Reprinted with the kind permission of Celeste Cooper 
Pain generators of things like headache, pelvic muscle and organ dysfunction, spinal disease, joint dysfunction, TMJ, and much more can often be attributed to the presence of myofascial trigger points in over-stressed or injured muscles. So what are they, how are they classified, and why are the stages of a trigger point (TrP) important?
WHAT IS A MYOFASCIAL TRIGGER POINT?
A myofascial trigger point (MTrP) is a “self-sustaining” hyper-irritable area of muscle fiber in a taut band of muscle that is felt as a nodule or bump. This aggravated spot causes gradual shortening of the muscle involved and interferes with normal muscle function. Myofascial TrPs can also put pressure on the surrounding nerves, lymph and blood vessels, causing other symptoms in addition to pain.
MTrP = myofascial trigger point
TrP = trigger point
Trigger points can usually be felt unless the muscle is too tight, they are buried under other large muscles or they are located behind bone. They radiate pain in a consistent pattern according to their location in the muscle affected and hidden TrPs are often located by their pain referral pattern. Additionally, TrP/s can be well away from the pain referral zone.
National Association of Myofascial Trigger Point Therapists handy SYMPTOM CHECKER 
It’s important to know what a trigger point feels like, so get familiar with your body. Feel around to see if you find any lumps or bumps that reproduce your pain. In average size muscles, TrPs are about the size of a pea and there can certainly be more than one. If a muscle is too tight, massage may be necessary to relax the muscle so we can isolate the TrP. Muscles around the face are very tiny. In this case, they may feel like a tight string, while larger muscles feel like a tight rope.
Acute myofascial pain from trigger points differs from chronic myofascial pain. You can read more in our book, Integrative Therapies for Fibromyalgia, Chronic Fatigue Syndrome, and Myofascial Pain  and in this article, What You Should Know About Chronic Myofascial Pain .
Trigger points that are adequately stimulated by compression or needle insertion exhibit a visible local twitch response. However, Perreault T, Dunning J, and Butts R, 2017, report  “several studies show that eliciting a local twitch response does not correlate with changes in pain and disability, and multiple systematic reviews have failed to conclude whether the twitch response is relevant to the to the outcome of TrP dry needling.”
Note: The size of the muscle involved, whether it is an active primary or active secondary TrP is not relative to pain intensity. Sometimes, TrPs in small muscles can cause more pain than those found in larger muscles. Such is the case of TrPs in the piriformis muscle, which is a small deep muscle in the buttocks that cradles the sciatic nerve. Trigger points in the piriformis muscle mimic sciatica pain which is created by compression of the sciatic nerve as it exits the spine. The treatments are very different.
CLASSIFICATIONS OF TRIGGER POINTS
The three familiar classifications of TrPs are primary, secondary, and/or satellite. Knowing the classification of a TrP is important to the person providing treatment, including us.
A primary TrP is the one that starts the painful event. A primary TrP can be active or latent. (More coming on what that means.)
A secondary trigger point is born when compensatory muscles, on the same side of the body, the opposite side, or even in the same group, also become overloaded in an attempt to temporarily help muscles that are affected by primary TrPs.
As we sit at a computer our neck bends as our head drifts forward from its weight. Before long we find ourselves rubbing the back of our neck. Our uncorrected posture has put undue stress on muscles that are trying to lift our head to a neutral anatomical position. We are in pain because muscle fibers have abnormally shortened in an effort to keep our face from smashing into the keyboard. You can imagine how much work is placed on ancillary muscles that have also been pulled out of their normal functioning position. For me, not continually adjusting my posture or taking frequent breaks will trigger a migraine attack because of the presence of occipital neuralgia and the pressure on my occipital nerve. That leads to a predictable cascade of events. My nose begins to run on the right side, my eyelid droops, my right eye crosses, and more.
After note: Muscles on the front, back, and sides of our neck not only hold up our head, they also contribute to other muscles that support function of things like our jaw, shoulders, scapula, and so on. The opportunity for secondary TrPs to develop is extensive.
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Secondary TrPs can also be active or latent.
Trigger point charts  you can download
A satellite trigger point is a type of secondary TrP that is located in the referral zone of the primary trigger point.
STAGES OF TRIGGER POINTS
Active Trigger Point
Active TrPs hurt without being touched, radiate pain, restrict motion, and can cause other symptoms depending on the surrounding nerves, lymph system, and blood vessels. They call for our attention.
Latent Trigger Point
A latent trigger is a dormant TrP that causes pain only when it is compressed or manipulated. Latent TrPs do all the same things as active TrPs and cause stiffness and weakness that persist for years after apparent recovery. They can go unnoticed only to be reactivated from a seemingly minor overstretching, repetitive use, overuse, or injury. In some people, a harmless event can cause dormant TrPs to light up like a firecracker instigating a cascade of events including referral of pain, and other symptoms.
WHAT IS A TRIGGER POINT?
My interview with myofascial trigger point specialist, Frank Gresham
Myofascial trigger points restrict muscle movement and cause weakness that can persist for years after apparent recovery, so don’t be lured into thinking you can only treat active TrPs. A latent primary TrP can remain dormant wreaking havoc on our musculoskeletal system and unless it is also treated, it will remain in the same state. This means secondary TrPs will redevelop and present in the active stage (causing pain without pressing on it), leaving us to think treatment doesn’t work. Have patience, it’s difficult to define a primary from a secondary trigger point, even for experts and especially when multiple muscles are dysfunctional. But, with the help of a professional trained in myofascial medicine, such as physician, physical therapist, chiropractor and/or myofascial therapist , we can conquer these unruly pieces of muscle fiber that cause us so much pain.
Coming soon: Prevention and treatment
Celeste Cooper, RN
Think adversity? See opportunity!
Celeste Cooper, RN, is a frequent contributor to ProHealth. She is an advocate, writer and published author, and a person living with chronic pain. Celeste is lead author of Integrative Therapies for Fibromyalgia, Chronic Fatigue Syndrome, and Myofascial Pain and Broken Body, Wounded Spirit, and Balancing the See Saw of Chronic Pain (a four-book series). She spends her time enjoying her family and the rewards she receives from interacting with nature through her writing and photography. You can learn more about Celeste’s writing, advocacy work, helpful tips, and social network connections at CelesteCooper.com .