No. Two stiffness devices measuring the same muscle will give you two different numbers, and the gap between them is not random error you can average away. Published comparisons report systematic bias between methods. A reading is meaningful against other readings from the same device and protocol, not against a number from a different tool.
Why do two devices disagree on the same muscle?
Because they are not measuring the same thing. They share the word stiffness, which hides the fact that each device answers a different physical question.
Surface myotonometry applies a brief mechanical impulse and reads the oscillation that comes back, then reports dynamic stiffness in newtons per metre. Shear wave elastography sends a wave through tissue and calculates a modulus in kilopascals from how fast that wave travels. One reads the response of an entire tissue column. The other reads a selected region inside it.
A 2026 study in Diagnostics using layer-specific shear wave elastography looked at where surface stiffness readings actually originate and found that the superficial layers, including skin and fascia, contribute meaningfully to what a surface device reports. That is not a flaw. It is a different measurement target.
How large is the disagreement in practice?
Large enough to change a clinical conclusion. Correlations between methods are often moderate in relaxed superficial muscle and fall off with depth or contraction.
Work measuring stiffness in major muscles with shear wave elastography shows values that vary substantially by muscle and by whether tissue is at rest. Comparisons against mechanical reference testing have been less encouraging still. A 2026 paper in the Journal of Biomechanics comparing MyotonPRO-derived stiffness against tensile measures reported poor agreement, which is a direct argument against treating one device's output as a stand-in for another's.
| Method | Unit reported | What it primarily reads | Safe to compare against another method? |
|---|---|---|---|
| Surface myotonometry | N/m | Oscillation of the full tissue column under the probe | No |
| Shear wave elastography | kPa | Wave speed within a selected region of interest | No |
| Tissue hardness meter or indentometer | Force or displacement | Resistance to a known indentation force | No |
| Tensiomyography | mm displacement | Mechanical response to electrical stimulation | No |
| Same device, same site, same protocol | Consistent | Change in that patient over time | Yes, this is the valid comparison |
Does a strong correlation mean the numbers are interchangeable?
No, and this is the mistake worth avoiding. Correlation and agreement are separate questions.
A correlation coefficient tells you two devices rank patients in roughly the same order. Agreement asks whether device A and device B produce the same value on the same tissue. A method can rank patients perfectly while reading consistently high by a fixed offset, and the correlation would look excellent. Bland-Altman analyses across these comparisons commonly show exactly that pattern: reasonable correlation, real systematic bias.
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 this make stiffness measurement unreliable?
No. Reliability within one device is the part the evidence supports well. The weak link is cross-device comparison, not repeated measurement.
A 2024 systematic review evaluating MyotonPro reliability pooled dozens of studies and reported consistently high intra-rater and inter-rater reliability across muscle groups. That is the property that matters for tracking one patient across a course of care. You are asking whether this muscle changed since March, not whether this device agrees with a machine in a different building.
What should you do in your own clinic?
Pick one device, fix the protocol, and never plot two devices on the same chart. Three habits cover almost all of the risk.
First, record the device model alongside every reading, not just the number. Second, if you replace or upgrade the device, close the old series and start a new baseline on the changeover date. Third, when you use published reference values, check that the reference study used your device, your muscle, and comparable positioning. Reference value work for muscle stiffness and asymmetry using myotonometry is device-specific and protocol-specific for exactly this reason.
What you show a patient should follow the same rule. A patient comparing their March number to their August number on one device is looking at real change. A patient comparing a number from your device to a number from another provider's device may be looking at nothing but the difference between two tools.
Frequently Asked Questions
Can you compare muscle stiffness readings from two different devices?
No. Different measurement methods report different units and probe different tissue depths, and published comparisons show systematic bias between them. Readings from one device should be compared only to other readings from that same device.
Why do two devices disagree on the same muscle?
Surface myotonometry measures the oscillation of the whole tissue column under the probe, including skin and fascia. Shear wave elastography measures wave speed inside a selected region. They are answering related but different physical questions.
Is there a conversion formula between N/m and kPa?
There is no general conversion you can apply in clinic. Published agreement varies by muscle, depth, and contraction state, so a single conversion constant would be wrong for most sites.
What if I upgrade to a newer device mid-treatment?
Treat the switch as a new baseline. Record the last reading from the old device, note the changeover date in the chart, and start a fresh comparison series rather than plotting both on one trend line.
Do correlations between devices mean the numbers are interchangeable?
No. Correlation says two devices tend to rank patients similarly. Interchangeability requires agreement in absolute value, which Bland-Altman analyses generally do not support.
Does this mean stiffness measurement is unreliable?
No. Reliability within a single device and protocol is consistently reported as good to excellent. The limitation is cross-device comparison, not repeat measurement on one device.
Can I compare my patient to published reference values from another device?
Only if the reference set used the same device, muscle, and positioning protocol. Otherwise the comparison introduces a device offset you cannot quantify.
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.