What is actually inside a trigger point? What imaging and biochemistry studies show

The trigger point started life as a purely clinical object: a tender spot in a taut band that reproduces a patient's familiar pain when compressed, described in the injection literature of the 1940s and formalised decades later as the integrated hypothesis — sustained low-level overload, excessive acetylcholine release at the endplate, a locally contracted sarcomere segment, ischaemia and sensitisation (Travell 1942; Simons 2004; Gerwin 2004).

That model made testable predictions: something should be locally stiffer, locally hypoxic, and locally rich in pain-related chemistry. Two lines of research went looking.

Using grey-scale imaging, elastography and Doppler on the upper trapezius, investigators reported that palpable trigger points corresponded to focal hypoechoic areas that were stiffer than adjacent muscle, with altered blood-flow waveforms in the immediate vicinity (Sikdar 2009). In other words, the spot your thumb finds is not purely a perceptual artefact: at least in an accessible muscle like the trapezius, there is a small region with different mechanical and vascular properties.

The limits matter as much as the finding. These are small samples in one or two superficial muscles, imaging is operator-dependent, and no threshold exists that lets a scan diagnose a trigger point on its own. Ultrasound supports the idea of a local lesion; it does not yet replace the examining hand.

A microdialysis needle small enough to sample interstitial fluid in living muscle allowed direct measurement of the local milieu (Shah 2005). At active trigger points in the upper trapezius, pH was lower and substance P, CGRP, bradykinin, serotonin, noradrenaline and several cytokines were elevated compared with latent points and normal muscle — and, importantly, some of these changes were also detectable in remote, non-tender muscle in the same people (Shah 2008).

Two practical readings follow. First, an active point is a chemically sensitised environment, which fits why sustained pressure, needling or movement that restores circulation can change symptoms quickly. Second, myofascial pain is not strictly local: a single tender spot sits inside a wider sensitised system, which is why treating one point in isolation often disappoints.

The weakest link is not the biology but the examination. Systematic reviews of manual palpation find that agreement between examiners on where a trigger point is, and on signs such as the local twitch response or the jump sign, ranges from poor to moderate depending on muscle, training and the criteria used (Lucas 2009; Rathbone 2017). Critics use exactly this to argue that the construct is unreliable and that the findings above can be explained by generalised sensitisation rather than a discrete lesion (Quintner 2015).

A fair summary in 2026: the tender spot and its referred pattern are real, useful and reproducible enough to guide treatment; the mechanism is partly mapped and probably mixed, peripheral and central; and the single most reliable clinical criterion remains reproduction of the patient's own pain, not the feel of a nodule.

Stop chasing nodules and start chasing patterns. Palpate the muscles that refer into your symptom area — for neck stiffness that means upper trapezius and levator scapulae, for deep shoulder pain infraspinatus, for jaw and temple pain masseter, for one-sided low back pain quadratus lumborum and gluteus medius — and keep only the points that reproduce your familiar pain.

Then treat the whole loop rather than the spot: moderate sustained pressure or needling to reduce local sensitisation, movement through full range to restore circulation, and correction of the sustained low-level load that keeps feeding it. The biochemistry explains why the last of those three is the one that decides whether the change lasts.