Deep Myofascial Kinetic Lines in Horses, Comparative Dissection Studies Derived from Humans

Seven superficial myofascial kinetic lines have been described earlier in horses in a comparative dissection study to the human lines. The lines act as an anatomical basis for understanding locomotion, stabilization, and posture. Further dissections verified three profound equine lines comparable to those described in humans and a fourth line not described previously. Forty-four horses of different breed and gender were dissected, imaged and video recorded. The horses were euthanized due to reasons not related to this study. A Deep Ventral Line (DVL) very similar to that in the human was verified in these studies. The line spans from the insertion of the profound flexor tendon in the hindlimb to the base of the cranium and oral part of the cavities of the head. It includes the profound, hypaxial myofascial structures, the ventral coccygeal muscles, the psoas muscles, the diaphragm, the longus colli/capitis muscles and the ventral capital muscles. The inner lining of the pelvic, abdominal and thoracic cavities with all the organs, vessels and nerves are also included. The line is closely connected to the autonomic nervous system by the vagus nerve, the pelvic nerves, the sympathetic trunk and several of the prevertebral nerves and ganglia. The new line identified in this study, is a Deep Dorsal Line (DDL), which starts in the dorsal tail muscles. It comprises myofascial structures of the spinocostotransversal system from the tail to the head including the nuchal ligament. It connects to the dura mater and has a major role in controlling the motion and stabilization of the Columna vertebralis. Both the DDL and the DVL include the coccygeal myofascia and periosteum of the skull. Due to differences in biped and quadruped anatomy the Front Limb Adduction Line (FADL) and the Front Limb Abduction Line (FABL) differ from the human lines. The lines are identified as slings in the brachial and antebrachial regions. The FABL includes structures for abduction How to cite this paper: Elbrønd, V.S. and Schultz, R.M. (2021) Deep Myofascial Kinetic Lines in Horses, Comparative Dissection Studies Derived from Humans. Open Journal of Veterinary Medicine, 11, 14-40. https://doi.org/10.4236/ojvm.2021.111002 Received: November 29, 2020 Accepted: January 26, 2021 Published: January 29, 2021 Copyright © 2021 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
Seven superficial equine myofascial kinetic lines consisting of long rows of interconnected anatomical structures have earlier been described and published by Elbrønd and Schultz [1]. The lines are composed of integrated muscles and fascia and are arranged in long rows spanning from the head to the hoofs in the equine body. Each line is responsible for connecting movements between the spine and the extremities e.g. the Superficial Dorsal Line extends the spine and flexes the hind limbs, and the Superficial Ventral Line flexes the spine and extends the hind limbs. The structures of the lines serve to functionally direct basic locomotion patterns, support the musculoskeletal system as well as balance and describe the posture and motion of the whole body. In addition, these lines are essential if one is to fully understand the basic anatomy of the whole body and especially such generally accepted concepts as head to hoof and latero-lateral connections and their integrations in the horse. The human equivalents, the myofascial anatomy trains, composed of interconnected muscles, tendons and fascia, have already been dissected and described by Myers [2]. Besides the seven human superficial trains, equivalent to the already published equine lines [1] [2] another three profound lines are presented. These three lines are all situated profound, that is to say deep, within the body where they complement the seven superficial lines. Myers, [2], identifies one line as passing through the body cavities connecting the somatic body with the viscera, the Deep Front Line, as well as two deep lines in the arms. In the horses the corresponding lines are the Deep Ventral Line (DVL) and two lines related to front limb motion. These latter complement the already dissected Front Limb Protraction (FLPL) and Front Limb Retraction Lines (FLRL) [1]. In our opinion a deep dorsal line, complementary to the DVL seems to be lacking in the horse. Therefore, we V. S. Elbrønd, R. M. Schultz additionally focused on a Deep Dorsal Line situated in the deep epaxial back muscles.
Biomechanically and structurally the Human Deep Front Line is essential in balancing the spine through the hypaxial muscles. With the presence of this line Myers [2] has established an understanding of the connection between the fascia lining of the inner body cavities and the organs, to extensions into profound myofascial structures in the directions of the head, neck and limbs. Connections between somatic and visceral systems have been discussed in relation to many topics, e.g. viscero-somatic reflexes based on integration in the nervous system, and indeed viscero-somatic pain [3]- [12]. A new approach in this respect is raised by the findings by Stecco et al. [13] in a histological study of the subserosal fascia related to the inner organs. Here the group found that the subserosal fascia can be divided into two major groups, the investing fascia, and the insertional fascia. The latter is characterised by sheets forming the compartments to the organs, being composed of several layers of larger bundles of collagen fibres, including both smaller and myelinated large diameter nerves. According to Stecco et al. [13] the presence of such nerves indicates a connection between the insertional fascia and the somatic/musculoskeletal parts of the body. The insertional type of fascia is present both in relation to the pericardium, the abdominal organs and very distinct around the kidneys. A deeper layer of subserosal fascia, named the investing fascia, organizes, and shapes the organs and supports the parenchyma. It is elastic and rich in small blood vessels and small diameter nerves. The thickness and presence of the two types of layers differs between organs.
