The muscular system provides support and movement due to the muscles that compose it (cardiac, striated and smooth), through muscular contraction, which is divided into slow (repetitive) and fast (for high intensity and resistance), also by Isometric and isotonic tension (eccentric and concentric). Muscle injuries (tears) are related to the practice of physical and sports activity, which can occur at any time due to bruising (trauma) or bloating (hyperextension), and when they occur they generate edema, ecchymosis - hematoma, functional impotence and deformation in its architecture. Timely management as well as classification and optimal imaging approach allow an improvement in therapeutics - recovery and return to physical activity - competition of the patient.

skeletal muscle fibers, skeletal muscle, magnetic resonance imaging, ultrasonography, muscle injuries, classification of muscle injuries in sports, muscle injuries in athletes, imaging classification of muscle injuries and treatment of muscle injuries

The muscular system is in charge of providing support - body movement, and is made up of more than 650 muscles, which are divided into 3 categories (cardiac, smooth and striated). The muscles are divided into 10% smooth and cardiac muscle, and 40% skeletal or striated muscle, which in turn is made up of numerous fibers with a diameter of 10-80 µm made up of smaller subunits. Each muscle fiber has a membrane, sarcomere, bands and a disk; and it is made up of thousands of myofibrils (1500 myosin filaments and approximately 3000 actin filaments), adjacent to each other, being large polymerized protein molecules responsible for muscle contraction. These filaments partially interdigitate, thus giving alternate, light (actin filaments known as bands I, isotropic in polarized light) and dark (myosin filaments or A bands, anisotropic in polarized light) overlapping each other and with projections. (Crossed bridges), giving an interaction that causes muscle contraction.^1–5 The muscular contraction is divided according to the contractile function of the fiber, in type I or slow contraction, which is repetitive, resistant to fatigue, with a greater amount of mitochondria and capillary action, ideal for low intensity exercise; and type II or rapid contraction, for high intensity and short time activity, with high resistance, capable of producing greater force.

It is also divided according to the tension into isometric or without change in length, and isotonic, that is, from tension associated with the change in fiber length, which in turn is subdivided into concentric (fiber shortening) and eccentric (elongation of fiber).^1–5 Muscle tears or injuries are generally related to the practice of physical and sports activity - competitive, however these can occur at any time or daily activity. A muscle tear or injury is the loss of the continuity of the muscle tissue or its connective tissue, that is, a traumatic injury to the interior of the muscle, which consists of a laceration of greater or lesser number of muscle fibers (myofibrils) that break, can bleed and cause bleeding.^1–5 The injury can be caused by contusion (trauma) or distension (hyperextension during contraction) of the muscle fibers responsible for muscle contraction, as well as by the connective tissue that surrounds them and can present with symptoms such as even acute localized pain, accompanied by inflammation , ecchymosis / hematoma, functional impotence and structural deformation.^1–5 The objective of this bibliography review is to describe the types of muscle tears and the phases of recovery from muscle injuries for the care of patients who carry out light-moderate physical activity as well as elite athletes, as well as to inform the innovation of the classification and approach updated imaging for better therapy.

Bibliographic review is performed by searching Google Academic, NCBI databases; PMC and sports federations (football clubs, confederations) to obtain research material, using. The reviews in each document were read, which allowed determining their use for this review, and 3 books, 1 video and 13 articles were obtained in their entirety. Publications of extensive and detailed material from the multiple classifications, approach and treatment of muscle injuries were included, as well as little detailed publications from the approach and classification of muscle injuries were excluded.

Physiology of muscle contraction

The muscle fiber is made up of multiple myofibrils (made up of multiple sarcomeres, which is the contractile unit of the striated muscle). The sarcomere is made up of thick filaments (myosin exclusively) and thin filaments (tropomyosin, troponin, and actin), which in turn makes up various structures such as the M line (being the central structure of each fiber), H band (thick filaments only) , band A (both filaments: actin and myosin), band I (thin filaments only) and line Z (disk Z, which is the anchoring zone for actins); in turn between Z lines is titin, the largest protein in the human body.^1–6 Muscle contraction is due to the interaction of multiple structures, the thin filament (the actin enveloped by tropomyosin and the sites of action are covered by troponin and its subunits) and the thick filament (myosin covered by ADP molecules; the which will keep it stable and firm without contracting, an action that they possess for themselves, prepared for contraction).^1–6

