There are many definitions describing myofascial trigger points (TrP). The most commonly referred to is Travell and Simons (1983, pp.4) definition that a myofascial trigger point is: "A hyperirritable spot, usually within a taut band of skeletal muscle or in the muscles fascia. The spot is painful on compression and can give rise to characteristic referred pain, tenderness, and autonomic phenomena.” Janet Travell M.D. is one of the biggest influences behind finding and researching TrP’s and she helped write  “Myofascial Pain and Dysfunction: The TrP manual, upper half of the body & The TrP manual, lower half of the body”
Other definitions of a TrP
1.        Dringber (1997) - Hyperirritable bundles of fibers within a muscle, which becomes "knotted" and inelastic, unable to contract or relax, due to an injury.
2.        Hong, et al. (1997, pp.957) - Hyperirritable spot in a palpable taut band of skeletal muscle fibers.
3.        Maigne and Maigne (1991, pp.734) - Maximal tenderness, where pressure reproduces actual pain.
4.        Nice, et al. (1992, pp.893) - Hyperirritable area in soft tissue that, when palpated by the examiner, produces a predictable pattern of pain.
5.        Schneider (1995, pp.39) - Hyperirritable spots located within a taut band of skeletal muscle that are painful upon compression and give rise to characteristic referred pain and autonomic phenomena.

TrP facts
There are approximately 696 muscles in the body (347 paired and 2 unpaired). Skeletal muscle accounts for 40% of body weight, and about 85% of human pain complaints. The commonest muscles affected are those in the neck, shoulder girdle, low back, and hip girdle.
A trigger point can shorten and or weaken the muscle. A trigger point usually lies within a taut band of a muscle, which prevents the muscle from achieving full length. Achieving full muscle length is key to the release of a trigger point. Trigger point therapy is actually a theory and not true fact (Most things are just theories). TrP’s is a subset of neuromuscular therapy, which examines and treats muscles and muscle attachments in layers from superficial to deep, ligaments and connective tissues. If you dislike giving deep pressure, this form of treatment might
not work for you. TrP’s are not visible with traditional medical testing such as MRI or X-ray. Some people say that up to 80 % of the trigger point locations are common with acupuncture treatment locations. Trigger point massage is not a relaxing, "fluff and buff" technique.
The voluntary skeletal muscles make up the largest organ of the body and are directly controlled by the nervous system. Any one of the 684 muscles--more than 40% of the body's weight--can develop trigger points that refer pain and distressing symptoms to other body areas.

The “Tools” of Myofascial Trigger Point Therapy:
Thumb:  The thumb is desired over other fingers and body parts unless otherwise stated.  The thumb can be developed as an extremely sensitive tool for palpation, this is due to the number of nerve receptors are located within the thumb.  
Elbow:  Used for large muscles such as erector spinae group and gluteal group.
T-bars: A manual instrument used to apply pressure in lieu of your hands.
•        Large T-bar: Used over the lumbosacral and gluteal regions (i.e.: larger muscle bellies and tendons).
•        Small Beveled T-bar: For use in the lamina groove for intersegmental muscles, as well as for intercostal muscles.  Note: not for use on muscle bellies.
•        L-bar:  Appropriate for a variety of applications including but not limited to deep pressure and transverse friction massage.   
Fingertips: Used for TMJ and smaller muscles of the neck and shoulder.
Pincer-grip: Used on superficial, bulky muscles such as the pectoralis major, latissimus dorsi, sternocleidomastoid, and upper trapezius.

Finding a TrP
You can use your thumbs, all four fingers, one hand, two hands, knuckles or your elbow to find the TrP. If an area of the body cannot handle digital pressure, try pinching the skin or use palmar pressure on the area to desensitize that area so you can eventually use trigger point therapy. Using hard objects to palpate TrP’s is more difficult to feel the release of pain and you need to be aware that it is more likely to bruise the client and that you are using different body mechanics to use pressure.
TrP’s are sometimes easier to locate when muscles are stretched. Most trigger points produce a deep aching pain and are usually bilateral; with one side being more painful (Both sides need to be treated). TrP’s are more commonly found at the sites of the greatest mechanical and postural stress. They may develop in all
associated muscles. If you are treating only the area of pain and the cause is myofascial trigger points, you are in the wrong spot nearly 75% of the time!
To determine if the joint is locked open or closed, the therapist simply flexes and extends the involved area while palpating the hypertonic knot. If the bony knot pushes back when flexion is introduced, the joint is not opening on that side. If the knot pushes back during extension movements, the joint is not closing on the opposite side, forcing the vertebra to rotate back against the therapist's palpating fingers.
The idea with deep stroking massage is to work slowly, about one stroke every two seconds. The strokes need to be only an inch or so long, just enough to move from one side of the trigger point to the other. Rather than sliding your finger across the skin, move the skin with the fingers. Release at the end of the stroke, and then go back to where you started, reset and repeat. Each time you release the pressure fresh blood immediately flows in bringing a renewing charge of oxygen and nutrients. The trigger point has been deprived of these essential substances because the knotted-up muscle fibers have been keeping a stranglehold on the capillaries that supply them.
Although we have been taught to always move the fluid toward the heart, it's not a critical issue with these very short strokes. You can depend on the system to carry the junk away, once you get it squeezed out of the trigger point. In doing self-applied massage, apply the strokes in whatever direction is easiest. If you don't find ways to make self-massage easy, you won't do it.
Referral
Not all TrP’s refer pain some are just localized and the referral patterns are not the same in any 2 people. Referred sensations can include pain, tingling, numbness burning, or itching. Muscles, tendons, and ligaments often refer pain down the arms and legs in ways that are similar to nerve compression injuries.

