Myofascialtriggerpoint(MTrP)sizeandelasticitypropertiescanbeusedtodifferentiatecharacteristicsofMTrPsinlowerbackskeletalmuscle

Myofascial trigger points (MTrPs) are painful contraction knots that develop in muscles after overuse or trauma, and they are a major cause of myofascial pain syndrome (MPS). It is estimated that 30–85% of patients visiting primary care or pain clinics suffer from MPS. Despite this high prevalence, effective non-invasive diagnostic tools for MTrPs remain limited. Current diagnosis relies heavily on clinician palpation skills and patient self-report, which are subjective and require significant experience.

Many primary care physicians lack the training to identify MTrPs properly, leading to potential undertreatment of patients. The lack of objective measurement tools also hinders the development of training protocols for providers and educational materials for patients. This study addresses this critical gap by developing ultrasound-based diagnostic metrics that can quantitatively distinguish the biophysical properties of MTrPs in lower back muscles, one of the most commonly affected body regions.

The researchers recruited 25 participants with non-specific low back pain from two university campuses using snowball sampling. An osteopathic physician examined each participant in a prone position, using physical examination and palpation to identify MTrPs in the low back muscles between the L1 and S1 vertebrae. The physician then randomly selected three MTrPs per participant for ultrasound evaluation using a Sonosite Edge II system. For each MTrP, two ultrasound images were captured: one without applied force and one after applying approximately 4.5 N of weight (about 1 pound).

The researchers measured MTrP depth, transverse height (thickness), and longitudinal length, then calculated a stiffness parameter defined as the ratio of MTrP height with pressure to height without pressure. They categorized MTrPs into four groups based on patient-reported pain and observed twitch response: Category 1 (pain and twitch), Category 2 (no pain but twitch), Category 3 (pain but no twitch), and Category 4 (neither pain nor twitch). Statistical analyses included paired t-tests, repeated-measures t-tests, and chi-squared analysis.

The study analyzed 60 single MTrPs and 14 images containing two MTrPs (after excluding one outlier with three MTrPs). Under applied pressure, MTrP circularity ratio decreased by approximately 22% (from 0.54 ± 0.027 to 0.42 ± 0.03; P = 0.0023). The researchers found that MTrP stiffness increased with depth in muscle tissue, following Saint-Venant's principle where force dissipates quadratically with depth. Approximately 70% of MTrPs occurred in the top 25% of muscle depth.

Patients with reported pain had MTrPs with higher stiffness (0.73 ± 0.03) compared to those without pain (0.67 ± 0.03). Average MTrP size was 0.6 ± 0.03 cm, and stiffness decreased exponentially with increasing MTrP size up to 0.6 cm, after which it remained constant. For patients reporting pain, MTrPs were approximately 15 to 20% stiffer than those from patients without pain, for the same fraction of muscle tissue. When two MTrPs were present within 1 cm of each other, each individual MTrP exhibited greater stiffness than single MTrPs alone—strain was 0.20 ± 0.03 for two MTrPs versus 0.29 ± 0.02 for single MTrPs.

Stiffness also decreased exponentially with longitudinal distance from applied force, dropping 25% for every 0.25 cm away, with all force lost beyond 1 cm. For clinical diagnosis, Group 1 MTrPs (pain + twitch) tended to be simultaneously deeper, smaller, and stiffer than Group 2 MTrPs (no pain + twitch), with depth-corrected strain < 0.2 for Group 1 versus > 0.2 for Group 2.

This study establishes that ultrasound-based measurements of MTrP depth, thickness, and stiffness can serve as objective diagnostic tools to supplement traditional clinical assessment. Clinicians can use these three combined metrics to differentiate between MTrP types during diagnosis and potentially track treatment progress. The finding that MTrPs in patients with pain are stiffer and less elastic than those without pain provides a quantifiable biomarker that does not rely solely on patient self-report. The discovery that nearby MTrPs influence each other's elastic properties suggests that clinicians should consider the local MTrP environment when planning interventions.

The rapid loss of effective force with distance from the MTrP (all force lost beyond 1 cm) has important implications for force-based treatments like manual therapy, dry needling, or trigger point injections—precision in targeting the MTrP location is critical for effectiveness. The depth-dependent findings explain why deeper MTrPs may be harder to treat with surface-applied force and why some MTrPs may not produce a twitch response during palpation. For patients, this research moves toward more objective, reliable diagnosis of their condition rather than depending on the variable skill of individual clinicians. Future studies should establish how these same metrics change after successful treatment, which would enable clinicians to objectively verify treatment effectiveness.