Myotonometry shows excellent reliability for superficial trunk and limb muscles (ICC above 0.85), and moderate reliability for deep muscles and the vastus medialis. The most rigorous 2024 evidence is a systematic review of 48 studies published in MDPI Medicina by Lettner et al. that found consistently high intra- and inter-rater reliability for stiffness and frequency across most muscle groups when examiners followed a standardized landmark protocol.
Which muscles show the highest myotonometry reliability?
Superficial trunk and limb muscles produce the most reproducible readings. Across the published literature on the MyotonPRO (the most-studied myotonometer), the muscles with consistent ICC values above 0.85 for both stiffness and frequency include:
- Upper trapezius
- Biceps brachii
- Rectus femoris
- Gastrocnemius (medial and lateral heads)
- Erector spinae (at the L3-L4 level)
- Tibialis anterior
These muscles share three characteristics: clear surface landmarks, minimal overlying fat or fascia, and a relatively predictable fiber orientation under the measurement site. A 2024 Military Medicine study by Pruyn et al. confirmed that trunk and thigh muscles can be reliably measured in a military physical performance context, supporting the use of myotonometry for athletic and rehabilitation screening.
Which muscles are harder to measure reliably?
Deep muscles, layered muscles, and muscles covered by thick subcutaneous fat are the most challenging. The published reliability data point to a few specific problem cases:
| Muscle | Typical stiffness ICC | Why reliability drops |
|---|---|---|
| Vastus medialis (distal) | 0.50-0.75 | Overlapping vastus medialis obliquus fibers, variable fiber angle |
| Multifidus (lumbar) | 0.60-0.80 | Lies deep to thoracolumbar fascia and erector spinae |
| Gluteus medius | 0.65-0.85 | Variable subcutaneous fat thickness, landmark drift |
| Pectoralis major (sternal) | 0.60-0.80 | Influenced by ribcage, fiber orientation changes |
| Deep cervical flexors | Not reliably measurable | Lie deep to sternocleidomastoid and platysma |
A 2024 reliability study in Frontiers in Sports and Active Living compared myotonometer readings across athletes from football, volleyball, handball, and MMA against untrained controls. Pearson correlations ranged from 0.82-0.88 and ICCs from 0.74-0.99 across most muscles, but the vastus medialis stiffness ICC came in lower than the rest of the thigh, which the authors attributed to fiber-orientation variability.
How does subcutaneous fat affect the reading?
Thicker subcutaneous fat dampens the impulse and adds variability. Myotonometers work by delivering a brief mechanical impulse and measuring the tissue's damped oscillation in response. When the impulse passes through more fat before reaching the muscle, the recorded signal reflects a mixed tissue response rather than pure muscle behavior.
The practical consequence is that absolute stiffness thresholds (e.g., "any reading above X means dysfunction") do not generalize across body compositions. Within-patient change over time is more defensible than comparing one patient to a normative table. This is consistent with the 2025 Journal of Manual & Manipulative Therapy pilot study on reference values, which found that sex was a significant predictor of baseline stiffness and recommended caution when applying single-cohort norms to other populations.
How does the examiner affect reliability?
With a written landmark protocol, inter-rater reliability approaches intra-rater reliability. Without one, examiner variability widens substantially. The 2024 MDPI Medicina systematic review pulled apart the studies that reported high inter-rater ICCs from those that did not. The pattern was consistent: high-reliability studies described the landmark, the patient position, and the probe orientation in detail. Low-reliability studies left those choices to the examiner.
The practical takeaway for clinic use: write down where you measure each patient (anatomic landmark, patient position, side), and stick to it on every follow-up visit. The reliability data assumes you do this. If you skip it, your readings will drift even if the muscle has not changed.
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 many measurements per site should you take?
Three to five averaged measurements per site is the validated protocol. The MyotonPRO default of 5 successive measurements at 1-second intervals is what most of the published reliability data is based on. Reporting a single measurement throws away the reproducibility advantage and inflates session-to-session noise.
If a single reading in the series falls more than 2 standard deviations from the others (often flagged by the device), discard it and re-measure. Outliers usually mean the probe drifted, the patient breathed in mid-measurement, or the contact pressure changed.
How does myotonometry compare to manual palpation reliability?
The reliability gap is large. Manual palpation of stiffness has poor to fair inter-rater agreement; myotonometry of the same muscle is in the excellent range. Across multiple studies of palpation reliability, kappa values for "is this muscle stiff" agreement between examiners typically fall below 0.4. The same muscle measured with a myotonometer typically returns ICCs above 0.85.
This does not mean palpation is useless. It locates tender points, detects trigger points, and informs hands-on treatment in ways an instrument cannot. But for tracking change in tissue mechanics across visits, a reproducible number wins over a clinician's recalled impression.
Frequently Asked Questions
Which muscle groups have the highest myotonometry reliability?
Superficial trunk and limb muscles like the upper trapezius, biceps brachii, rectus femoris, gastrocnemius, and erector spinae consistently show ICCs above 0.85. These muscles are easy to locate, have minimal overlying tissue, and produce highly repeatable readings.
Which muscles are harder to measure reliably with myotonometry?
Deep muscles like the multifidus, the vastus medialis (especially distally), and muscles covered by thick subcutaneous fat tend to show lower reliability. A 2024 Frontiers in Sports and Active Living study found that vastus medialis stiffness in particular showed a lower ICC than other thigh muscles.
Does myotonometry reliability depend more on the muscle or the examiner?
Both matter, but landmark consistency dominates. A 2024 MDPI Medicina systematic review of 48 studies found that with a standardized landmark protocol, inter-rater reliability approached intra-rater reliability for most muscles. Without that protocol, examiner variability widens substantially.
Does subcutaneous fat affect myotonometry readings?
Yes. Thicker subcutaneous fat dampens the impulse and adds variability. Reliability tends to be best in lean populations and lower in patients with higher subcutaneous fat. Within-patient change over time is more defensible than applying universal thresholds across body compositions.
How many measurements should you take per site to get a reliable reading?
Most validated protocols take 3 to 5 measurements per site and average them. The MyotonPRO defaults to a series of 5 measurements with 1-second intervals. Single measurements should not be reported.
Is myotonometry more reliable than manual palpation?
Yes. Manual palpation of stiffness has poor to fair inter-rater reliability, with kappa values often below 0.4. Myotonometry of the same muscles routinely reports ICCs above 0.85 with a standardized landmark protocol.
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.