Significant differences are present in the anatomy and motion of the arm/front limb between the horse, which has a quadruped posture and that of the human with its biped posture. The four human arm lines [2] comprise all the major motions over the full length of the arm. In the horse, however, the front limbs are constructed for stable movement at low energy costs which is only possible due to specifically designed myofascial structures. These features are clearly reflected in the anatomy of the front limb, which from the elbow and distally are specifically addressed to flex and extend and perform protraction and retraction movements. The only exception is the coffin joint presenting very limited abduction/adduction and internal/external rotation.
The equine FLPL and FLRL [1] clearly reflect these movements but descriptions of lines addressing the ab-and adduction as well as in-and external rotation (pro-and supination in humans) [14], are clearly missing in the horse. Due to the anatomy of the limb from the elbow and distally including hinge joints and fusion of the antebrachial bones [15] [16], only myofascial structures in the proximal part of the front limb are involved in such movements. Indeed, this indicates that the deep arm lines are located around the scapula, humerus and the thoracic sling (e.g. m. serratus ventralis and mm. pectoralis).
To understand the context of the myofascial connections of the myofascial lines Yucesoy et al. [17] explain in their study that intact muscles with endo-, peri-and epimysial linings do not work as isolated structures. Due to the 3-D fascia skeleton they are to be dealt with as linked and connected units with properties designed for force transmission, energy loading and energy conservation as well as shock absorption. Several research groups present this feature in both rats and humans, in intact muscles in vivo, and in muscles detached from the intermuscular fascia connections. These results change the up to now stated biomechanical interpretation of the relation between muscles as being agonists-antagonists-synergist etc. [18] [19] [20] [21] [22], and opens the door for a new perspective on the functional muscle-fascia interactions which also include an integration of the full body.
Conjoined with a development of the locomotory system and motion patterns the body adapts the engaged structures e.g. muscles, bones and the fascia/myofascia. The concept "fascia plasticity" describes a reflexion and adjustment of not only the fascia/ myofascia and myofascia lines based on the principle of "Supply and demand". A biomechanically induced tension in myofascial structures can induce adaptations of e.g. deposition of collagen fibres along the lines of tension thereby adjusting the strength between muscle groups, fascia sheets etc. [23]. The fascia can adapt in other ways, at different levels and with different time frames. At the cellular level the fibroblasts can contract within a matter of minutes [24] [25] [26] [27] and at the macroscopic level an increased deposition of collagen fibres, as mentioned above, within a significantly longer time period. This latter process is initiated by only a few repetitions of tensions "over the load" [28]. With ongoing tension, renewal of the collagen fibres in a healthy human occurs within six months [29]. Indeed, a rough extrapolation from these data estimates the intrafascial dynamics with a time frame of replacement of 30% within six months and 75% within two years in humans [24]. When loading myofascia, the interplay and the exchange of tension between the muscles and fascia favours a smooth and elastic loading which protects the fascia and keeps it "fit" and supple. Younger individuals seem to possess a high degree of flexibility and elasticity of their fascia, a phenomenon seen and explained by the histological architecture and organization of the collagen fibres [30]. The fibre bundles are found to undulate in a wavy pattern ("the crimping effect") and elastic fibres arranged as an interwoven network provide a structure that enables lengthening without overstretching the collagen fibres. Initially such loading results in lengthening, but subsequently it allows the collagen fibres to regain their original organization, supported by the elastic fibres [31] [32]. Thus, in this way fascial plasticity supports the body in several ways to adapt to acute and more permanent tensional changes.
The myofascial kinetic lines are arranged and overlapping one and another. Therefore, mobility and flexibility are essential for the lines to interact and to balance the body. Loose irregular connective tissue containing high amounts of hyaluronan (HA) meets this demand and is situated between and on surfaces of muscles, myofascia, aponeuroses, fascia layers as well as many other vital structures such as nerves, blood and lymph vessels etc. [32] [33]. The tissue lubricates and makes the surfaces slippery and maintains the vital movements e.g. in the locomotion system, including the myofascial lines. In human specimens, Stecco et al. [34] [35] identified not only the presence of HA but also HA secreting fasciacytes, which they assume to be specialized fibroblast-like cells of monocyte/macrophage like origin. Recently, the presence of HA between the fascia layers has been confirmed at the microscopical level in horse specimens [33]. Moreover, the conformation of HA is known to influence the capability of structures to move in relation to each other, with changes in the HA-matrix leading to an inhibition/change in movements, the induction of pain and inflammatory conditions which then lead to more pathological conditions [36].
The aim of this study was to reveal the inter-connective functionality of the locomotor system of the profound lines in the horse in continuation with the superficial lines previously described by [1]. This study has therefore been Line, which serves to establish a balance around the vertebral column, the tail and the head.

