In the current study, the origin (assumed A-TrP site) of the brachioradialis muscle was more irritable than the middle of the muscle belly (endplate zone, assumed MTrP site), and the muscle-tendon junction of the control site. This occurred in children of both genders 4 years and older. Therefore, we have assumed that children may develop latent MTrP at the belly of the brachioradialis muscle at 4 years old. However, the exact timing of the development of the latent MTrP is still unclear and requires further investigation. The timing of development of a latent MTrP may differ from muscle to muscle.
In a previous similar study on the brachioradialis muscles of 60 newborns comparing to 60 healthy adults, Kao et al [19
] found no significant differences in mean PPT values measured at site A, site B, and site C in infants, but the mean PPT values at site B was significantly lower than that in site A or site C.
The irritability of all sites in the brachioradialis muscle tended to decrease gradually with age until 9 years old. Thus, children younger than 5 years old have a very low pain threshold (Table ), but this pain threshold gradually increases as they grow up. The pain threshold reaches a plateau at 9 years old. However, the pain threshold (mean PPT) at the endplate zone of muscle belly (assumed MTrP region) of the brachioradialis muscle in children aged 9-11 years is still much lower than that in adults [19
]. In an earlier study on the same muscle, brachioradialis, Kao et al found that the mean PPT values at the site B (assumed MTrP region) was 2.54 ± 0.87 kg/cm2
in the right side and 2.87 ± 0.52 kg/cm2
in the left side of brachial radialis in 60 normal adults, but much lower in the 60 infants [19
]. However, whether any deviation resulted from variations due to different measurers remains unclear. A survey of these age groups after 11 years old is necessary to determine when the PPT values approximate adult levels.
A child may have already developed an A-TrP at the origin of the brachioradialis muscle at 4 years old, although the exact timing of the development of the A-TrP is unclear. For each age group of children, the A-TrP of the brachioradialis muscle is more irritable than the corresponding MTrP. The tendon attachment of a muscle may likely be still immature (not strong enough) during childhood so that it suffers injuries easily. However, analysis of the gender differences reveal no significant differences between the genders in terms of the mean pain thresholds at the three measured sites even though boys are generally more active than girls in Oriental culture with respect to daily activities. In a similar study by Kao et al on an adult population, the MTrP of the brachioradialis muscle was determined to be more irritable than the A-TrP [19
]. The timing of this change in muscle irritability remains unclear. Further studies are required to clarify these questions.
Dommerholt thoughtfully discussed the issue regarding the concept of A-TrP [30
]. Simons proposed the concept of the A-TrP to explain pain at the muscle-tendon junction in people with MTrPs, based on the assumption that taut bands would generate sufficient sustained force to create localized enthesopathies. However, to date, no convincing evidence has proven that the tension generated in shortened sarcomeres in a muscle belly could generate passive or resting force throughout the entire taut band [30
]. On the contrary, the force generated by individual motor units is always transmitted laterally to the muscle's connective tissue matrix [31
]. Dommerholt also argues that there is considerable evidence to suggest that the assumption that muscle fibers pass from tendon to tendon is without basis, concluding that the development of the so-called "attachment trigger points" as a result of increased tension by contracted sarcomeres in MTrPs is not clear, and more research is needed to explain the clinical observation that MTrPs appear to be linked to pain at the muscle-tendon junction [30
]. However, the term "A-TrP" is still used even if the mechanism of hyperirritability is unclear because the site is the tendon "attachment" region and it is definitely a hyperirritable spot.
