Latent Trigger Point ResearchLatent myofascial trigger points: their effects on muscle activation and movement efficiency
Introduction
Many clinicians claim that the body can be viewed as a series of segments that link together to form a kinetic chain. This chain then acts to transfer force and energy in a coordinated manner in order to produce a movement outcome at the most distal segment of the chain. Kibler (1998) suggested that where there is a deficiency in a proximal segment of this kinetic chain, changed workloads may be required in more distal segments in order to preserve the same movement outcome at the most distal segment. If this is the case, patients presenting with overuse or overload injuries of the limbs may also be experiencing dysfunction in more proximal segments.
Kibler (1998) proceeds to speak specifically about the upper limb. He suggests that the scapula and the muscles that attach it to the vertebrae and ribs, comprising the trapezius, serratus anterior (SA), rhomboids, levator scapulae and pectoralis minor, serve as the segment that links the trunk to the upper limb. In order for the scapula to be positioned effectively to perform its role in transfer of force from the trunk to the upper limb, the scapular positioning muscles must be recruited in the optimal muscle activation pattern (MAP). Therefore, motor control of this muscle group becomes important for successful upper limb function.
Wadsworth and Bullock-Saxton (1997) performed experiments with young elite male swimmers with unilateral chronic shoulder impingement syndrome. They found that chronic shoulder pain was associated with altered timing of muscle activation in the upward scapular rotator muscles. They did not, however, have the opportunity to establish a cause and effect relationship between these two variables. That is, did the presence of the painful joint condition cause the scapular rotator muscles to be activated differently from swimmers with no shoulder pain or does the activation sequence of the scapular rotator muscles alter first, causing a biomechanical change that leads to shoulder joint overload and eventually pain, in alignment with Kibler's theory?
The presence of a painful condition could account for any changes in MAPs during a movement test. In order to investigate Kibler's theory, it was necessary to find pain-free subjects, with some sort of deficiency of function in a proximal segment of the kinetic chain.
Latent myofascial trigger points (LTrPs) are pain-free neuromuscular lesions that are associated with muscle overload and decreased contractile efficiency (Simons et al., 1999, p. 12). There is also some evidence (Lucas et al., 2001; Simons et al., 1999, p. 12) that these lesions are prevalent in the community, rendering LTrPs as relevant lesions to investigate, as understanding their potential effects would be useful for many community members.
Accordingly, our research questions became:
- 1.
Do LTrPs in the scapular rotator muscles alter the timing of muscle activation in this muscle group?
- 2.
What is the effect on the timing of activation of muscles more distal in the kinetic chain of the upper limb?
Section snippets
Subjects
After gaining approval from the University Human Research Ethics Committee, 154 pain-free university staff and students volunteered to be examined for ‘normal’ shoulder girdles and subsequently, the presence of LTrPs in their scapular rotator muscles bilaterally. Subjects (n=112) were excluded from the study if they did not have 160° of arm elevation, had a positive apprehension test (glenohumeral instability), upper limb tension test (neurological dysfunction), significantly increased thoracic
Results
Group data are shown in Fig. 5. The group mean activation times and SD of activation times are shown for each muscle. The control group displays a relatively stable, sequential MAP where the UT is always activated first, on average 115 ms prior to movement start. Immediately after the arm leaves the side of the body, the Inf (mean=75 ms) and the MD (mean=201 ms) are activated. The SA and LT were activated 433 ms and 776 ms after movement start, respectively, and displayed more variability in
Discussion
The control group displayed a relatively stable and sequential MAP. In this group, the UT is consistently activated before movement start. As activity was measured in the UT, but the arm had not moved from the side of the body, perhaps the UT is tensioned prior to movement start in order to create subacromial space in which the humeral head can rotate.
In the control group, the Inf was activated 75 ms after movement start on average. This seems reasonable, considering that the primary role of the
Clinical significance
MTrPs are not just contracted muscle fibres but neuromuscular lesions that form part of a neurological loop that affects and is affected by the CNS. This is evidenced by the fact that removing LTrPs normalises the MAP.
The presence of LTrPs in the scapular rotator muscles is associated with changes in motor control prior to the presence of pain. The changes described above may predispose individuals to increased risk of subacromial impingement, overuse of the inf and decreased efficiency of
Conclusions
LTrPs in the scapular rotator muscles do alter the timing and decrease the consistency of the MAP of this muscle group and muscles more distal in the upper limb chain. These findings occurred in the absence of pain and may have implications in the areas of shoulder impingement syndrome, rotator cuff overuse and in training optimal movement efficiency of the upper limb.
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