Classification of Trigger Points
Trigger points are described in various ways according to location, tenderness and chronicity: central (or primary); satellite (or secondary); attachment; diffuse; inactive (or latent); and active. Central (or primary) trigger points are the most well established and ‘florid’ when they are active, and are usually what people refer to when they talk about trigger points. The central trigger points always exist in the centre of the muscle belly; where the motor end plate enters the muscle. N.B. Muscle shape and fibre arrangement is of importance in this regard. For example, in multipennate muscles, there may be several central points. Also, if muscle fibres run diagonally, this may lead to variations in trigger point location.
Satellite (or secondary) trigger points may be ‘created’ as a response to the central trigger point in neighbouring muscles that lie within the referred pain zone. In such cases, the primary trigger point is still the key to therapeutic intervention and the satellite trigger points often resolve once the primary point has been effectively rendered inactive. The corollary is also true in that satellite points may prove resilient to treatment until the primary central focus is weakened; such is often the case in the para-spinal and/or abdominal muscles.
It has been noted that the area where the tendon inserts into the bone (tendino-osseous) is often ‘exquisitely’ tender. (Travell & Simons, 1999; Davies, 2004). This may well be the result of the existing forces travelling across these regions. It has been also suggested that this may result from an associated chronic, active myofascial trigger point. This is because the tenderness has been demonstrated to reduce once the primary central trigger point has been treated; in such cases, the point is described as an attachment trigger point.
Furthermore, it has been suggested that if a chronic situation occurs where the primary and attachment trigger points remain untreated, ‘degenerative changes’ within the joint may be precipitated and accelerated. (Travell & Simons, 1999). Diffuse trigger points can sometimes occur where multiple satellite trigger points exist secondary to multiple central trigger points. This is often the case when there is a severe postural deformity such as a scoliosis, and an entire quadrant of the body is involved. In this scenario, the secondary points are said to be diffuse. These diffuse trigger points often develop along lines of altered stress and/or strain patterns.
Inactive (or latent) trigger points apply to lumps and nodules that feel like trigger points. These can develop anywhere in the body; and are often secondary. However, these trigger points are not painful, and do not elicit a referred pain pathway. The presence of inactive trigger points within muscles may lead to increased muscular stiffness. It has been suggested that these points are more common in those who live a sedentary lifestyle. (Starlanyl, 2000).
It is worth noting that these points may re-activate if the central or primary trigger point is (re-)stimulated, or following trauma and injury. Active trigger points can apply to central and satellite trigger points. A variety of stimulants can activate an in-active trigger point such as forcing muscular activity through pain. This situation is common when increasing activity post road traffic accident (RTA), where multiple and diffuse trigger points may have developed. The term denotes that the trigger point is both tender to palpation and elicits a referred pain pattern.