Background
Musculoskeletal impairments are the leading cause of work disability in middle-aged adults, and low back pain is the most common form. There are two broad clinical categories of musculoskeletal impairments: articular and non-articular. The articular group includes joint diseases (intra-articular) involving inflammation or injuries due to trauma or degenerative processes. Typical examples are rheumatoid arthritis or shoulder instability. The non-articular group includes disorders that primarily affect soft tissue (peri-articular), such as muscles, ligaments, and tendons. Examples are fibromyalgia, myofascial pain syndrome, and tendonitis. It is thought that up to 85% of patients that visit pain clinics complain of non-articular musculoskeletal impairments such as the myofascial pain syndrome. The first authors who systematically described the myofascial pain syndrome were Travell and Simons, who theorised that this painful condition is due to the presence of myofascial trigger points (MTrPs).
MTrPs are hyperirritable points located within a taut band (TB) of skeletal muscle that cause referred pain, local tenderness, and sometimes autonomic changes. An active MTrP is characherized by spontaneus pain or pain response to movement, while a latent MTrP is a sensitive spot with pain only elicited in response to compression. The MTrP diagnosis requires detailed history taking and physical examination to confirm the presence or absence of an original set of diagnostic criteria (i.e., taut band, spot tenderness, referred pain, pain recognition, local twitch response).
MTrP pathophysiology appears to be associated with the motor endplate zone. This region, also known as the innervation zone (IZ), is where the α-motor neuron divides into a number of branches and synapses onto target muscle fibres. The IZ is usually described as being in the middle region of the muscle belly, but more complex IZ spatial distibutions have been reported in muscles with unipennate or multipennate fibre arrangements (e.g., gastrocnemius and soleus). Needle electromyography (EMG) studies have demonstrated that MTrPs contain minute loci that produce characteristic low-amplitude electrical activity (i.e. active locus), which is described by in the literature as spontaneous electrical activity (SEA).
The origin of SEA has been extensively debated among experts. Initially, Hubbard and Berkoff attributed the source of SEA action potentials to intrafusal muscle spindles located near MTrPs. Later, Simons considered previous work by Liley and hypothesised that SEA originates from motor endplates and defined it as endplate noise.
In support of the last hypothesis, a needle EMG study showed that endplate noise was more prevalent in MTrPs than in adjacent sites.
This "motor endplate" hypothesis was also tested by Kuan et al., who injected MTrPs with botulinum toxin type A to block acetylcholine release into the synaptic cleft and found that the injection diminished SEA in MTrPs.
Finally, the described electrophysiological findings have been correlated with histological changes and local biochemical alterations (e.g., inflammatory mediators, neuropeptides, catecholamines, and cytokines) to support MTrP pathophysiology.
The described experimental findings of MTrPs suggest that they are located close to the IZ. In a research context aimed to clarify the MTrP etiology a confirmation of the overlapping between MTrP and IZ will help to clarified their interaction (i.e. the active locus and the sensitive locus). Furthermore manual identification of MTrPs could become useful to optimize treatments addressed to the IZ, like botulinum injections for both cervical dystonia and myofascial pain.
A technology useful for detecting the IZ in vivo has recently proposed, and to-date, no study has assessed both MTrP and IZ locations. Therefore, the purpose of this work was to describe MTrP and IZ locations in the upper trapezius muscle.