Horses
In total forty-four horses of different breeds, aged ( Sciences not only approved this project but also provided facilities and monitored its progress.

Pre-Dissection Approach
The profound human lines (Myers 2013

Practical Aspects of the Dissection
After euthanization, the dissections were performed on skinned horses following the specific rules of the lines as stated by Myers [2], 1) The lines of tension

Transverse Sections
For the transverse sections the horses were euthanized, shaved and the gastro-intestinal-tract was removed. The horses were marked segmentally along the spine and positioned either hanging by their legs or lying prone on a table before being frozen at −20˚C for a week. The horses were finally cut into segments using a bandsaw and the segments were kept separated until they were photographed for more detailed image analysis.

Results
The results of the dissections will be described in detail for each and every line.

Deep Ventral Line (DVL)
The deep ventral line is illustrated in Figure 1 Table 1).
The peripheral fascia capsule around the retroperitoneal positioned kidneys and the retropleural positioned heart (the pericardial sac) were found to have the same macroscopic characteristics as the inserting fascia described in humans [13]. These macroscopically identifiable compartments favour the possibility that these organs can move freely yet still remain protected. The insertional fascia layers are also present in close connection to the rest of the abdominal organs and provide the organs with mobility and protection. Generally, the insertional fascia connects the organs to the musculoskeletal system [13]. The  Table 1.
The cranial continuation of this second pathway enters the thorax and on into the subserosal fascia lining of the mediastinum and the pericardium. Collagenous tension lines are present from the base of the heart and in the dorsal mediastinum with a direction pointing towards the mid thoracic vertebrae (Th 7 -10). Here the line includes the subpleural fascia enclosing the thoracic organs e.g. the esophagus, the trachea, the lungs, and the heart. An intimate contact is observed with the parasympathetic N. vagus, which is situated in the mediastinum from the Apertura thoracis cranialis to the diaphragm. In the closure of the Apertura thoracis cranialis the prevertebral sympathetic ganglions are situated e.g. the Ggl. Stellatum cranially and dorsally under the Costa prima and secunda. Open Journal of Veterinary Medicine  The line continues in the Cavum thoracis through the Apertura thoracis cranialis and follows along the Trachea and Esophagus in the Fascia cervicalis, lamina pretrachealis to the ventral cervical region. The fascia arranges bilaterally into a right and a left Vagina carotis, which include the A. carotis communis, V. cava cranialis, the Truncus vagosympaticus and the N. recurrens. At the Apertura thoracis cranialis the line also comprises M. scalenus from the Costa prima to C4-C7 (the ventral part) and to C7 (the middle part) between which the Plexus brachialis passes through. The right and left Mm. scalenii embrace the trachea. The line continues to the Pharynx, Larynx, Diverticulum tubae auditivae and into the profound masticatory muscles of the oral cavity and the tongue (Figure 2(d)).
3) The third and ventral pathway follows the subserosal lining of the Lamina parietalis of the walls and ventral surface in the Cavum pelvis, abdominis and thoracis (Figure 2(b)). In the thorax, the subpleural fascia also includes the dense and strong Fascia endothoracica. Over the caudal part of the sternum the endothoracic fascia splits and directs dorsally, transforming into the Lig. sternopericardiaca and at the apex of the heart it transforms into the Pericardium fibrosum. From the thorax in a cranial direction the line continues over the Manubrium sterni and through the M. sternohyoideus to the Apparatus hyoideus and the Articulatio temporo-hyoidea. It also continues rostrally through the hyoid muscles inserting at the rostral part of Mandibula and the structures within the Cavum oris (Figure 2(d)). The two latter pathways unite within the Lamina pretrachealis of the Fascia cervicalis.
The function of the DVL is to serve as a flexor and stabilizer of the spine through the hypaxial muscles. The psoas muscles are responsible for the flexion and stabilization of the thoraco-lumbar and the lumbo-sacral region and the longus colli/capitis muscles for the flexion of the cervico-thoracic and the cervico-capital junction. This part of the line also integrates several structural connections of the viscera to the somatic body.
The full line can be divided into three major parts: The two ends (caudal/hindlimb and cranial/neck and head), which include structures mostly related to the somatic part of the body, and the middle part (the abdomen and thorax), which in majority is related to the visceral part of the body.

