SI Joint Pain in Active People: Why "Weak Core" Is the Wrong Diagnosis
If you've been told your sacroiliac joint pain is caused by a "weak core" or "SI joint instability," you've probably spent months doing planks, bird dogs, and dead bugs without meaningful improvement. You might have been prescribed a belt, told to avoid asymmetrical movements, or advised that your pelvis is "out of alignment" and needs regular manual therapy and manipulation to stay in place.
This explanation is attractive because it's simple and offers a clear fix. But for most active adults and athletes with SI joint pain, it's fundamentally incomplete, and often wrong.
SI joint pain in active people isn't typically caused by weakness or instability. It's usually a load tolerance issue: your sacroiliac joint and surrounding structures are being asked to manage forces they're not currently conditioned to handle. The solution isn't just core exercises or avoiding asymmetrical loading forever. It's progressively building capacity to tolerate the specific demands your training, sport, and life require.
What Is SI Joint Pain?
The sacroiliac joint sits at the junction between your sacrum (the triangular bone at the base of your spine) and your ilium (the large pelvic bone on each side). You have two SI joints, one on the left, one on the right. These joints are relatively immobile compared to other joints in your body, moving only a few millimetres during normal function.
The SI joint's primary role is load transfer, distributing forces between your upper body and legs during movement. Every time you walk, run, jump, lift, or rotate, forces travel through your SI joints. They're designed to handle substantial loads when functioning optimally.
SI joint pain typically presents as one-sided lower back or pelvic pain, often around the belt line or just lateral to the sacrum. The pain can refer into the buttock, groin, or upper thigh, but rarely extends below the knee in a dermatomal pattern like radiculopathy would.
Pain in this region is complex because multiple structures overlap: the SI joint itself, lumbar facet joints, surrounding ligaments and muscles, and nerve pathways all occupy similar territory. This is why a thorough assessment is key in gaining the correct diagnosis.
Why "Weak Core" Became the Default Explanation
The "weak core causes SI joint pain" narrative emerged from early core stability research in the 1990s, which suggested that dysfunction in deep stabilising muscles (transversus abdominis, multifidus) contributed to lower back pain and pelvic instability.
This research was valuable and shifted physiotherapy thinking toward active rehabilitation. However, it also created an oversimplified narrative: if you have back or pelvic pain, your core must be weak, and therefore isolated core strengthening will fix it.
This explanation is attractive for several reasons. It's simple and easy to communicate. It provides a clear "problem" (weakness) with an obvious "solution" (strengthen your core). It gives patients something actionable to work on.
But for active adults and athletes, people who already train regularly, lift weights, run, or play sport, the idea that their core is fundamentally weak doesn't always make sense. These individuals can have significant trunk strength and control. They're not deconditioned. Yet they're told to spend months doing beginner-level core exercises that don't resemble their training demands.
The "weak core" explanation fails because it ignores load specificity, training volume and intensity, unilateral and asymmetrical loading patterns, fatigue and recovery capacity, and movement strategy and load distribution.
Active people don't typically develop SI joint pain because they lack foundational strength. They develop it because cumulative load in specific patterns has exceeded their tissue's current capacity to adapt.
Why Active People Actually Get SI Joint Pain
SI joint pain in athletes and active adults typically develops when training load, volume, or intensity increases faster than tissue adaptation. This isn't about weakness, it's about load tolerance.
Training Asymmetry and Sport-Specific Demands
Many sports and activities involve repetitive unilateral loading or asymmetrical movement patterns. Running (repetitive unilateral impact), single-leg strength training (split squats, lunges, step-ups), rotational sports (golf, tennis, throwing), and unilateral carrying or loaded positions all create higher demands on one SI joint compared to the other.
If you've recently increased running mileage, started heavy single-leg work, changed your training split, or returned to sport after time off, your SI joint might be experiencing loads it's not currently conditioned to handle. This isn't instability, it's load intolerance.