To activate the filaments, the membrane action potential must be given by stimulation of acetylcholine from the alpha motor nerone, by voltage-dependent sodium channels, reaching the threshold and the polarity change being unleashed, allowing the opening of channels (mainly tubule T, closest site to the sarcoplasmic reticulum) where the dihydropyridine receptor (voltage-dependent calcium channel) is located and this allows the entry of calcium.^1–6 With the entry of calcium and its interaction with the sarcoplasmic reticulum, intracellular calcium is released, which seeks to bind to the c subunit of troponin and release the actin action site, which in turn releases the site of action of myosin, to join it and finally generate the contraction.^1–6 With muscle contraction, ATP is required for re-entry of intracellular calcium into the reticulum. This is given by the SERCA pump of the reticular membrane, by means of an opposite concentration gradient (attracts calcium to its interior), releasing ATP for its binding again to myosin and becoming ADP. In turn, the c subunit stops receiving the calcium stimulus and the troponin covers the actin action site again, returning to the initial stage.^1–6

Pathological anatomy: phases of muscle regeneration

Knowing the phases by which the muscular injury develops when the trauma occurs or an alteration in the adequate contraction of the same, helps in a better recovery of the injury caused, because the therapy can positively or negatively affect this process. ^7–9 It should be noted that muscle tissue is very dynamic and therefore can be affected by external factors of its environment as well as positive stimuli to it, that is, it is manipulable by mechanical processes such as massages, electromagnetic fields as well as chemical processes (use of NSAIDs). Furthermore, it is a source of different types of somatic stem cells, myogenic stem cells (here are the satellite cells: main responsible for muscle growth, muscle regeneration) and multipotent muscle derived stem cells.^7–9 The regeneration capacity of the muscles decreases with age, while fibrosis increases, and it is known that the repair of a muscle injury does not tend to be uniform or homogeneous, therefore, it is affected with passage of time (Table 1) & (Figure 1).^7–9 Recovery period chronology and the components that interfere in each period of time. Source: own elaboration based on Pedreta: Muscle injuries in sport.⁷

Figure 1 Own elaboration based on Pedreta: Muscle injuries in sports.

Tear classification: anatomic criteria

The criteria that provide the classification of the types of muscle injury (fiber tears) by anatomical injury, mechanism of injury and image visualization should be separated, since each category has various key components for its identification (Table 2).⁸ The classification by anatomical criteria, provides a comprehensive visualization of the muscle injury, while the classification by image criteria, provides a common language in the medical field for a generalized approach to a simple but at the same time complex picture such as the injury of the muscle tissue, as it is a dynamic complex (Table 3).^7,8 Image of each type of tear (ultrasound).

Source: own elaboration based on Verdugo: Ultrasonographic classification of muscle tears⁸

Tear classification: image criteria

The classification by anatomical criteria provides a more detailed classification of the muscle injury, the classification by image criteria is a way to simplify the characterization of the muscle injury, excluding size and prognostic factor, which is why in certain communities of the medical union, it tends to be difficult to accept, more used for its ease. This classification takes into account the degree of preparation of the individual, qualitatively the tear (prognosis and recovery time) and measurement of the injury (quantitatively); This means an approximation but not an exact data.^3,8 With it, it is a matter of unifying criteria for greater facilitation of understanding among related personnel. This classification excludes lacerations or injuries caused by external agents, and even tear-simulating injuries (delayed muscle pain syndromes, compartment syndrome, pathologies due to muscle overload) (Table 4).^3,8

Injury mechanisms

The mechanisms involved in the formation of a muscle injury are due to the intervention of intrinsic and indirect factors (muscle properties) and extrinsic factors - direct on the surface (due to trauma, laceration) (Table 5).^8–11

Risk factor's

Multiple external factors intervene in promoting a muscle injury, increasing the risk of it happening as well as complications of this pathology. They can be classified into intrinsic risk factors (age, sex, body composition, genetic predisposition, medical history of previous injuries, general condition), that is, those related to the patient (morbidities) and extrinsic factors (Error when training, material that is uses the environment, psychological conditions, season, toxic habits) or totally external to it, increasing the incidence of injury.⁷ Factors that are not part of the classification must also be taken into account, but rather are components developed during physical activity such as alterations in the mechanical axes of the extremities (irregular support), support zone (coordination and footfall), muscle balance - coordination (execution of movement), muscle fatigue and degree of elasticity.⁷ 

Frequency and location of injuries

The frequency of the appearance of injuries can be estimated in 31% of the cases of athletes, while the relapse is estimated between 14-32%, depending on the type of sport performed; however, these data can be modified by the location and incidence of the injury. The location tends to associate mostly the lower limbs (preferably thigh) due to the large number of muscles that make up this anatomical area, the hamstrings and quadriceps being the most affected. The abductors and posterior leg region are followed.⁷

