Because the studies are not measuring the same thing. Method, tissue depth, muscle site, patient population, and psychological state all shift the result. Even in healthy adults, normal side-to-side asymmetry runs about 10% to 14%, which is large enough to swamp a small effect in an uncontrolled study.
What do the conflicting findings actually look like?
One body of work finds an association, another finds nothing, and both are competently done. A 2025 review in Frontiers in Pain Research on fascia in myofascial pain syndrome reported that affected patients often show stiffer, less elastic fascia, and that treatment targeting fascial mobility may reduce both pain and stiffness.
Then the same review notes that the specific relationships remain under-researched and that more clinical study is needed. That hedge is not politeness. It reflects how unstable the effect is across study designs.
Why does the measurement method change the answer?
Different instruments read different tissue layers. Handheld myotonometry reads superficial tissue under the probe. Shear wave elastography reaches deeper structures. Indentometry measures resistance to a fixed deformation.
A 2023 study in Scientific Reports comparing stiffness measurement tools found that reliability and validity vary by tool and by site. Two teams studying low back pain with different instruments at different depths are answering different questions while publishing under the same topic heading.
| Source of disagreement | How it distorts the result |
|---|---|
| Instrument and tissue depth | Superficial and deep readings can diverge at the same site |
| Muscle site selection | An effect at upper trapezius may not appear at gastrocnemius |
| Resting versus contracted state | Contracted muscle reads substantially stiffer |
| Sex distribution of the sample | Sex is a significant predictor of resting stiffness |
| Uncontrolled normal asymmetry | Healthy side-to-side difference of 10% to 14% can be read as a finding |
| Psychological state of participants | Attitude toward pain modulates reported effect |
How much does the patient's own belief interfere?
Enough to separate a measured tissue change from a reported pain change. A 2025 study in Healthcare on attitude toward pain and the effects of foam rolling found that psychological factors modulated the relationship between measured tissue property changes and pain perception.
This is why a study can show a real drop in stiffness with no matching drop in reported pain, and vice versa. The tissue variable and the reported variable are connected by a person who has opinions about their own body.
Survey data: In a 2026 survey of 455 patients who stopped chiropractic care, 58% cited perception-based reasons: 36% felt no progress, and 22% felt better and stopped. Neither group was shown that their soft tissue stiffness was still elevated.
Does normal variation explain part of the conflict?
A meaningful part of it. A 2025 pilot study in the Journal of Manual and Manipulative Therapy measured seven muscle groups in 26 healthy young adults and found sex was the only significant predictor of resting stiffness. Age, BMI, and weekly exercise were not.
The same study reported average bilateral asymmetry of roughly 10% to 14% in people with no complaint. A study comparing a painful side to a non-painful side without accounting for that baseline difference can produce a positive finding out of normal physiology.
What should a clinician take from this?
Treat stiffness and pain as two independent variables you track separately. That is the conservative reading of a conflicted literature, and it is also the more useful one at the table.
The population question, whether stiffness explains pain in general, is unsettled. The clinical question is narrower. Did this patient's reading at this site change from their own baseline, measured the same way both times. That comparison does not depend on resolving the wider debate.
Keep the claims hedged accordingly. A change in a stiffness reading may reflect a change in the tissue. It does not diagnose a condition, it does not predict recurrence, and it is not a proxy for how much the patient hurts.
Frequently Asked Questions
Do studies agree that muscle stiffness causes pain?
No. Some studies report an association between elevated soft tissue stiffness and pain, others find none. No well-designed study has established that stiffness causes pain, and association is the strongest claim the literature supports.
What is the biggest source of disagreement between studies?
Measurement method and tissue depth. Handheld myotonometry, shear wave elastography, and indentometry read different tissue layers with different sensitivity, so two studies on the same condition can report different results.
Do psychological factors affect the relationship?
They appear to. A 2025 study in Healthcare found that a participant's attitude toward pain modulated the perceived effect of foam rolling, even where tissue property changes were measured directly. Belief sits between tissue and reported pain.
Does normal variation explain some of the conflict?
Yes. A 2025 pilot study found bilateral asymmetry of roughly 10% to 14% in healthy adults and identified sex as a significant predictor of resting stiffness. Studies that do not control for these can read normal variation as a finding.
Is the fascia research more consistent than the muscle research?
Somewhat, in myofascial pain syndrome specifically. A 2025 Frontiers in Pain Research review reported that patients with myofascial pain syndrome often show stiffer, less elastic fascia, though the review also calls the mechanisms under-researched.
What can a clinician safely conclude from the current evidence?
That stiffness and pain are independent variables worth tracking separately. A stiffness reading may reflect the state of the tissue. It does not stand in for the patient's pain, and it does not diagnose.
Does the conflict make stiffness measurement useless?
No. The disagreement is about whether stiffness explains pain across populations. Tracking one patient against their own baseline over a course of care is a different and much narrower question.
One approach is to add a second channel of objective data alongside subjective pain reports. Options include soft tissue stiffness measurement (such as MuscleMap), range-of-motion testing, and posture analysis. Each gives you something concrete to show the patient rather than asking them to take your word for it.