Sudden weakness in one leg, especially with facial drooping, arm weakness, slurred speech, loss of bladder or bowel control, or numbness spreading rapidly, can be a sign of a stroke or spinal cord compression. Call 999, go to A&E, or contact 111 immediately. Do not wait to book a scan.
Why Does One Leg Feel Weaker Than the Other?
Weakness in one leg usually happens because a nerve is being compressed in your lower back, because the muscles on one side have become weaker from disuse, or because pain is stopping you from using that leg fully. Standing whole-body imaging can reveal alignment problems that explain one-sided leg weakness when standard scans look normal.
Key takeaways
- One leg feeling weaker can be "true" muscle weakness or "pain inhibition" where your body avoids loading that leg.
- The lower back is one of the most common sources, even when you feel no back pain.
- Gradual weakness often points to nerve compression, muscle imbalance, or joint misalignment.
- Sudden weakness is a red flag and needs urgent medical review.
- Standing imaging shows how your alignment under gravity may be contributing.
It is surprisingly common. You might notice one leg giving way when walking, a feeling of heaviness on one side, or that you simply cannot trust one leg to hold your weight. Sometimes the weakness builds so slowly that you only realise it when you catch yourself limping or favouring one side.
Understanding why it is happening matters, because the cause is not always where you feel it.
What does weakness in one leg actually feel like?
It can show up in several ways, and not all of them feel like obvious "weakness." Your leg may feel heavy, unreliable, or slow to respond rather than completely unable to move.
One leg feels like it needs more effort to lift or swing through when you walk.
Your knee buckles or you stumble unexpectedly, especially on stairs or uneven ground.
Trouble rising from a chair, climbing stairs, or pushing off when walking.
You notice one thigh or calf looks thinner than the other, a sign of muscle wasting.
What causes one leg to be weaker than the other?
There are two broad categories. "True weakness" means the muscle itself, or the nerve supplying it, is not working properly. "Pain inhibition" means the muscle is capable, but your nervous system dials down its power to protect you from pain. Both feel the same to you, but they need different approaches.
Common causes in more detail
A disc bulge or bony narrowing in the lower spine presses on a nerve, reducing the signal to specific leg muscles. This is often called lumbar radiculopathy.
Narrowing of the spinal canal, often in older adults, can compress multiple nerves. Symptoms typically worsen with standing and walking. Learn more about spinal stenosis.
If you have been favouring one leg for weeks or months, the muscles on the weaker side shrink. This creates a cycle of weakness and further avoidance.
If your pelvis, hips or spine are out of alignment, one leg carries more load than the other. Over time, this imbalance fatigues one side and weakens it.
Can back problems cause weakness in one leg?
Yes, and this is one of the most overlooked connections. Each nerve that leaves your lower spine controls specific muscles in your leg. Clinicians call these muscle groups "myotomes." When a nerve is pinched, the muscles it supplies lose some of their power, even if you feel no back pain at all.
This is why someone with leg weakness and back pain may find that treating the back, not the leg, is what helps. And it is also why someone with no back pain can still have a spinal cause. The body is a connected chain from your feet to your head. A shift in your pelvis or a curve in your lower spine can press on a nerve root on one side, sending weakness down one leg while the other leg compensates and stays strong.
The tricky part is that standard lying-down scans relax your muscles and take the load off your spine. A nerve that is compressed when you are standing and weight-bearing may look perfectly fine when you are lying in a scanner. This is one reason people are told their scan is "normal" when they clearly have symptoms.
How does standing imaging help find the cause of leg weakness?
A standing, weight-bearing scan captures your skeleton the way it actually works, under gravity, with your muscles engaged. This reveals alignment shifts, pelvic tilts and loading imbalances that disappear when you lie down.
You stand naturally while the EOS system captures your full body, from head to feet, in a single image. It uses about 90% less radiation than a standard X-ray, around ten times less. Because you are upright, the image shows how your spine, pelvis, hips and legs relate to each other under load. If one side of your pelvis is dropping, or your spine is shifting to one side and compressing a nerve, it shows clearly.
Conventional X-rays, CT and MRI are usually taken lying down. This relaxes the muscles, decompresses the spine and levels the pelvis. The alignment problems causing your one-sided weakness may not be visible in that position. The scan itself is accurate, but it is not showing your body the way you use it.