Load Spikes and Inadequate Recovery
SI joint pain often emerges after training volume spikes, competition periods with inadequate recovery, returning to training after a break, or adding new movement patterns without gradual progression.
The joint isn't inherently weak or unstable, it's been asked to do more than it's adapted for, often with insufficient recovery between sessions. This is a normal physiological response to excessive demand, not evidence of structural dysfunction.
Why High-Capacity Athletes Still Get SI Joint Symptoms
You don't need to be weak or deconditioned to develop SI joint pain. Elite athletes with exceptional trunk strength and stability still experience load-related pelvic pain when demands exceed capacity. The difference between elite athletes and recreational gym-goers isn't that elites don't get injured, it's that they're exposing their bodies to significantly higher loads and volumes.
If anything, active people are more likely to push into load intolerance because they have the capacity and willingness to train hard. The solution isn't avoiding load, it's managing progression intelligently and building specific tolerance to the movements that provoke symptoms.
Common Symptoms and Presentation
SI joint pain typically presents with characteristic patterns that help differentiate it from other causes of lower back and pelvic pain.
Typical Pain Patterns
Pain is usually one-sided, located around the posterior superior iliac spine (the bony prominence at your belt line), into the buttock, or along the lateral sacrum. It can refer to the groin or upper thigh.
Athletes commonly describe a deep, achy discomfort that can become sharp with specific movements. The pain often worsens with unilateral loading (single-leg stance, running, climbing stairs), transitioning from sitting to standing, prolonged standing on one leg, asymmetrical lifting or carrying, and impact activities (running, jumping, landing).
Many athletes report that symptoms start subtly during or after training, improve with rest, but return predictably with the same activities. Unlike disc-related pain that often worsens with sitting and forward bending, SI joint pain typically worsens with weight-bearing asymmetry and prolonged standing.
How Symptoms Behave With Training
Athletes notice that pain increases with training volume accumulation throughout the week, appears during or immediately after specific movements (lunges, single-leg deadlifts, running), improves with strategic rest but returns when resuming previous training loads, and often feels better with bilateral loading (squats, deadlifts) compared to unilateral work.
This pattern, load-related, position-specific pain that settles with rest, suggests a capacity issue, not structural instability.
SI Joint Pain vs Disc Pain vs Radiculopathy
Understanding these distinctions helps clarify what you're dealing with. SI joint pain stays localised to the pelvic region or refers non-specifically to the buttock/groin, worsens with unilateral weight-bearing and impact, and doesn't cause significant leg symptoms below the knee, numbness, or weakness.
Disc-related pain often worsens with sitting, forward bending, and prolonged flexed postures, may refer pain into the leg but typically in non-dermatomal patterns, and can cause centralised symptoms.
Radiculopathy involves nerve root compression, causing sharp, shooting pain following a dermatomal distribution (often below the knee), and may include specific patterns of numbness, tingling, or weakness.
Many athletes experience combinations of these presentations, which is why clinical assessment based on symptom behaviour matters more than relying solely on imaging.
How SI Joint Pain Is Diagnosed Clinically
SI joint pain is primarily a clinical diagnosis based on symptom behaviour, movement patterns, and load response, not just imaging findings.
Limitations of Imaging and SI Joint Tests
MRI, X-ray and CT scans can show degenerative changes, inflammation, or structural variations in the SI joint, but these findings can occur in pain-free individuals. Imaging confirms what's there anatomically but doesn't reliably predict who has pain or who will respond to treatment.
"SI joint tests" (provocation tests like FABER, Gaenslen's, distraction) are commonly used but have limited diagnostic accuracy when performed in isolation. A positive test doesn't definitively confirm SI joint pain, and a negative test doesn't rule it out. Instead clustering tests together increases the likelihood of accuracy. Positive test results with repeated, and different tests, targeting the same structure improves the probability of an accurate diagnosis.
