Although there are only a few reliable tests for shoulder pain, it is determined in association with the subacromial bursa, rotator cuff tendon, and tendon of the long head of biceps muscle, which are the anatomical structures of the subacromial space (28–30). This assumption does not take into account the fact that muscle tissue may induce pain in the shoulder area (31).
A recent study on the referral of muscle pain is based on the fact that the synaptic connection of central dorsal horn neurons may be altered as a result of nociceptive input (Mense, Simons, Hoheisel, & Quenzer, 2003).
Dorsal horn neurons have an effective and ineffective synaptic connection including afferent neurons. An effective synapse accepts information regarding trigger points and forms an existing receptive field. Local pain is moderated through this pathway. An ineffective synapse fires an insufficient potential to induce a response in the dorsal horn neuron. However, when an ineffective synapse is placed in a pathological environment, neurons stimulated by nociceptive input create a new receptive field and it is converted into an effective synapse, a process known as central sensitization. When a new receptive field emerges, non-nociceptive input in a location other than the existing location of pain can be felt as pain (32).
Trigger points of the rotator cuff may provoke local referral pain deep in the shoulder joints. As a result, shoulder pain caused by trigger points may be misunderstood as subarcomial buritis or tendinitis, and consequent inflammation-related treatment may diminish the efficacy of treatment (22). Therefore, direct treatment of MTrPs of the infraspinatus muscle can be an alternative for shoulder pain.
The ICT applied in this study reduced VAS by 49.36% and improved PPT by 8.07%. LLLT reduced VAS for shoulder pain by 38.80% and improved PPT by 6.42%. These results suggest that both treatments alleviate shoulder pain.
Carel Bron et al. (2011) applied manual pressure, passive muscle stretching, and cold application during stretching on trigger points of shoulder muscles (22). The results showed that VAS and K-DASH scores were significantly improved, and the number of active trigger points decreased. Carrasco (2009) reported that LLLT on trigger points of the masseter and temporalis was effective in reducing temporomandibular joint pain (33). Although the type of treatment used and muscle involved differed between our study and previous studies, one similarity is that the studies aimed to identify pain by treating trigger points.
Although the exact mechanism underlying the pain-reducing effects observed in this study is yet unclear, it can be understood through various hypotheses. A recent study reported that pain mechanism is related to glial cells. Microglia and astrocytes have been reported to be activated by peripheral pathological changes, including inflammation (34). Data on whether trigger points generally impact glial cells are scarce. However, because pain receptors activate glial cells, pain receptors in trigger points may have an impact on glial cells (35). During an ICT, compression induces a momentary ischemic state in the trigger points and once the compression is removed, reactive hyperemia occurs, in which increased blood flow to the muscle fibers facilitates circulation (36). LLLT controls microcirculation and increases oxygen supply to the trigger points, thereby normalizing the metabolic rate of tissues (37). The enhanced blood circulation as a result of these treatments is speculated to impact the pain receptors in the trigger points by reducing inflammation in the body and thus affecting glial cells.
In the present study, we assessed shoulder ROM (flexion, abduction), K-DASH, and rotator cuff strength to evaluate shoulder function. ICT increased the range of shoulder flexion by 1.34% and shoulder abduction by 0.98%, decreased K-DASH score by 28.23%, and improved rotator cuff strength by 4.39%. LLLT increased the range of shoulder flexion by 1.26% and shoulder abduction by 0.65%, decreased K-DASH score by 19.05%, and improved rotator cuff strength by 4.71%. These results show that all shoulder functions, with the exception of range of abduction, were improved. However, both ICT and LLLT seem to have clinical effects on the range of shoulder abduction, though the effect sizes are small, at 0.31 for ICT and 0.27 for LLLT.
Hains et al. (2010) applied sham compression and ICT on trigger points of the supraspinatus, infraspinatus, deltoid, and biceps brachii muscles, which led to significant improvements in shoulder pain and dysfunction index (SPADI) (21). Chow et al. (2006) applied LLLT on trigger points around the neck in patients with chronic neck pain and reported that there were therapeutic effects, with significant changes in the Neck Pain and Disability Scale (NPAD) and McGill Pain Questionnaire (MPQ) (38). The present study also confirmed improvements in shoulder functions after MTrP treatment, but our study differs in that shoulder function was assessed after treatment of MTrPs in a single muscle.
Subacromial pain reduces voluntary activity of the infraspinatus muscle and external rotation of the shoulder joint. Inhibition of the infraspinatus muscle by subacromial pain may lead to abnormal Gh joint motion and translation (39). As a result, normal muscle activation patterns may be affected, which may result in muscle weakening and motor disabilities (40) as well as reduced ROM. Lucas et al. (2010) reported that passive stretching and dry needling to reduce MTrPs led to normalized motor activation pattern within 20–30 minutes (41).
In the present study, treatment of MTrPs of the infraspinatus muscle is believed to have improved external rotation and normalized shoulder joint movement. In the long term, normal motor activation patterns appeared, which seems to have increased rotator cuff strength. However, these speculations are based on previous findings, so additional studies are needed to establish a theoretical mechanism.
Because there were no differences between the two groups in our study, we compared their effect sizes as well. ICT had greater effect sizes than those of LLLT, with the exception of effect sizes for VAS and rotator cuff strength. This suggests that both treatments were helpful for mitigating pain and improving shoulder function in patients with shoulder pain. Therefore, selective treatment of MTrPs of the infraspinatus muscle is effective, and ICT and LLLT could be clinically effective when used appropriately according to patient characteristics.
This study has a few limitations. First, the sample size was not sufficient to generalize the study findings. Second, the infraspinatus muscle is not the only muscle that can provoke shoulder pain and limit shoulder functions. Third, we examined the ROM of shoulder movements that are frequently used in daily life (flexion, abduction), so we could not determine external rotation, the major function of the infraspinatus muscle. Finally, due to a lack of a control group, we could not examine changes in the variables over time. Subsequent studies should address these limitations for further comparisons.