Leg Length Discrepancies: When Is Asymmetry a Problem?
Human Bodies Are Not Symmetrical
Humans are inherently asymmetrical—inside and out. For instance:
Heart position: Our heart sits slightly to the left in the chest.
Lung lobes: The right lung typically has three lobes, whereas the left lung usually has two.
Liver and spleen: The liver, a fairly large organ, is on the right side of the abdomen, and the spleen is on the left.
These asymmetries are considered completely normal, rarely affecting function in a negative way. In this same vein, there’s increasing discussion—and occasional worry—about whether having different leg lengths (leg length discrepancy, or LLD) is something we should all be concerned about. How common is LLD? Does it cause low back pain or hip/knee problems? Let’s dive into what the research actually shows.
---
Prevalence of Leg Length Discrepancies
Multiple studies have examined how common LLD is in the general population, with findings like:
Up to 90% of people have a measurable difference in leg length, though most are under 1 cm (~10 mm).
Research suggests that many individuals have an LLD of 5 mm or more—often without even knowing it.
Crucially, most of these differences are small (often less than 1 cm). While exact numbers vary, it’s safe to say that a majority of adults do not have perfectly equal leg lengths. Some minor discrepancy is the norm, not the exception.
---
Types of Leg Length Discrepancies
There are broadly two categories to consider:
1. Anatomical (Structural) Discrepancy
This means there’s a difference in the actual bony structures—for example, the femur or tibia of one leg is physically shorter. This might stem from genetics, or from injuries or surgeries that alter bone growth.
2. Functional (Apparent) Discrepancy
This doesn’t arise from the bone itself being shorter, but rather from soft-tissue tightness, muscle imbalances, or pelvic tilt/postural habits. Even if both legs are exactly the same length structurally, the way you stand or move may create the appearance of uneven lengths.
Many people have a combination of both structural and functional factors playing a role—further blurring the distinction.
---
Correlation With Pain and Dysfunction
(1) Low Back Pain
Some older clinical studies (e.g., Gait & Posture, Spine) examined potential links between mild LLD (less than 2 cm) and low back pain. While certain subgroups showed a possible association, most individuals with mild LLD did not have any increase in back pain compared to those with perfectly matched leg lengths.
Larger discrepancies—like differences exceeding 2 cm (20 mm)—can contribute to altered gait patterns and potentially place asymmetrical stress on the spine or pelvis. Yet even in these cases, the link to back pain is far from automatic.
(2) Hip & Knee Issues
The knee and hip joints can experience slightly altered loading from an LLD. According to some biomechanical studies, the hip joint on the shorter leg may bear more compressive forces, and the opposite leg may develop compensatory patterns.
However, systematic reviews often find that mild to moderate LLD does not necessarily predict the onset of degenerative changes (like arthritis) or pain. Individuals adapt to small asymmetries quite well.
(3) Running & Sports Injuries
In the running community, leg length discrepancies spark concerns about stress fractures, IT band syndrome, or other overuse injuries. While it’s plausible that a significant LLD could increase risk, evidence suggests that minor discrepancies (under 1 cm) rarely matter for the average runner.
Elite athletes often undergo individualized gait analyses, and small corrections (e.g., shoe orthotics) may help optimize performance—but for the general population, no universal rule indicates that you’ll develop an injury simply because of a small LLD.
Bottom Line: The correlation between LLD and pain/dysfunction isn’t straightforward or automatic. Many people live with slight LLDs without any related musculoskeletal issues.
---
When & Why LLD Might Matter
Despite most cases being benign, there are a few scenarios in which an LLD is more likely to be functionally relevant:
1. Significant Length Difference
Discrepancies greater than 2 cm (20 mm) can create a noticeable tilt in the pelvis and a compensatory spinal curve, which might contribute to discomfort over time.
2. Specific Injury History
Individuals recovering from hip or knee replacement surgery, fractures, or conditions like Perthes disease may end up with a real structural difference that alters movement patterns significantly.
3. Performance Demands
Professional dancers, runners, or athletes might notice even subtle differences if they’re training at high volumes or intensities—where small inefficiencies become magnified.
4. Chronic Pain Presentations
In some cases of long-standing pain or repeated injuries, investigating LLD could be part of a thorough assessment. Even here, though, it’s one piece of a bigger puzzle, not always the primary culprit.
---
Assessment & Accuracy: Challenges in Measuring LLD
Measuring leg length discrepancies (LLDs) in a clinical setting can be surprisingly tricky—especially for smaller differences. A few factors contribute:
1. Tape Measure Methods
ASIS to Medial Malleolus: One traditional approach is measuring from the anterior superior iliac spine (ASIS) on the pelvis down to the medial malleolus at the ankle. Though common, studies show variability based on patient positioning, soft-tissue thickness, and even the practitioner’s technique.