Other useful diagnostic information comes from understanding what provokes your pain (single-leg loading, impact, asymmetrical movements), what relieves it (rest, bilateral positions, specific positions), how it behaves throughout the day and week, and how it responds to progressive loading.
Evidence-Based Rehab for SI Joint Pain
Rehabilitating SI joint pain isn't about isolated core strengthening until some arbitrary strength threshold is achieved. It's about progressively building capacity to tolerate the specific loads and movements your sport and training demand.
Phase 1: Intelligent Load Modification
This isn't complete rest or avoidance of all asymmetrical movement. It's strategic modification of variables that are currently exceeding tissue tolerance.
Temporarily reduce volume or intensity of single-leg work that significantly provokes symptoms. Modify running mileage, intensity, or frequency if impact loading is aggravating. Maintain strength training through bilateral movements that are well-tolerated (squats, deadlifts, bilateral carries). Continue cardiovascular work that doesn't provoke symptoms (cycling, swimming, rowing). Maintain as much training as possible whilst managing cumulative load on the irritated structures.
The goal is settling acute reactivity whilst maintaining fitness and capacity elsewhere. You're buying time for tissue adaptation whilst staying active and engaged with training.
Phase 2: Restoring Effective Load Transfer
Once acute symptoms settle, focus shifts to improving how forces are distributed through your pelvis and trunk during movement. This isn't just about isolated core exercises, it's also about training movement patterns under progressive load.
Develop control and strength in positions that challenge pelvic stability (single-leg stance progressions, split stance work, carries). Improve hip and thoracic mobility to reduce compensatory motion through the pelvis. Build bilateral strength capacity (squats, deadlifts, hip thrusts) to establish foundational load tolerance. Begin introducing controlled unilateral loading with manageable loads (step-ups, split squats with reduced range or load).
This phase emphasises improving movement quality and control under load, not achieving perfect alignment or isolating specific muscles. The body works as an integrated system, training should reflect that.
Phase 3: Progressive Unilateral and Sport-Specific Loading
SI joint pain often creates fear of asymmetrical movement. Athletes avoid single-leg work, reduce running volume indefinitely, or modify training to eliminate all unilateral loading. This perpetuates the problem.
Progressive re-exposure involves systematically reintroducing previously aggravating movements with controlled progression. Gradually increase load, volume, and intensity in single-leg exercises (lunges, split squats, single-leg deadlifts). Progressively build running volume, frequency, and intensity if running was provocative. Introduce sport-specific movements with graduated complexity (cutting, landing, rotational work). Build confidence through successful exposure to previously feared movements.
Progression isn't linear. You'll have challenging days and minor setbacks. What matters is the overall trend toward improved tolerance and capacity, not perfect symptom-free status every session.
Phase 4: Building Long-Term Resilience and Capacity
The final phase focuses on developing greater capacity than you had before symptoms developed. This means building unilateral strength beyond previous levels, increasing training volume tolerance in asymmetrical movements, developing robust load management strategies you can implement independently, and understanding early warning signs and how to adjust training when needed.
Successful rehabilitation doesn't guarantee you'll never experience discomfort again. It means you understand your body's load tolerance, recognise when you're approaching capacity limits, can modify variables appropriately, and maintain training without ongoing professional supervision.
Common Rehab Mistakes That Keep People Stuck
Endless Low-Level Core Work
Spending months doing planks, dead bugs, and bird dogs doesn't build the capacity needed to tolerate running, heavy lifting, or sport-specific demands. These exercises have value in early rehab or for people developing foundational control, but they don't replicate the loads active people need to manage.
If you're an athlete or regular gym-goer still doing beginner core exercises six months into rehab without progressing to heavier, more specific loading, you're not building relevant capacity.
Avoiding Asymmetrical Loading Indefinitely
Fear of single-leg work, running, or asymmetrical movements perpetuates the problem. Your body adapts to the demands you expose it to. If you avoid unilateral loading because it provokes symptoms, you'll never build tolerance to those movements.
