Lower back pain during ab exercises is one of the most common gym complaints, and it almost always traces back to the same problem: your hip flexors are doing the work your abs should be handling, and those hip flexors attach directly to your lumbar spine. When muscles like the iliopsoas fire hard to move your torso or legs, they pull on the vertebrae of your lower back, creating compressive and shear forces that cause pain. The story gets more interesting when you look at which exercises are the biggest culprits, why some people experience it and others don’t, and what you can do about it without abandoning core training altogether.
The Hip Flexor Problem
Your hip flexors are a group of muscles that run from your lower spine and pelvis down to your thighbone. The most relevant one here is the iliopsoas, which originates directly on the lumbar vertebrae. Every time you do a movement that involves pulling your legs toward your torso or your torso toward your legs, the iliopsoas activates. In a perfect world, your abdominal muscles would do most of the trunk-curling work while the hip flexors handled only the hip-bending portion. In practice, especially when your abs fatigue or are weak relative to your hip flexors, the iliopsoas takes over and yanks on your lumbar spine.
Research comparing sit-ups and leg raises has confirmed this split. Sit-ups activated the upper and lower rectus abdominis and external obliques more effectively, while straight leg raises produced significantly more activation of the iliopsoas and rectus femoris. The same study noted that excessive contraction of the iliopsoas during sit-ups can damage the lumbar vertebrae.1Journal of Physical Therapy Science. Comparison of muscular activities in the abdomen and lower limbs while performing sit-up and leg-raise That’s the core of the issue: the exercise you chose to strengthen your abs may actually be loading your spine through a muscle that has no business being the primary mover.
Compressive and Shear Forces on the Spine
Even when your abs are doing a reasonable share of the work, many traditional ab exercises place the lower spine under forces it wasn’t designed to handle repetitively. Sit-ups and curl-ups involve repeated spinal flexion, and the combination of trunk weight, speed, and the angle of your spine generates both compressive forces (pushing vertebrae together) and shear forces (sliding them past each other). Previous research on sit-up and curl-up mechanics has indicated that these exercises produce high shear and compressive forces in the lower back, forces sufficient to be injurious with enough repetition.2PubMed Central. Abdominal and hip flexor muscle activity during 2 minutes of sit-ups and curl-ups
The discs between your vertebrae absorb these forces, but they have limits. Repeatedly flexing the spine under load stresses the posterior and posterolateral portions of the disc in particular. Add any rotation to the mix and things get worse. A study modeling forces on intervertebral discs found that lateral bending combined with axial rotation produced the highest increase in fiber strains, with the most strain concentrated posterolaterally.3PubMed. Intradiscal pressure, shear strain, and fiber strain in the intervertebral disc under combined loading This is relevant because many popular ab exercises involve rotation, such as Russian twists, bicycle crunches, and oblique sit-ups. If you’re already experiencing back pain from straight-ahead flexion exercises, rotational variations can amplify the problem by stressing the disc from additional angles simultaneously.
When Your Core Fails to Stabilize
Your deep core muscles, particularly the transversus abdominis, act like a natural corset around your midsection. They’re supposed to fire just before you move your arms or legs, stiffening the spine so it stays stable while your limbs do their thing. But in people with low back pain, this timing is off. A landmark study found that the onset of transversus abdominis contraction was significantly delayed in people with low back pain, indicating a deficit of motor control that results in inefficient muscular stabilization of the spine.4PubMed. Inefficient muscular stabilization of the lumbar spine associated with low back pain. A motor control evaluation of transversus abdominis
This creates a frustrating cycle. You have back pain, so your deep stabilizers start firing late. Because they fire late, your spine is less stable during movement. Because your spine is less stable, you’re more likely to experience pain during ab exercises. And because those exercises hurt, you might avoid core training, which further weakens the stabilizers. People in this cycle often feel their back “give” during a leg raise or crunch, a brief moment where the lumbar spine moves more than it should because the stabilizing muscles weren’t ready.
The good news is that this motor control deficit is trainable. It’s not a permanent structural problem but a coordination issue, and targeted exercises can retrain the timing. The challenge is that you often can’t fix it by doing more of the exercises that are aggravating it.
The Anterior Pelvic Tilt Trap
One of the sneakiest ways ab exercises cause back pain involves your pelvis. Many exercises, particularly leg raises and their variations, require your abdominal muscles to hold your pelvis in a stable, slightly posteriorly tilted position while your hip flexors move your legs. When the abs aren’t strong enough to maintain that position, the pelvis tips forward into an anterior tilt. This deepens the curve in your lower back, compressing the facet joints and loading the discs unevenly.
