There is no single normal value for muscle stiffness. Normal varies by muscle, sex, posture, and device. A 2025 pilot study using myotonometry established preliminary reference values in healthy young adults and confirmed that males show greater global stiffness than females, while age and BMI within young adults were not significant predictors.
Where do published reference values come from?
The most useful starting point is a 2025 PubMed-indexed pilot study that measured resting muscle stiffness across multiple muscles in healthy young adults using the MyotonPRO. The authors reported sex differences, evaluated bilateral asymmetry, and tested age, BMI, and exercise level as predictors. The takeaway was practical: sex matters, the other variables did not in this sample, and the values can serve as a baseline for comparison until larger normative studies catch up.
For lumbar muscles specifically, a 2024 study in PeerJ reported good-to-excellent intra-rater and inter-rater reliability of handheld myotonometry on the lumbar erector spinae in both prone and sitting positions, with ICCs between 0.84 and 0.96. Reliability that high means a stiffness reading you take today can be compared to a reading you take six weeks from now without the difference being swamped by measurement noise.
What units should you expect on a stiffness report?
| Property | Handheld myotonometry (MyotonPRO) | Shear wave elastography |
|---|---|---|
| Stiffness | Newtons per meter (N/m) | Shear modulus in kilopascals (kPa) |
| Tone / frequency | Hertz (Hz) | Not directly reported |
| Elasticity | Log decrement (unitless) | Not directly reported |
| Wave speed | Not reported | Meters per second (m/s) |
Values from different devices are not directly comparable. If you switch tools mid-case, you reset the baseline. Pick one method and stay with it for the duration of care.
What about clinically meaningful asymmetry?
Most published protocols treat left-right asymmetry above roughly 10 percent on the same muscle as worth investigating, measured under standardized conditions (same position, same hydration state, same time of day, multiple readings averaged). The exact threshold depends on the muscle. Asymmetry by itself does not diagnose anything. It gives you a starting point for comparison over time.
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 told their stiffness was still elevated.
How should you use reference values in practice?
Treat published norms as a sanity check, not a diagnostic threshold. The most useful comparison is the patient to themselves over time. Take the first reading at baseline. Re-measure under the same conditions at re-exam. Show the patient the trajectory, not the raw number against a population average.
A 2024 systematic review of MyotonPRO across 48 studies found that intra-rater and inter-rater reliability are consistently high in upper and lower extremity muscles. The implication: your in-clinic baseline is a defensible reference point even when external normative data is thin for the specific muscle you measured.
Frequently Asked Questions
Should I publish a reference range in the patient's report?
If you have a defensible normative source for that muscle, yes. If not, show the patient's baseline value, the current value, and the percentage change. Patients understand "your right upper trap is 14 percent stiffer than your left" better than a raw N/m number with no context.
How many readings should I take to establish a baseline?
Most clinical protocols use three to five readings per muscle and report the mean. The MyotonPRO multi-scan setting handles this automatically. Single readings are noisier and harder to defend as a comparator.
Do hydration and recent activity affect stiffness readings?
Yes. Dehydration may elevate stiffness. Recent exercise may transiently raise stiffness in the worked muscles. Standardize the visit conditions: same time of day, no workouts within two hours of measurement, normal hydration. Note any deviations in the chart.
Is there a difference between resting and active stiffness?
Yes. Resting stiffness is measured with the muscle relaxed. Active stiffness is measured during voluntary contraction. Most clinical protocols use resting stiffness because it is more reproducible. Reference values in the literature are almost always resting values unless otherwise stated.
Can I compare a patient's reading to a value from a different study population?
Cautiously. Population differences in sex distribution, age, and activity level can shift means substantially. Use external norms as orientation, not as a diagnostic cutoff. The most defensible comparison is the patient to their own baseline.
Where do normative values stand for trunk and pelvis muscles?
Reliability evidence is strongest for lumbar erector spinae and upper trapezius. Quadratus lumborum, gluteals, and deep abdominal muscles have less normative data published. Treat readings on those muscles as patient-internal comparators rather than against population norms until larger studies are published.
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.