The Deep Front Limb Lines
In contrast to the FLPL and FLRL [1] the deep front limb lines are arranged as slings and comprise several broad aponeurotic fascia sheets related to the scapular and brachial region. The deep lines are focused around the pivot joint in the upper third of the scapula similar to the pro-and retraction front limb lines.
The Deep Front Limb Lines are illustrated in the Figure 1

The Front Limb Adduction Line (FADL)
The FADL starts at the Margo cranialis et Facies cranio-dorsalis scapula with the M. subclavius and M. supraspinatus. The line follows these muscles towards the Sternum accompanied by M. pectoralis descendens and M. pectoralis transversus from Os humeri. Just proximal to the Articulatio humeri at the transitional zone between the scapula and the brachium and on the medial side, is a broad fascia connection which is present between M. subclavius and M. teres major ( Figure  3(d)). From here the line proceeds along M. teres major and merges proximally to the Angulus caudalis scapula (Figure 3(a), Figure 3(d)) where the fascia to direct the front limb into adduction and an external rotation during the last phase of protraction (Figure 1(b)), just before landing occurs on the caudo-lateral part of the hoof. During the landing phase the direction changes into an internal rotation.

The Front Limb Abduction Line (FABL)
The FABL starts dorsal to the scapula in M. trapezius pars cervicalis and thoracalis and a superficial layer of the Fascia spinocostotransversus s. Lig. dorso-scapularis. This fascia sheet continues in a distal direction on the abaxial side of the scapula into M. deltoideus, which surpasses the craniolateral part of the Articulatio humeri towards the Tuberositas deltoideus humeri (Figure 3(b)). A fascia sheet continues from here in a medial direction over M. biceps brachii (Figure 3(b), Figure 3(c)). The fascia continues in an axial direction and blends into M. pectoralis ascendens (Figure 3