A case study of a patient with back pain and excessive lordosis illustrated this clearly. When the patient’s abdominal muscles were weak, leg raises caused an anterior pelvic tilt that worsened her pain. Modifying the exercise by adding neck flexion helped activate the abdominal muscles, allowing the patient to stabilize her pelvis and avoid moving her lower trunk during the exercise.5Journal of Physical Therapy Science. Effect of modified leg-raising exercise on the pain and pelvic angle of a patient with back pain and excessive lordosis The takeaway is practical: if your lower back arches off the floor during leg raises, your abs have lost control of the movement and your spine is paying for it.
You can test this yourself. Lie on your back and slide your hand under the small of your back. Now slowly lower one leg toward the floor, as in a leg-lowering exercise. If your lower back lifts off your hand and the gap increases, your pelvis is tilting anteriorly and your abs aren’t holding. That’s the moment back pain starts. Many people push through it, adding reps and sets to an exercise their core can’t actually control, and wonder why they leave the gym with a sore back instead of sore abs.
Which Exercises Cause the Most Trouble
Not all ab exercises are equally risky for your lower back. The worst offenders share certain characteristics: they involve a long lever arm (like extended legs), repeated full spinal flexion, or combined rotation with flexion.
- Full sit-ups: These involve the most hip flexor contribution and the greatest spinal compression. The second half of the sit-up, once your shoulders are already off the ground, is almost entirely hip flexor work, not abdominal work.
- Straight leg raises: With both legs extended, the lever arm is long and heavy, demanding significant hip flexor activation that pulls directly on the lumbar spine. Research confirms they activate the iliopsoas and rectus femoris far more than the abdominals.1Journal of Physical Therapy Science. Comparison of muscular activities in the abdomen and lower limbs while performing sit-up and leg-raise
- Rotational exercises under load: Russian twists, weighted oblique crunches, and similar movements combine flexion with rotation, the loading pattern shown to produce the highest disc fiber strains.3PubMed. Intradiscal pressure, shear strain, and fiber strain in the intervertebral disc under combined loading
- Hanging leg raises: These add gravity and momentum to an already hip-flexor-dominant pattern. Without exceptional core control, most people swing into lumbar hyperextension at the bottom of each rep.
Exercises like planks, dead bugs, and bird dogs tend to be gentler on the lower back because they train the core in an anti-movement role, resisting extension or rotation rather than producing flexion. The spine stays in a more neutral position, and the hip flexors don’t get the opportunity to dominate.
How Abdominal Bracing Protects the Spine
If you’ve ever heard a trainer tell you to “brace your core,” they’re referring to a technique where you tighten all of your abdominal muscles at once, as if you’re about to take a punch. It’s distinct from “drawing in,” which focuses on pulling your belly button toward your spine. Bracing involves co-contracting the muscles around the entire trunk, and the research behind it is convincing.
A study testing the effects of abdominal bracing on spinal motion during sudden loading found that bracing reduced movement between individual vertebral segments by 45% to 94%, depending on the spinal level. At L4/L5, one of the most common sites of lower back pain, the reduction was about 45%. Bracing also resulted in greater baseline activation of all abdominal and back muscles before any load was applied.6PubMed Central. Brace yourself: How abdominal bracing affects intersegmental lumbar spine kinematics in response to sudden loading That pre-activation is the key. It means the spine is already stabilized before any force hits it, rather than trying to catch up after the fact.
Separately, raising intra-abdominal pressure, the internal pressure that increases naturally when you brace hard, has been shown to increase trunk stiffness in flexion by 21% at moderate levels and 42% at high levels.7SpringerLink / European Spine Journal. Lumbar spine stability can be augmented with an abdominal belt and/or increased intra-abdominal pressure That stiffening effect is exactly what your lower back needs during an ab exercise: enough rigidity to prevent the small, damaging micro-movements between vertebrae that accumulate into pain over time.
The practical implication is straightforward. Before you begin any ab exercise, brace. Not a gentle tightening but a real contraction of your entire midsection. Maintain it throughout the rep. If you can’t hold the brace through a full set, end the set early rather than letting your spine go unsupported for the final reps. Those last few grinding reps with a fatigued core are where most back injuries during ab work happen.
