Fascia is a membrane that extends throughout the whole body and numerous muscular expansions maintain it in a basal tension. During a muscular contraction these expansions could also transmit the effect of the stretch to a specific area of the fascia, stimulating the proprioceptors in that area.
Histological characteristics of the deep fascia of the upper limb:
Specimens taken from the antebrachial and brachial fasciae demonstrated that fasciae are formed of numerous layers of undulating collagen fibre bundles. In each layer, the bundles are parallel to each other, whereas adjacent layers show different orientations.
Each layer is separated from the adjacent one by a thin layer of adipose tissue, like plywood. Many elastic fibres and a variety of both free and encapsulated nerve endings, especially Ruffini and Pacini corpuscles, are also present, suggesting a proprioceptive capacity of the deep fascia.
Specimens taken from different areas of upper limb [taken at the level of: (a) the expansion of pectoralis major onto the bicipital fascia, (b) the middle third of the brachial fascia, (c) the lacertus fibrosus, (d) the middle third of the antebrachial fascia, (e) the flexor retinaculum.] revel abundant innervation of the fascia consisting in both free nerve endings and encapsulated receptors, in particular, Ruffini and Pacini corpuscles.
However, differences in innervation were seen between retinaculum the more innervated element whilst the muscle (pectoralis major) expansions the less innervated. This suggests that the retinaculum has more a perceptive function whereas the tendinous expansions onto the fascia have mostly a mechanical role in the transmission of tension that confirms that the fascia plays an important role in proprioception, especially dynamic proprioception.
MRI study shows:
1. The clavicular part of the pectoralis major muscle sent a fibrous expansion onto the anterior portion of the brachial fascia, its costal part onto the medial portion and medial intermuscular septum. Thus the pectoralis major fascia always continues with the brachial fascia in two distinct ways: the fascia overlying the clavicular part of pectoralis major had an expansion towards the anterior brachial fascia, whereas the fascia covering its costal part extended into the medial brachial fascia and the medial intermuscular septum.
2. The latissimus dorsi muscle showed a triangular fibrous expansion onto the posterior portion of the brachial fascia. In the posterior region of the arm, the fascia of the latissimus dorsi sent a fibrous lamina to the triceps brachial fascia.
3. The posterior part of the deltoid muscle inserted muscular fibres directly onto the posterior portion of the brachial fascia, its lateral part onto the lateral portion and the lateral intermuscular septum.
4. The lacertus fibrosus was also composed by two groups of fibres: the main group was oriented downwards and medially, the second group longitudinally.
5. The triceps tendon inserted partially into the antebrachial fascia, while the extensor carpi ulnaris sent a tendinous expansion to the fascia of the hypothenar eminence.
6. The palmaris longus opened out into a fan-shape in the palm of the hand and sent some tendinous expansions to the flexor retinaculum and fascia overlying the thenar eminence muscles.
Function of Pectoral girdle muscles i.e. pectoralis major, latissimus dorsi and deltoid are connected not only through it’s muscular attachment to the humerus but also through the deep fascia i.e. the brachial fascia of arm. Hence during the various movements of the arm, these expansions stretch selective portions of the brachial fascia, with possible activation of specific patterns of fascial proprioceptors. It is hypothesized that the tendinous muscular insertions maintain the fascia at a basal tension and create myofascial continuity between the different muscles actuating flexion and extension of the upper limb, stretching the fascia in different ways according to the different motor directions.
Pathogenesis of pain from fascia:
1. The undulating collagen fibre bundles and elastic fibres, the fasciae can adapt to stretching, but this is only possible within certain limits, beyond which nerve terminations are activated by stretching. This mechanism allows a sort of "gate control" on the normal activation of intrafascial receptors.
2. The capacity of the various collagen layers to slide over each other may be altered in cases of over-use syndrome, trauma or surgery. In such cases, the mechanism of the fascia on the nervous terminations is lost, causing incorrect paradoxical activation of nerve receptors within the fascia, resulting in the propagation of a nociceptive signal even in situations of normal physiological stretch. At the same time, the layered collagen fibres allow transmission of tension according to the various lines of force. This structure of the muscular fascia guarantees perceptive and directional continuity along a particular myokinetic chain, acting like a transmission belt between two adjacent joints and also between synergic muscle groups.