are forced to work through a greater functional range during explosive movements, raising their injury risk.
For athletic trainers and physical therapists, the practical takeaway is that the Nachlas test can do double duty. In a pre-season screening context, it identifies athletes whose anterior hip and thigh flexibility is below the threshold needed for safe performance. In a clinical context, the same maneuver can flag lumbar or sacroiliac pathology that might otherwise be missed during a sports-focused exam.
How It Fits Into a Full Lumbar and Hip Examination
No orthopedic test works well in isolation, and the Nachlas test is no exception. A thorough lumbar and hip examination typically layers several provocation tests together, each one stressing a slightly different structure or nerve pathway. The Nachlas test is often paired with the straight leg raise, Kemp’s test, the Thomas test, and the femoral nerve stretch test to build a more complete clinical picture.
The straight leg raise primarily tensions the L5 and S1 nerve roots via the sciatic nerve. If the straight leg raise is negative but the Nachlas test reproduces back pain, the examiner has reason to suspect pathology at the L2-L4 levels rather than the lower lumbar segments. This is helpful because upper lumbar disc herniations are less common than lower ones and can be missed if the clinician relies too heavily on tests that only stress the sciatic nerve.
The case report of a sciatica patient mentioned earlier illustrates this layered approach. The clinician performed the straight leg raise, Bechterew’s test, Kemp’s test, the Nachlas test, and prone lumbar flexion. Several came back positive, and together they pointed toward a large disc herniation with nerve root compression.4Frontiers in Integrative Neuroscience. Spinal manipulation and mobilization forces delivered treating sciatica: a case report No single test in that battery could have made that determination alone. The Nachlas test’s contribution was specifically implicating upper lumbar involvement.
The Thomas test, by comparison, assesses hip flexor contracture in a supine position. It complements the Nachlas test by providing a second angle on the same muscles. When both tests show restricted range, the examiner can be more confident that the hip flexors are genuinely shortened rather than that one test simply provoked a guarded response.
Limitations and the Evidence Gap
The biggest limitation of the Nachlas test is the thin evidence base supporting its diagnostic accuracy when used for spinal pathology. Unlike the straight leg raise, which has been studied extensively and has published sensitivity and specificity values for lumbar disc herniation, the Nachlas test has not been subjected to large-scale diagnostic accuracy trials. Most of what clinicians know about it comes from anatomical reasoning, clinical experience, and its inclusion in case reports rather than from controlled studies comparing its results against a gold-standard reference like MRI findings.
This does not mean the test is unreliable, but it does mean clinicians should interpret its results with appropriate caution. A positive Nachlas test in a patient with upper lumbar radicular symptoms supports the clinical impression but does not confirm a diagnosis the way imaging would. A negative Nachlas test does not rule out pathology at L2-L4 either, because the sensitivity of the test in this context has not been established.
For the rectus femoris flexibility application, the evidence is somewhat better because the measurement is more straightforward. The examiner is assessing range of motion in degrees, which is reproducible and does not require inference about hidden internal structures. Even so, standardization remains a challenge. Different examiners apply different amounts of force, stabilize the pelvis to varying degrees, and use different landmarks to measure the angle. These variations make it difficult to compare Nachlas test results across clinics or studies.
When the Test Should Not Be Performed
There are situations where the Nachlas test should be avoided or modified. Patients with recent knee surgery, acute knee injuries, or significant knee effusion should not have their knee forcibly flexed. Similarly, patients with known unstable lumbar fractures or acute inflammatory conditions of the hip joint are not good candidates for a test that places mechanical stress on those areas.
In patients with total hip replacements, the prone position itself may be contraindicated depending on the surgical approach used, and the combined hip extension and knee flexion involved in the test could violate post-surgical movement precautions. Clinicians working with post-surgical populations typically substitute alternative flexibility and neurological tests that avoid these positions.
Patients who cannot lie prone comfortably, whether because of pregnancy, significant abdominal girth, or respiratory difficulty, present a practical barrier. Some clinicians attempt a sidelying modification, but the altered hip position changes what structures are stressed, and the modified version has not been validated separately. In these cases, the femoral nerve stretch test performed in a sidelying position is often a better substitute for evaluating upper lumbar nerve root involvement.
The Femoral Nerve Stretch Test and How It Differs
The femoral nerve stretch test shares enough anatomy with the Nachlas test that the two are sometimes confused, but the technique and emphasis differ. In the classic femoral nerve stretch test, the patient lies prone, the knee is flexed (as in the Nachlas test), and then the examiner adds hip extension by lifting the thigh off the table. That additional hip extension increases tension on the femoral nerve significantly more than knee flexion alone.
The cadaveric study mentioned earlier measured nerve root displacement under varying combinations of knee flexion and hip position. The data showed that combining knee flexion with hip extension produced greater L4 nerve root stretch than knee flexion with the hip in neutral.5PubMed. Does the L4 nerve root extend during femoral nerve stretch test? A cadaveric study of four cases analyzing the extension rates of the lumbar nerve root This confirms the clinical intuition that the femoral nerve stretch test is a more aggressive provocation of the upper lumbar nerve roots than the Nachlas test. In practice, clinicians often start with the Nachlas test as a screening maneuver and progress to the femoral nerve stretch test if they need a stronger provocation to reproduce symptoms.
The distinction matters for patients with borderline findings. Someone whose Nachlas test is negative but who has clinical signs pointing toward an upper lumbar radiculopathy may still test positive on the femoral nerve stretch test because of the added hip extension component. Clinicians who stop at the Nachlas test without progressing to the full femoral nerve stretch may miss subtle nerve root involvement.
Rectus Femoris Tightness in Non-Athletes
While sports medicine applications get the most attention, rectus femoris tightness identified by the Nachlas test is not limited to athletes. Prolonged sitting, a feature of many modern jobs, keeps the hip flexors in a shortened position for hours each day. Over time, this can lead to adaptive shortening of the rectus femoris and iliopsoas. The consequences extend beyond the thigh: chronically tight hip flexors pull the pelvis into an anterior tilt, which increases the lumbar lordosis and places more compressive load on the posterior elements of the lumbar spine, including the facet joints.
For physical therapists treating chronic low back pain, a positive Nachlas test showing anterior thigh tightness rather than lumbar symptoms can redirect the treatment plan. Instead of focusing exclusively on the lumbar spine, the therapist addresses hip flexor flexibility through targeted stretching and motor control exercises. This is a case where the test changes practical decisions: two patients with the same complaint of low back pain may get very different treatment programs based partly on what the Nachlas test reveals about their anterior chain flexibility.
Older adults often present with combined findings, showing both rectus femoris tightness from reduced activity levels and early lumbar degenerative changes. In these patients, the Nachlas test may produce symptoms from multiple sources simultaneously, making interpretation trickier. Clinicians sometimes add a stabilization step, pressing down on the pelvis during the maneuver to isolate the thigh component from the lumbar component. If pain in the low back disappears when the pelvis is stabilized but thigh tightness remains, the primary issue is likely muscular rather than spinal.