Surface Stability and Exercise Choice
Where you do your ab exercises can also affect how your muscles activate. A study examining planks performed on different surfaces, including a stable ground surface, a balance board, a Swiss ball, and a disc, found that surface instability changed the activation pattern of the abdominal muscles. Upper and lower rectus abdominis activity was significantly greater on the unstable disc surface compared to the stable surface and balance board conditions.8Journal of Human Kinetics / PubMed Central. Effects of Surface Stability on Muscle Activation during Plank Exercise
This sounds like an argument for doing your ab work on unstable surfaces, but there’s a catch. Greater activation isn’t always better, especially for someone with back pain. Unstable surfaces also increase the demand on your stabilizers, and if those stabilizers are already firing late or fatiguing early, adding instability can make things worse before they get better. For most people dealing with back pain during ab work, a stable surface and controlled movement is the smarter starting point. Instability training can be layered in later, once the foundational motor control issues are resolved.
Fixing the Problem With Smarter Programming
The most well-studied approach to training your core without destroying your back comes from spinal biomechanics research that emphasizes stability over movement. The idea is that the core’s primary job isn’t to create movement (like curling your torso up) but to resist it (preventing your spine from buckling under load). Exercises built around this principle include curl-ups with one knee bent and hands under the lower back, side planks, and bird dogs, all performed with an emphasis on holding a neutral spine position.
A six-week trial comparing this style of stabilization exercise with conventional physiotherapy in patients with chronic non-specific low back pain found that the stabilization group improved more in pain, functional disability, and back extension range of motion. Pain improved roughly 15% in the stabilization group versus 6% in the conventional group, and functional disability improved 13% versus 7%.9Journal of Physical Therapy Science. Effects of McGill stabilization exercises and conventional physiotherapy on pain, functional disability and active back range of motion in patients with chronic non-specific low back pain These are modest numbers, but they point in a consistent direction: exercises that prioritize spinal stability beat exercises that load the spine through flexion, at least for people who already have pain.
If you’re currently experiencing back pain during ab work, consider swapping your existing routine for a few weeks. Replace sit-ups and leg raises with variations that keep the spine in neutral. As the pain decreases and your motor control improves, you can gradually reintroduce more demanding exercises, starting with partial ranges of motion and lighter loads.
Tight Hip Flexors and the Flexibility Question
A common piece of gym wisdom is that tight hip flexors cause lower back pain during ab exercises, and that stretching them will fix the problem. The logic sounds reasonable: if the iliopsoas is short and stiff, it pulls on the lumbar spine even at rest, and exercises that activate it further would amplify that pull. But the evidence is less clear-cut than the theory suggests.
A correlational study of 64 adults with low back pain found only a weak association between hip flexor tightness and lumbar instability.10PubMed Central. Association Between Hip Flexor Tightness And Lumbar Instability In Adults That doesn’t mean hip flexor flexibility is irrelevant. It means tightness alone probably isn’t the primary driver for most people. Someone with very tight hip flexors, an excessive lumbar curve, and weak abdominals is likely to have more trouble than someone who is simply “tight.” But stretching your hip flexors and expecting your back pain to disappear during ab work is probably giving flexibility more credit than it deserves. Motor control and exercise selection matter more.
That said, maintaining reasonable hip flexor flexibility still makes sense as part of a broader approach. If your hip flexors are so tight that you can’t lie flat on your back without your lower back lifting off the ground, that resting anterior pelvic tilt is going to make every supine ab exercise harder to perform with good spinal alignment. Stretching won’t solve the problem on its own, but it can reduce one contributing factor.
When It’s Not Just Mechanics
Most lower back pain during ab exercises is mechanical: wrong exercise, poor form, insufficient stabilization. But persistent or worsening pain during core work that doesn’t improve with exercise modification deserves attention. Disc herniations, spondylolisthesis (where one vertebra slips forward over another), and facet joint irritation can all produce pain that mimics the “my back hurts when I do abs” pattern but won’t resolve with better programming alone.
Some red flags worth noting: pain that shoots down one or both legs, numbness or tingling in the feet, pain that’s worse in the morning and doesn’t warm up, or pain that persists for more than a few days after you’ve stopped the aggravating exercise. Any of these warrant evaluation by a healthcare provider rather than another round of YouTube exercise swaps. The vast majority of ab-related back pain is benign and fixable with the techniques described above, but the small percentage that isn’t can get significantly worse if you keep loading through it.
People with a history of disc issues should be especially cautious with repeated spinal flexion exercises. The combination of compression and flexion is the classic loading pattern that stresses the posterior disc wall, and if you already have a weakened or bulging disc, high-rep sit-ups or crunches can push it further. For this population, anti-extension and anti-rotation exercises aren’t just a preference but a genuine protective strategy, letting you train your core without repeatedly stressing the weakest point of your spine.