Progressive exposure in controlled doses is how you rebuild capacity, not indefinite avoidance.
Chasing Symptom Elimination Instead of Function
Waiting for complete pain resolution before progressing load or returning to sport often means waiting indefinitely. Pain isn't always a reliable indicator of tissue capacity, especially in later rehabilitation stages.
Progress is measured by improving function, can you tolerate more load, more volume, more complex movements? If yes, you're progressing, even if some discomfort persists.
Over-Reliance on Belts, Manual Therapy, or Passive Treatments
SI joint belts, regular manual therapy, or ongoing passive treatments might provide temporary symptom relief, but they don't build long-term capacity. If you're relying on these interventions week after week without improving your ability to train independently, you're not progressing toward meaningful outcomes.
Effective rehabilitation builds independence, your capacity to understand, manage, and progress your condition without continuous external support.
When Professional Support and Advocacy Matter
Most active adults with SI joint pain respond well to progressive, load-based rehabilitation. However, certain scenarios warrant professional assessment or coordinated care.
When to Seek Assessment
Consider assessment from a physiotherapist specialising in spinal and pelvic conditions if pain has persisted beyond 6-8 weeks despite intelligent load modification, you're experiencing recurrent flare-ups that don't settle with previous strategies, pain is progressively worsening rather than fluctuating or improving, you're developing new symptoms (neurological changes, significant leg pain), or you're unsure how to progress loading safely without aggravating symptoms.
Red Flags Requiring Medical Attention
Seek immediate medical assessment if you experience unexplained weight loss or feeling systemically unwell, significant trauma or new mechanism of injury, loss of bowel or bladder control, progressive bilateral leg weakness or numbness, or constant night pain unrelated to position.
The Role of Coordinated Care and Advocacy
Sometimes rehabilitation requires coordination between healthcare providers. A spine-focused physiotherapist can assess movement patterns and guide progressive loading, communicate with your GP regarding imaging interpretation or specialist referral if needed, liaise with coaches or trainers to modify programming during rehabilitation, and advocate for evidence-based care when you've been given overly simplistic explanations or conservative advice that doesn't align with current understanding.
Too many active people are told their SI joint is "unstable," that they need ongoing manual therapy to keep their pelvis "in place," or that they should avoid asymmetrical movements indefinitely. These explanations don't reflect current evidence and often perpetuate fear and dependence rather than building capacity and confidence.
Key Takeaways for Active People
- SI joint pain in active adults is usually a load tolerance issue, not always instability or weakness
- "Weak core" is an incomplete explanation that fails to account for training load, volume, and movement specificity
- Most cases respond well to progressive, capacity-focused rehabilitation, not just isolated core work
- Diagnosis is primarily clinical, based on symptom behaviour and load response, imaging findings can help correlate clinical findings but are not a diagnosis if clinical findings don’t match
- Rehabilitation focuses on intelligent load modification initially, then systematic rebuilding of unilateral and sport-specific tolerance
- The goal isn't avoiding asymmetrical movement forever, it's building capacity to handle it confidently
- Common mistakes include over-resting, endless low-level core exercises, and avoiding single-leg work indefinitely
- Progress is measured by improving function and load tolerance, not achieving zero pain
- Professional assessment is valuable when progress stalls, symptoms worsen, or you need advocacy navigating overly simplistic explanations
SI joint pain doesn't mean your pelvis is unstable or your core is fundamentally weak. It means you've encountered a load tolerance limit. With intelligent rehabilitation focused on progressive loading, movement quality, and building resilience, most active people return to full training, often with greater capacity and better movement strategies than before symptoms developed.
If you're dealing with persistent SI joint pain and want clarity on your specific situation, professional assessment can provide a structured pathway forward. Athletic Spine specialises in evidence-based spinal and pelvic rehabilitation for active adults and athletes, with a focus on measurable outcomes, building independence, and returning you to training better than ever. Contact us to discuss how we can help guide your recovery.