01/04/2026
People always ask why we check their leg lengths and have them stand on 2 scales to measure there weight balance.
This is why...
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🦵 Biomechanics Behind “One Leg Shorter Than the Other” – What’s Really Happening?
Many people are told they have one leg shorter than the other, but true structural leg-length difference is actually rare. In most cases, the issue is functional, meaning the bones are equal, but posture, joint position, and muscle imbalance create the illusion of unequal leg length.
Let’s break down the biomechanics 👇
🔹 1. Pelvic Tilt Is the Main Driver
When the pelvis tilts or rotates:
One innominate bone moves anteriorly or posteriorly
This changes the apparent height of the hip joint
The leg on the lower side looks “shorter” even though bone length is equal
➡️ This is why pelvic alignment is critical before diagnosing leg-length discrepancy.
🔹 2. Femoral Head Height Difference
The femoral heads (tops of the thigh bones) may sit at different vertical heights due to:
Pelvic obliquity
Lumbar side-bending
Asymmetrical hip muscle tone
📌 The diagram shows how even a small pelvic shift alters femoral head positioning, affecting lower-limb alignment.
🔹 3. Lumbar Spine Compensation
The spine adapts to pelvic asymmetry by:
Side-bending
Rotating
Increasing compressive load on one side
This compensation helps maintain eye-level posture but increases asymmetric loading, often leading to:
Low back pain
SI joint irritation
Hip or knee symptoms
🔹 4. Knee and Ankle Adaptations
Down the chain:
One knee may flex slightly
The other may hyperextend
One foot may pronate more to “reach the ground”
These compensations equalize stance height, not leg length.
🔹 5. Why Heel Lifts Often Fail
If the problem is functional:
Adding a heel lift treats the symptom, not the cause
It can worsen pelvic asymmetry
May increase spinal and hip stress over time
✅ Correction should focus on pelvic control, mobility, and muscle balance, not just shoe modifications.
🧠 Key Takeaway
✔️ Most “leg-length differences” are functional, not structural
✔️ Pelvic position and spinal mechanics matter more than bone length
✔️ Proper assessment must include posture, movement, and alignment
✔️ Treatment should restore symmetry, not force compensation
💬 Clinical tip:
Always assess pelvic landmarks, femoral head height, and movement patterns before labeling someone with a leg-length discrepancy.
