SustainedNociceptiveMechanicalStimulationofLatentMyofascialTriggerPointInducesCentralSensitizationinHealthySubjects

Myofascial trigger points (MTrPs) are widely recognized as important peripheral pain generators in musculoskeletal pain conditions. While active MTrPs produce spontaneous pain and are clearly implicated in chronic pain syndromes, latent MTrPs — which are clinically silent but share many physical characteristics with active points — have received less attention regarding their potential role in initiating central nervous system changes. Central sensitization, a key mechanism in chronic pain development, involves amplified pain processing in the spinal cord and brain that leads to widespread pain hypersensitivity. Previous research had established that MTrPs are associated with central sensitization in conditions like fibromyalgia and whiplash, and that inactivating active MTrPs can partially reverse this sensitization.

However, a critical unanswered question remained: whether MTrPs themselves can actually initiate central sensitization, rather than merely maintain it. This distinction matters enormously for prevention — if latent MTrPs can trigger central sensitization when activated, then early intervention before points become active could potentially prevent chronic pain development.

The researchers conducted a carefully designed laboratory experiment with 12 healthy volunteers (8 men, 4 women, average age 27 years) who had no current pain problems but were confirmed to have latent MTrPs in their finger extensor muscles. Each participant completed two separate sessions, at least 3 days apart, in randomized order. In one session, an intramuscular EMG needle was inserted and retained for 8 minutes in a latent MTrP; in the other session, the same procedure was performed at a non-MTrP control site in the opposite forearm. This within-subjects design allowed each person to serve as their own control.

During the 8-minute needle retention period, participants continuously rated their pain intensity on a 10-cm electronic visual analog scale. The researchers also recorded both surface and intramuscular EMG to detect muscle cramps, defined as episodes of significantly increased electrical activity accompanied by increased pain and visible muscle contraction. Referred pain patterns were mapped at the end of each session.

Critically, to test for widespread central sensitization, pressure pain threshold (PPT) was measured at the tibialis anterior muscle on the opposite leg — well outside the spinal segment of the forearm — before stimulation, immediately after, and at 10, 20, and 30 minutes post-stimulation. A decrease in PPT at this distant site would indicate extrasegmental mechanical hyperalgesia, a hallmark of central sensitization.

The results strongly supported the researchers' hypothesis. During sustained nociceptive mechanical stimulation, latent MTrPs produced significantly higher maximal pain intensity (P < .002) and greater total pain as measured by area under the VAS curve (P = .02) compared to non-MTrP sites. Mean pain intensity over time was also significantly higher at latent MTrPs (P < .001), with particularly elevated pain at 2 and 3 minutes post-insertion.

Referred pain was induced in 8 of 12 subjects following latent MTrP stimulation but in no subjects following non-MTrP stimulation. The referred pain typically radiated to adjacent areas of the forearm, elbow, palm, or wrist.

Most importantly for the central sensitization question, PPT in the contralateral tibialis anterior muscle showed significant changes over time specifically after latent MTrP stimulation (P < .001), with significant interaction between time and group (P < .001). Post-hoc analysis revealed that PPT values at 10, 20, and 30 minutes post-stimulation were significantly lower than baseline and immediate post-stimulation values (all P < .05), and were significantly lower than corresponding values in the non-MTrP group (all P < .05). This demonstrates that brief noxious stimulation of a latent MTrP produced widespread mechanical hyperalgesia lasting at least 30 minutes.

Muscle cramps occurred in 10 of 12 subjects during latent MTrP stimulation but in no subjects during non-MTrP stimulation. The total time spent in muscle cramps was positively correlated with both referred pain area (r = .60, P = .03) and total pain (VASauc, r = .72, P = .009), suggesting cramps contributed to pain generation.

This study provides the first direct experimental evidence that latent MTrPs can initiate central sensitization when subjected to sustained noxious input. The finding that just 8 minutes of mechanical stimulation at a single latent MTrP produced measurable widespread hyperalgesia in a distant muscle has significant clinical implications. It suggests that latent MTrPs are not merely dormant precursors to active points, but represent genuine neurological vulnerabilities that can trigger central nervous system changes when activated by injury, overuse, or other stressors.

The role of muscle cramps identified in this study is particularly noteworthy. The strong correlations between cramp duration and both local and referred pain suggest that the motor dysfunction associated with MTrPs — specifically the heightened excitability that produces cramps — may be integral to their pain-generating capacity rather than merely a secondary phenomenon. This supports therapeutic approaches that address motor unit hyperexcitability, not just nociceptive input.

For patients, this research helps explain why a seemingly minor muscle strain or period of overuse can sometimes trigger widespread, persistent pain problems. The latent MTrPs many people unknowingly harbor may serve as neurological "kindling" that, when activated, ignites broader pain processing changes. This reinforces the value of preventive approaches — including appropriate exercise, posture correction, and early manual therapy — that might keep latent points from becoming activated.

For clinicians, the findings support aggressive early intervention when latent MTrPs are identified, even in patients not currently reporting pain. The study also suggests that treatments targeting muscle cramp prevention and motor unit stabilization may be particularly valuable. The authors note that therapeutic methods decreasing MTrP sensitivity and motor-unit excitability, including dry needling, lidocaine injection, and certain electrostimulation approaches, may help prevent the transition from latent to active MTrPs and thereby block a key pathway to chronic pain development.