Deep Dorsal Line (DDL)
The counterpart to the DVL is the Deep Dorsal Line (DDL), which has not been described in humans (Figure 1(a), Figures 4(a)-(d)). The line originates in the coccygeal region in the dorsal muscle compartments of the tail. The M. sacrocaudalis (coccygeus) dorsalis medialis (Figure 4(a), Figure 4(d)), which cranially continues to the Mm. mulitifidi (origo S2/3), and the M. sacrocaudalis dorsalis lateralis, which continues in the space between Mm. multifidi and M. longissimus lumborum at L6, are all parts of the line. In a caudoventral direction at the level of the Tuber ischiadicum the DDL connects to the SDL in two ways: 1) a fibrous contact between the caudal part of M. biceps femoris and the epimysium of M. sacrocaudalis medialis, and 2) a connection between M. semitendinosus (endo-and perimysial contact) and M. sacrocaudalis lateralis. In a cranial direction the line joins the myofascia of the transversospinal system e.g. Mm. multifidi (Figure 4(a), Figure 4(b), Figure 4(c)), which span with five fans over one, three and five vertebrae, respectively [39] [40], along the spine until C2. Here it proceeds into the epaxial suboccipital muscles, M. rectus capitis major and minor and M. obliquus capitis caudalis et cranialis of which the latter attaches additionally to the Crista occipitalis. The suboccipital muscles connect to both the atlanto-occipital and atlanto-axial membranes which again connect to the dura mater, the so-called myodural bridges (Figure 2(b) [41]). Similar connections are found in the dorsal intervertebral foraminae along the whole spine. Open Journal of Veterinary Medicine From the sacral region the line also extends into the intervertebral ligaments and dorsally into the fibrous Lig. supraspinale (Figure 3(c)). It continues in a cranial direction on top of the spinous processes and continues into a transitional zone into a more elastic composition at the level of Th3-Th9, continuing into the Lig. nuchae (Figure 3(b)). The Funiculus nuchae, of the Lig. nuchae, continues cranially and attaches to the Crista occipitalis ventral and medial to M. semispinalis. Open Journal of Veterinary Medicine The Lamina nuchae splits from the funiculi and attaches to the cervical spinous processes (C2 to C5 (C6-7)).
A left as well as a right DDL is present although they run in close proximity.
The function of the DDL is to take part in the proprioception and the movement (extension, rotation, and lateral flexion) and stabilization of the vertebrae [14] [39] [42] [43] and thereby the whole spine. The DDL has a close connection to the dura mater both through the myodural bridges but also through the close connection to the Os sacrum [41].

Discussion
The aim of this study was to dissect and visualize the profound equine kinetic myofascial lines in continuation with the superficial lines previously described [1]. The first two hypotheses studied have their base in the human lines presented by [2] namely the Deep Front Line and the Deep Arm Lines. The third hypothesis that this study has focused on is derived from own practical experiences and observations of the locomotion system as well as the posture of horses, namely the Deep Dorsal Line, which serves to join the spinocostotransversal muscle system. In all of the lines presented [2] we found and observed that the quadruped posture influenced the anatomy and the function of the lines as well as their composition.