ScanAlign standing EOS imaging captures your whole alignment in one session. Your specialist radiographer reviews how your pelvis, spine and legs line up under gravity. This gives your treating clinician the information they need to understand why one leg is weaker, and what to address.
When MRI is the better choice: EOS shows bones, joints and alignment. If your clinician suspects a disc pressing on a nerve, a spinal cord problem, or soft-tissue damage, MRI is usually the better tool. In many cases, both are helpful. EOS shows the alignment picture, and MRI shows the soft-tissue detail.
What should you do if one leg feels weaker?
The right next step depends on how your weakness started and how it behaves. Here is a simple guide.
- Your weakness has built up gradually over weeks or months
- It is worse when standing or walking and eases when sitting
- Standard scans have been reported as normal
- You also have back, hip or knee problems
- You notice a gait change or lean to one side
- Weakness came on suddenly
- You have numbness spreading in your leg or groin
- You have lost bladder or bowel control
- You also have facial drooping, arm weakness, or slurred speech
- You have a high temperature with worsening back pain
For gradual, unexplained weakness, understanding your alignment under load can be the missing piece. A physiotherapist testing your individual muscle groups can tell you which muscles are weak. A standing whole-body image can show your clinician why.
Not sure whether a standing scan is right for your symptoms? You can book an assessment with our specialist radiographer at no cost. It is a short video call to discuss your situation before committing to anything.
Book a free video consultationFrequently asked questions about weakness in one leg
It depends on the cause and how quickly it started. Sudden weakness is a red flag that needs urgent medical attention (call 999 or go to A&E). Gradual weakness over weeks or months is usually less urgent but still worth investigating, because the underlying cause can often be identified and addressed.
Yes. A compressed nerve root in the lower spine can reduce power to specific muscles in your leg, making it buckle or feel unreliable. This is a common feature of lumbar radiculopathy and spinal stenosis.
A heavy feeling usually means the muscles are not firing at full strength, or the nerve supply to them is partially reduced. It can also come from pain inhibition, where your body automatically limits effort on that side to avoid discomfort.
Yes. If your pelvis is tilted or your spine shifts to one side, the muscles and nerves on one side can be overloaded while the other side is underused. Over time, this creates noticeable differences in strength between your legs.
True weakness means the muscle or its nerve supply is damaged and cannot produce full force. Pain inhibition means the muscle is physically capable, but your nervous system reduces its output to protect a painful area. Both feel like weakness to you, but a clinician can often distinguish them with specific muscle tests.
When you lie down, gravity is no longer compressing your spine. Muscles relax, the pelvis levels out, and a nerve that is pinched under load may appear free. A standing, weight-bearing scan captures your body as it actually functions, making alignment-related causes visible.
EOS uses about 90% less radiation than a standard X-ray, around ten times less. It captures your full body in one pass without repeated exposures. It is suitable for adults and is performed in a quick, comfortable standing position.
No. ScanAlign is a private, self-pay clinic. You can refer yourself directly. We recommend starting with a free video consultation so our radiographer can check whether a standing scan is appropriate for your symptoms.
Pricing details are available on our FAQs and cost page. The free video consultation will also confirm which scan package suits your situation.
ScanAlign is a diagnostic imaging clinic, not a treatment provider. Your report and images are shared with you and, with your consent, your treating clinician, whether that is a physiotherapist, orthopaedic surgeon, or GP. They use the findings to guide the right next step for you.
Understand why one leg feels weaker
A free video consultation with our specialist radiographer can help you work out whether standing alignment imaging is the right next step.
Book a free video consultationSources
- NHS. "Leg weakness." NHS Conditions, nhs.uk. Accessed 2024.
- Tarulli, A.W. and Raynor, E.M. "Lumbosacral radiculopathy." Neurologic Clinics, 2007; 25(2): 387-405.
- Illés, T. and Somoskeöy, S. "The EOS imaging system and its uses in daily orthopaedic practice." International Orthopaedics, 2012; 36(7): 1325-1331.
Written by Abbas Dhami (Specialist Diagnostic Radiographer)
This page is general information and not a diagnosis.
ScanAlign is the trading name of CoreMed Solutions Ltd and operates under The Harley Street Hospital's CQC licence.