One-Sided Low Back Pain? It Might Be Your Sacroiliac (SI) Joint, Not Your Low Back
Have you ever bent over to pick something up, rolled out of bed, or simply stood on one leg and felt pain on one side of your lower back?
Have you tried countless low back stretches and exercises with little to no relief?
The answer may be that your low back isn't actually the primary source of your pain.
Instead, you may be experiencing sacroiliac (SI) joint dysfunction, with your low back simply becoming one piece of a much larger puzzle.
At Tonic & Phasic, we often remind patients that the body functions as one interconnected system. Rather than chasing symptoms, we want to understand why your body isn't moving or transferring load efficiently.
Once we identify the "why," we can build a treatment plan that addresses the root cause, not just the pain.
Understanding the Sacroiliac (SI) Joint
The sacroiliac (SI) joint is where your sacrum, the triangular-shaped bone at the bottom of your spine, meets your ilium, the two large bones that make up your pelvis. Together, these structures create the center of what we call the pelvic girdle.
The SI joint isn't working alone. It is surrounded by a network of ligaments, fascia, muscles, and connective tissue that all work together to stabilize your body and transfer force between your spine and your legs.
Its primary role is load transfer.
Think about walking. Every step requires one leg to move forward while the other moves behind you. Your pelvis naturally rotates, shifts, and transfers your body weight from side to side. This asymmetrical movement is normal and necessary.
When your hips, lumbar spine, or surrounding tissues become restricted, your pelvis has a much harder time transferring that load. Instead of efficiently distributing force, compression begins building around the SI joint, often resulting in pain.
One of the biggest misconceptions we hear is that the SI joint "doesn't move" or that it only moves a tiny amount.
While the amount of motion is relatively small, we encourage patients to focus less on how much the joint moves and more on how well your body transfers load.
Can you comfortably shift your weight?
Can you stand on one leg?
Can you walk, bend over, or roll in bed without pain?
Those questions often tell us much more than worrying about whether the SI joint is "out of place."
Why Is SI Joint Pain So Common During Pregnancy?
Pregnancy creates significant changes throughout your body.
As your uterus grows, your center of gravity shifts forward, changing how your spine, pelvis, and hips manage load throughout the day.
One contributor to SI joint discomfort is the uterosacral ligament, which connects the uterus to the sacrum. As the uterus expands during pregnancy, tension through these supporting ligaments and the changing mechanics of the growing abdomen can increase stress around the sacrum and back of the pelvis.
At the same time, your body is adapting to changing posture, increased abdominal pressure, and the additional growth of pregnancy, which can cause stress on the low back.
All of these changes make efficient load transfer even more important, which is why SI joint pain is so commonly experienced during pregnancy.
How Does This Connect to the Pelvic Floor?
One of the most common questions we hear is:
"If my pain is coming from my SI joint, why am I seeing a pelvic floor therapist?"
The answer is because everything works together. The sacrum sits at the center of the pelvis, and the pelvic floor attaches directly into this system.
If the SI joint has been irritated for a prolonged period, the pelvic floor may begin increasing its resting tension to help stabilize the pelvis or compensate for altered movement patterns.
This doesn't necessarily mean your pelvic floor caused the problem.
Instead, the pelvic floor simply becomes another piece of the puzzle, just like the lumbar spine, hips, abdominal wall, or surrounding connective tissues.
This is something we assess frequently in pelvic floor therapy because treating only one area often misses the bigger picture.
How Do We Know If It's SI Joint Pain?
SI joint pain can mimic traditional low back pain, which is why an evaluation is so important.
Common symptoms include:
Pain on one side of the low back
Pain around the dimples of the pelvis
Groin pain
Lower abdominal discomfort
Pain with forward bending
Pain when rolling in bed
Difficulty standing on one leg
Pain when transferring weight onto the painful side
During an assessment, we don't rely on just one test.
We evaluate your movement, your ability to transfer load, mobility throughout the hips and lumbar spine, palpate the SI joint to see if your symptoms are reproduced, and perform specialized clinical tests that help determine whether the SI joint is contributing to your pain.
A Simple Exercise to Try
One of our favorite starting exercises for SI joint discomfort uses isometric contractions.
An isometric exercise allows your muscles to generate force without producing movement.
Lie on your back with both knees bent.
Although every person is different, which is why an assessment is important, we commonly begin by activating:
The hamstring on the painful side
The hip flexor on the opposite side
On the painful side, dig your heel into the floor as though you are trying to slide it toward your glutes without allowing it to actually move. Imagine creating tension without movement.
On the opposite leg, lift your knee slightly and place your hand against it. Push your knee into your hand while your hand pushes back into your knee with equal pressure.
Perform both contractions at the same time.
Hold for 10 seconds, relax, and repeat 10 times while breathing normally.
If your pain increases immediately, stop the exercise. You may try switching the sides once, but if your symptoms continue, discontinue the exercise until you've had an evaluation.
If the exercise decreases your discomfort, it may be helping your body improve load transfer and temporarily reduce stress around the SI joint.
The Bigger Picture
While this exercise can be a great starting point, it is exactly that, a starting point.
If your symptoms improve, that's wonderful.
But it's also your body's way of telling you that there is likely something else contributing to the problem.
Is it your hips?
Your lumbar spine?
Your breathing mechanics?
Your pelvic floor?
Scar tissue?
Muscle coordination?
Pregnancy-related changes?
These are the questions we answer during a comprehensive evaluation.
Our goal isn't to reduce your pain for a day.
Our goal is to identify why your body isn't transferring load efficiently so you can confidently return to walking, exercising, lifting your children, working, and doing the activities you love.
When Should You Seek Help?
If you've been experiencing one-sided low back pain, pain around your SI joint, or persistent discomfort during pregnancy that isn't responding to general stretches or exercises, it may be time for a more comprehensive assessment.
At Tonic & Phasic Holistic Pelvic Health, we evaluate how your spine, pelvis, hips, core, connective tissues, breathing mechanics, and pelvic floor work together to identify the factors contributing to your symptoms.
Because lasting relief starts with understanding the whole picture, not just treating the place that hurts.
Medical Disclaimer:
This blog is for educational purposes only and is not intended to replace medical advice, diagnosis, or treatment. Every body and situation is different—if you have specific concerns, symptoms, or questions, we recommend working with a licensed healthcare provider or pelvic health specialist.