Based on clinical and basic studies on MTrPs, the pathophysiology of MTrP has been much clarified [1
]. Hong and Simons hypothesized that there are multiple MTrP loci in an MTrP region [7
]. The sensory component of the MTrP locus is the sensitive locus or local twitch response (LTR) locus [2
] at which pain, referred pain, and local twitch responses can be elicited. Meanwhile, the motor component is the "active locus" [37
], from which spontaneous electrical activity(SEA)(mainly endplate noise [EPN]) can be electromyographically recorded. This was later defined as the "SEA locus" [7
] or "EPN locus" [38
]. An LTR locus is a sensitized nociceptor (free nerve ending) [27
] and an EPN locus is a dysfunctional endplate with excessive release of acetylcholine (ACh) quanta [33
]. An SEA locus is in close proximity to an LTR locus, and both interact to form the taut band and to facilitate MTrP formation [1
]. Excessive leakage of ACh molecules (not simultaneously, so that EPN can be recorded) can cause the focal contraction of sarcomeres in the endplate zone to form a contraction knot as a taut band, which has been demonstrated in several morphological studies [43
]. Impaired circulation and increased energy consumption in the contraction knot (trigger region) can cause a vicious "energy crisis" cycle [49
]. This phenomenon in the MTrP has been further supported by recent biochemical studies that demonstrated a high concentration of inflammation or pain related substances in active MTrP regions [50
]. Based on the above findings, Simons suggested an "integrated hypothesis for MTrP," that considers excessive ACh release, sarcomere shortening, and the release of sensitizing substances as the essential features of MTrPs [1
]. Tissue ischemia and hypoxia in the contraction knot may induce the secretion of sensitizing substances that cause pain. The sensitizing substances can further cause abnormal ACh release, which activates a vicious cycle. These three essential features relate to one another in a positive feedback cycle that is self-perpetuating once started [1
]. With effective MTrP therapy, this vicious cycle can be interrupted at several points in the cycle. However, researchers are still uncertain whether an "abnormal ACh release" initially occurs to sensitize the nociceptors via peripheral sensitization or whether an "inflammatory reaction" initially causes the release of inflammatory and pain substances, and then induces abnormal ACh release. In fact, the findings by Shah [50
] support either hypothesis because the inflammation reaction can also be elicited by the muscle ischemia in the contracture knot.
Gunn considered the neuropathic lesion to be the primary mechanism of MTrP formation because EPN can be recorded in the MTrP region and facet joint lesions can activate MTrPs [52
]. Hong agreed with this hypothesis for latent MTrP formation, but not for the process of latent MTrP activation [53
]. The formation of a latent MTrP may be due to minor radiculopathy from minor repetitive stress to the spine while a baby is growing up. Minor radiculopathy may cause excessive ACh secretion at the neuromuscular junction, subsequently inducing electrotonic potentials (endplate noise) in the neuromuscular junction to cause the focal contraction of sarcomeres in the endplate zone (contracture knot).
Partanen et al suggested a different mechanism of MTrP formation, and concluded that MTrPs are related to painful muscle spindles in taut bands [54
]. However, this hypothesis cannot explain the focal shortening of sarcomeres that occurs only in the endplate zone (contraction knot) [45
Repetitive activity of a muscle may or may not cause MTrP formation. In a previous study, the PPT of common finger extensors can be reduced after piano practice, i.e., with repetitive finger activity [16
]. A recent study demonstrated that an MTrP (a sensitive spot in a palpable taut band with reduced pressure pain threshold) of the extensor digitorum muscle could be induced by the repeated eccentric exercise of that muscle [55
]. Unfortunately, in our study, no correlation was observed between the activity level of the students and their mean PPT values, neither when the differences in PPT of MTrP between dominant side and non-dominant side nor when the differences between "active" and "normal" children were compared. In the study by Kao [19
], the PPT of MTrP between different sides had no significant difference. Although the dominant upper limb conducts more activity than the non-dominant one, the majority of daily activity requires the involvement of both sides.
Regarding the activity levels of children in the current study, the extra activity appears to be inadequate to change the irritability of an MTrP in the brachioradialis muscle. This is probably due to the very active lifestyle of children. Another possibility is that different types of sports may provide different levels of influence. We attempted to compare the difference in PPT of MTrP for children involved in different types of sports, but no obvious correlations were found. Further studies on different muscles across different age groups are required to clarify these questions.
The brachioradialis muscle was selected for this study because the PPT measurement of this muscle is relatively easy. In fact, many other muscles contract more frequently than the brachioradialis in daily living activities.
Only age groups from 4 to 11 years were involved in this study. The sample size for the 4 years old age group was smaller than those of the other groups because children below 5 years old are usually unwilling to participate in this type of study and are also less cooperative. Initially, we attempted to approach 12-year-old children, but for some unknown reasons, recruiting such children was difficult. One possible reason is that they are in their last year at elementary school and are involved in academic examinations, and another is that they are just beginning to be teenagers and therefore lack interest.
Another problem in this study is the reliability of the assessment of pain threshold in children. They might respond to pressure compression inconsistently. Consequently, the standard deviation of PPT is relatively large.
Further studies that include all possible age groups, larger sample sizes, and the use of a reliable measuring tool are necessary to conduct a complete survey.