Block Method: Having a patient stand on blocks of various heights until the pelvis appears level can help estimate leg-length differences. Still, visual estimation of pelvic alignment introduces potential error.
2. Palpation & Positioning
Palpating bony landmarks (ASIS, PSIS) has its own margin of error. Small differences in how a patient stands (or how a landmark is identified) can shift the numbers by a few millimeters.
3. Imaging Techniques
Radiographic and imaging-based measurements (e.g., full-length standing X-rays, CT scans) remain the most accurate for structural LLD. But these methods can be expensive, expose patients to radiation, and aren’t always practical for every clinical exam.
4. Clinician Experience
Even seasoned healthcare providers demonstrate inter-tester reliability issues when measuring leg lengths manually. In other words, two well-trained practitioners might arrive at slightly different measurements when using the same method on the same patient.
Why It Matters
Small Discrepancies May “Disappear”
If your LLD is just a few millimeters, it might be measured differently at each visit—one day it’s 5 mm, another it’s 7 mm, another 3 mm—depending on the method used. This can make it hard to establish a reliable baseline.
Symptom Correlation Remains Key
Given the margin of error in clinical measurements, it’s no surprise that small discrepancies often don’t correlate well with symptoms. If something can’t be measured reliably in the first place, it’s tricky to draw firm conclusions about its impact on pain or function.
For Large Discrepancies
When there’s a substantial difference (e.g., > 2 cm), even less-precise clinical methods tend to confirm something’s noticeably off. These bigger discrepancies are also more likely to be relevant to pain or gait changes—but good measurement techniques (ideally imaging) are still recommended to confirm the true degree of difference.
Take-Home Point: Minor LLD is not only common but also notoriously difficult to measure precisely. This helps explain why research often finds minimal correlation between small discrepancies and pain: if we can’t reliably measure such tiny differences—and if our bodies adapt fairly well to them—then they’re unlikely to be a major driver of dysfunction on their own.
---
Research Round-Up
Below are a few select references for further reading (note: these are representative studies and reviews, not an exhaustive list):
1. Gurney B. Leg length discrepancy. Gait & Posture. 2002;15(2):195-206.
A review highlighting the biomechanical impacts of mild vs. severe discrepancies.
2. Beal MC. The short leg syndrome. Journal of the American Podiatric Medical Association. 1984;74(2):59-67.
An older but frequently cited discussion on how leg length discrepancies are assessed and managed.
3. Knutson GA. Anatomic and functional leg-length inequality: A review and recommendation for clinical decision-making. Chiropractic & Osteopathy. 2005;13:11.
Explores the distinction between structural and functional LLD and their clinical relevance.
4. Kujala UM, et al. Leg-length inequality and low-back pain. Scand J Med Sci Sports. 1992;2(4):205-209.
A study focusing on the association between LLD and low back pain among athletes.
5. McCaw ST, Bates BT. Biomechanical implications of mild leg length inequality. Br J Sp Med. 1991;25(1):10-13.
Discusses subtle gait changes and whether small discrepancies matter.
---
Practical Takeaways
Minor Leg Length Differences Are Normal
A small LLD (less than 1 cm) is extremely common and typically not cause for alarm.
Symptoms Are Key
If you don’t have pain or functional limitations, discovering a slight LLD doesn’t necessarily require intervention.
Look at the Whole Picture
Because our bodies are interconnected, it’s crucial to consider pelvic alignment, foot mechanics, muscle imbalances, and your overall movement patterns rather than focusing solely on leg length.
Professional Assessment for Larger Discrepancies
If you suspect a significant LLD or have ongoing pain that doesn’t resolve with standard exercise or therapy, consult a healthcare professional for a thorough evaluation. Sometimes an in-shoe lift or targeted rehab exercises can help—but only in the context of a comprehensive treatment plan.
---
Wrap-Up: Embracing Natural Asymmetry
Just as our hearts and lungs are not mirror images, our legs often differ in length—more often than not, by very small margins that we naturally accommodate. Research consistently shows that for mild discrepancies, there’s limited correlation with pain or dysfunction. Larger differences can be relevant but still represent just one factor among many that may affect someone’s comfort or performance.
Take comfort in the fact that a little asymmetry—leg length included—is part of the human design. Unless accompanied by symptoms or a truly large discrepancy, it generally doesn’t require aggressive correction. Instead, a balanced approach (assessing strength, flexibility, and overall movement patterns) is the best plan for keeping your body functioning at its best—uneven lobes and all.
---
As always, thanks for reading, and stay tuned for more on movement science and the fascinating quirks of human anatomy!