Physiology & etiology of trigger points

The musculoskeletal system develops trigger points in response to a variety of causes. Repetitive muscle flexion, such the use of the upper fibers of the trapezius to elevate the scapulae by persons working at a keyboard all day, is a common example in our culture. Prolonged or inappropriate contraction, sudden trauma, or, paradoxically, unfamiliar use of a muscle are also causes. Finally, any prolonged upset of homeostasis due to disorders of any other system of the body can result in myofascial trigger points. Thus trigger points can appear in the traveler carrying heavy bags for a day, the weekend athlete, the whiplash victim, or especially and most notably in the patient hospitalized for a condition unrelated to musculoskeletal disorders. Even the act of sitting with one's back to a blowing fan for over an hour can set up trigger points in the neck and back muscles.

Previous hypotheses for the development of trigger points focused on cycles in the nervous system, where the initial trauma would produce pain, causing the brain to attempt to splint the muscle, creating a spasm which caused more pain, creating more splinting, etc. However, the theory that pain causes increased muscle contraction has been disproved, and thus ideas such as the "tension headache" may be too generalized.

The current hypothesis theorizes that the activity that causes trigger points is more localized to individual motor nerves and their related muscle segments, resulting in pain, rather than being caused by it. Specifically, a local energy crisis occurs in the muscle fibers caused by a vicious circle beginning at the motor endplate between the nerve and the muscle fiber.

It works like this:
  1. Excess acetylcholine (ACh) is released by a damaged synaptic terminal due to muscle overuse or trauma.
  2. Excessive calcium is released from the sarcoplasmic reticulum, stimulated by the excess ACh.
  3. The calcium causes a sustained contraction of actin and myosin fibers.
  4. Sustained contraction causes local ischemia, resulting in a lack of nutrients to produce ATP.
  5. The calcium pump cannot function without ATP, and calcium continues to be present, causing sustained sarcomere contracture.

In addition, substances that can activate nociceptors are released, causing pain sensations upon palpation, or, in the case of active trigger points, even at rest.

This action is dependent upon myosin and actin filaments being in close enough proximity to "lock" in a contracture. If muscles are used in daily, varying activity and regularly stretched as seen in active persons such as athletes, then these filaments are constantly being separated, and trigger point activity cannot occur. Healthy, well used muscles do not exhibit trigger points. Consequently, therapy always involves an interruption of this myosin-actin connection, through pressure and stretches.

One of the goals of massage therapy is to help clients toward conscious awareness of their muscles, first through massage itself and then with suggestions for home care. It is trauma or unconscious use (other than normal tone) that can lead to trigger point activity. Increased muscle awareness through exercise and flexibility activities such as yoga or active stretching can prevent these small contractures from forming. The job of the massage therapist is to know the feel of healthy muscle tissue, and to be able to return muscles affected by trigger points to their proper tone.

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