DVL and DDL
The present study has confirmed the presence of an equine DVL very similar to that of the human Deep Front Line [2]. According to Myers [2] the line comprises three major divisions: two end parts (the legs and the neck) mostly related to the somatic body system as also seen in the superficial lines, and one middle part, a "true 3-D part" (the cavum pelvis, abdominis and thoracis) mostly related to the visceral system of the body.
Several differences between the human (DFL) and equine DVL lines are present and many of them are influenced by the postural difference, biped versus quadruped. One is the inclusion of the M. sartorius in the horse. This connection brings the medial fascia genus in close contact to Mm. psoas and the hypaxial surface of the lumbar vertebrae. The importance of the equine Mm. psoas has been shown by Hyytiäinen et al. [43], who found a muscle fiber composition that indicates that besides flexion of the lumbo-sacral region and flexion of the hip, these muscles act as stabilisers of the lumbar region. The movement or tension from the hind quarter passes directly into the hypaxial compartment of the trunk via the psoas muscles in a cranial direction onto the thoracolumbar junction, where the psoas muscles (origo at the lower thoracic vertebrae) overlap with the diaphragm approaching from a cranial direction (see include also lateral flexion coupled to axial rotation in the thoracic region [14]. It is at this point that the line changes from being mostly related to somatic/locomotive structures into more visceral. In a review [44] and in a study with manipulation of the thoracolumbar region Haussler et al. [45] discussed and showed that reduced movement in the lumbosacral junction transmitted the major movement of the spine into the thoracolumbar junction. The biomechanical consequence of such an overload at this junction has to our knowledge not been studied further. Nor have we been able to find research studies on the influence on equine performance, e.g. the respiration, the functionality of the diaphragm, the regulation of the autonomic nervous system, and thereby the regulation of the inner organs etc. When dissecting the profound and the subserosal fascia, the physical tightness between the organs and somatic structures raises questions about the role of the fascia in these viscerosomatic interactions. As presented in Table 1 The close connection between Mm.psoas and Crus diaphragmatis at the thoraco-lumbar junction makes the diaphragm a very central muscle and not only a divider between the thorax and the abdomen, but also a factor influencing respiration via the somatic body from a distance. Myers [2] explains how the pulling parts of the line in the legs (deep flexor muscles and adductors) are involved in coordinating a rhythm between respiration and locomotion in humans. A feature well known as e.g. in the gallop the push-off and extension of the trunk in the suspensory phase induces an inspiration, whilst landing, the flexion and collection phases induce an expiration [46].
Looked at from another perspective, a respiratory lung problem or rib cage trauma (influencing the function of the diaphragm) could potentially interact with mobility in the hindquarters. The connection between female urogenital or kidney problems to lumbar pain is a frequent condition seen in horses and it makes good sense when studying the subserosal connections to both the hypaxial muscles but also to the fascia connecting to the lateral raphe in the lumbar region.
Minor adaptations in the organ topography are observed with changes in posture, just as well in horses as in humans. In humans, the gastrointestinal tract is connected to the subvertebral region via the mesenterium (which in terms of human anatomy is now included as a separate organ/a fascia organ in Grey's Anatomy [47]. Similarly, in horses the mesenterium attaches the gastrointestinal tract to the dorsum of the abdominal cavity [15] [16]. Open Journal of Veterinary Medicine Denoix and Paillioux [14], illustrates in their book the interplay between viscera and the locomotion system. They explain that the abdomino-pelvic cavity is to be looked upon as an abdominal chamber in which the abdominal muscles contract against the resistance in the chamber created by the diaphragm and internal organs as also [46]. An interaction between the back muscles (SDL and DDL) as well as the abdominal muscles (SVL, LL, SP, FL) and the viscera, It might reasonably be argued as to whether Mm. scalenus should be part of the line as described in humans [2], spanning as it does from the first rib to C3 [49]. In horses M. scalenus medialis spans from the costa prima to C7 and M. scalenus ventralis spans from the costa prima to C4 -C7 [16]. Especially the medial scalene does not follow the rule of dissection [2] of multiple joint muscles, however, their close proximity to the Apertura thoracis cranialis and the fascial attachment to the trachea makes it difficult to separate them from the DVL. These muscles are especially important because the plexus brachialis passes between their ventral and medial parts. Thus, should these muscles be kept in a state of static contraction, they could quite easily have a considerable impact on the neurological efferent and afferent output to and from the front extremities.
The Apertura thoracis cranialis is a region in which the wide and complicated part of the 3-dimensional DVL transmits from the thoracic cavity and into a more pulling part of the line through the cervical region. The region is important as the aperture surrounds the en-and extrance of vital organs and connections (included in the DVL), but also because all other myofascial lines (see Table 2) but the FL embrace and outbalance this region. Additionally, the Columna vertebralis is arranged here in a secondary curve, the extension curve,   [56].
Influence on the cerebrospinal pulsation is also assumed to be supported by the MDBs situated in the atlanto-occipital and atlanto-axial regions. These structures are important parts of the DDL, and the pumping mechanism is influenced by the M. rectus capitis major and minor as well as the cranial and caudal oblique capital muscles and the Lig. nuchae, which in the horse is well developed [41].  [16]. The lack of the lower laminae nuchae increases the vulnerability of the cervicothoracic region as also mentioned previously.

Front Limb Lines
The  Table 3). Thus, in situations where there is a change in motion pattern in the front limbs, compensatory motion can affect other lines through these connections, contributing to imbalance. In this way it is easy to understand how front limb imbalance can interfere with the posture of the rest of the body and vice versa. Indeed, just such an interaction could explain the compensatory V. S. Elbrønd, R. M. Schultz

Conclusion
The Deep Ventral Line of the horse closely resembles the Deep Front Line described by Myers [2]

Author Contribution
VSE and RMS designed the project, contributed data, analysed the data, made the figures and wrote the manuscript.