Without proper therapeutic intervention, the effects of inflammation on damaged soft tissues are as follows:
•        Weaker tissues
•        Less elastic (stiffer) tissues
•        Greater likelihood of periodic exacerbation during periods of increased stress (flare-ups)
•        Varying degrees of residual alterations of functional capacity (does not work like it used to)
•        Chronic pain and disability
•        The tissues are more pain sensitive

Pressure
Pressure application varies in quantity and may start from a few pounds, up to 10 pounds, sometimes more. Have the client use a pain scale (1-10, 10 being the worst) and try not to go above a 7-8. When you are using pressure, make sure you are not using your upper body strength and not tensing up (Your hand will shake if you are using too much pressure).

Appropriate pressure depends on the following factors:
•        The age of the patient.
•        Extent of trauma to the tissues.
•        How long the patient has had the dysfunction.
•        The patient's overall general nutritional and health status.
•        The toxicity of the tissues (traumatic biochemical exudates)
•        Abnormal postural patterns.

Muscles most effected
The postural muscles of the body are most affected by trigger points, which occur in the neck, the scapula, the soft tissue of the thoracic and lumbar spine and the sacroiliac joint.

Possible causes
•        Travell proved that a major cause of myofascial pain is one or more trigger points in the muscle. A trigger point can (a) shorten and or (b) weaken the muscle.
A trigger point usually lies within a taut band of a muscle, which prevents the muscle from achieving full length. Achieving full muscle length is key to the release of a trigger point.
•        Here are some possible causes: B-6 deficiencies, magnesium deficiencies, vitamin C deficiencies, folic acid deficiencies, which are common after injuries or trauma, may cause triggerpoints.
•        Here are some more possible causes: Acute overload, overwork, fatigue, direct trauma, chilling skeletal asymmetry such as short leg or pelvic imbalances may cause trigger points.
•        Still some more: Visceral diseases such as ulcers, renal colic, myocardial infarction, gallstones, kidney problems, and irritable bowel syndrome can cause trigger points.        

Symptoms of TrP’s
Trigger points can present themselves as referred patterns of sensation such as sharp pain, dull ache, tingling, pins and needles, hot or cold, as well as can create symptoms such as nausea, ear ache, equilibrium disturbance, or blurred vision.

More Symptoms of TrP’s
•        Here are some basic signs: Stiffness, Muscle tightness and weakness, Localized sweating, Eye tearing, Copious salivation, Poor balance, Dizziness, Nausea, Goose bumps, Runny nose, Buckling knees, Weak ankles, Illegible handwriting, Occasionally increased perspiration in reference zone, Passive or active stretching increases pain, Staggering gait and Headaches.
•        Other signs are: Client "jumps" when pressure applied to trigger point, Resisted contraction causes pain. Taut palpable band in the affected muscle, Temperature changes-area is usually colder, Muscle cramps & others.

Factors that can worsen TrP’s
Fatigue & improper sleep, Chronic infection, Severe stress (Mental, emotional, physical), Nerve entrapment, compression, Excessive creatine in urine, Postural imbalances, Nutritional health of the tissue, Food allergy & inhalant allergy, Visceral (Organ) disease (Gall bladder problems, ulcers, kidney problems, irritable
bowel syndrome) & Exercise may worsen an active triggerpoint, but helps heal a latent trigger points.In a small proportion of people the TrP remains active long after the original injury. The reason for this is not fully understood, but it appears that a self-perpetuating loop operates making it possible for trigger points to remain active for decades. There is often also a complex interactive between fear of the pain, excessive guarding of the part, and abnormal beliefs about the cause of the pain. Many patients are told that the cause of the pain is due to arthritis, especially when it has gone on for many months after the original injury.

Some trigger point therapists believe that a point is not a trigger point unless it meets the requirements cited by Travell including:
1.        Elicits pain with firm pressure applied.
2.        Has a nodule in a palpable band of taut tissue.
3.        The patient may exhibit jump sign.
4.        Pressure on the trigger point may produce a muscle twitch.