Clinical manifestations

The muscle tear or injury has variations in its presentation, which vary in intensity in each patient and depends on its severity. Associated symptoms include: spontaneous or sudden onset pain, tenderness, and functional impotence; These may appear in the first 12-24 hours after exercise and in some cases tend to resolve spontaneously slowly as days go by.⁷ Among the signs that a patient may manifest, pain on palpation, pain in a relaxed state, is the hematoma called the seal of the injury of a muscle fiber, which can vary in location, where it is more frequent intermuscular characterized by the presence of blood that runs between the muscle fascial planes. Ultrasonographically, it tends to vary if it is active or recent (diffusely hyperechogenic), and after a period of time it becomes a homogeneous and hypogenic mass, and even when the blood elements are separated, there is a liquid-liquid level. As the days go by, this image will be from a uniform anechoic collection that can be absorbable (depending on the severity and extent of the injury).¹⁰ However, when it varies, there may be cases where the trauma is minimal that the patient does not perceive and has spontaneous resolution, where it occurs with discomfort at the end of physical activity or in the next 48 hours, without major affectation. It should be remembered that the lesions with the worst prognosis are those where the muscle is in a state of relaxation.⁷

Diagnosis - imagenological approach

For muscle injuries, their diagnosis is made clinically, that is, with a medical history and physical examination, so the anamnesis must contain detailed information supplemented by a medical physical examination. These elements are continuously supported by ultrasound studies and magnetic resonance imaging, ideally, because a plain radiograph shows calcifications, it could even be said that it visualizes a bit of an increase in muscle volume and fat fascia but it is not very useful, and computed axial tomography has limited use, and is supportive of plain radiography when it detects calcifications.^11–13 In the anamnesis it must contain local and general antecedents (similar and previous events, if there was pharmacological treatment), description of the injury moment (such as the event, moment of manifestation: before, during or after the activity), immediate evolution (continuity of the activity or abandonment of it, evolution of pain).^11–13 During the physical examination, 4 aspects must be taken into account: inspection (ecchymosis or deformities), palpation (painful points or muscle spasms), requesting the active contraction of the affected muscle, first with the muscle with stretching, more sensitive in minor injuries and then with manual resistance (observe whether it is possible, whether or not it is painful and the type of contraction), and assess the analytical flexibility of the muscle (pain on passive stretching).^11–13

As the evolution of the lesion occurs, the first procedure that is performed immediately is the medical history with physical examination. 12 hours after this stage, a physical examination is performed again together with ultrasound and biochemical markers. After 24 hours, the patient underwent a physical examination, ultrasound, magnetic resonance imaging with biochemical markers, and after 48 hours, only an ultrasound with physical examination was performed (Table 6).^11–13 Within the approach, the type of injury, its evolution and the approach available must be taken into account. On multiple occasions, the variation in the use of one imaging technique or another differs in how the picture presented by the patient (athlete - elite athlete) will be approached in the end. For a long time, ultrasound-ultrasound criteria based on histopathological and imaging evidence (ultrasound and magnetic resonance imaging) have been used, thus limiting the adequate reading of highly dynamic tissue.^9,11–13

Imaging criteria: ultrasound/ultrasound

It is a dynamic study that complements the clinical examination, that is, an evolutionary follow-up study. The guided evacuation of cavities and complemented with painful echo palpation of the determined muscle (previously identified by ultrasound) is of great help for topographic diagnosis (Table 7).^11–13 

Imaging criteria: magnetic resonance

It is a high sensitivity and precision imaging study to identify the affected structure (Table 8).^11–13  Currently, a technique for evaluating “muscle tone” called tensiomyography has been proposed, but there is little scientific evidence for this methodology.^11–13

Innovation in the classification

Currently, different sports entities have been commissioned to carry out research focused on a more optimal classification of muscle injuries, which allows a universal language (the most recent ones are based on ultrasound and magnetic resonance imaging), including the FC Barcelona medical team, which has started a new category for the interpretation of muscle injuries, allowing for a more objective approach.¹² The use of this new classification allows to avoid the ambiguity of the interpretation and to give it a greater specificity of the presented table (Table 9). The expanded version allows for greater specificity because the letters of each category allow a more exact location of the lesion, as well as a more precise description structure for the table presented, without ambiguity or subjectivity of operation (Table 10).^12,13 The muscular classification system allows a better reading of muscular injuries, offering a unique and simple nomenclature, which unifies various concepts in the medical field; It facilitates the proper approach of a patient with a muscle injury and that this is not misinterpreted, and therefore his recovery is optimal and fast. It should be noted that this system has not yet been used in controlled clinical studies and therefore not validated, despite being implemented in the sports community (Table 10).^9,11,12,14 

Complications

Among the complications that can occur in muscle injuries (depending on their severity and complexity) can be found muscle scar, muscle hernia, late-onset muscle pain (some do not consider it a complete complication), compartment syndrome and myositis ossificans (Table 10).^3,9,10 

Treatment - recovery

Muscle tears or injuries heal between 3-16 weeks according to the magnitude of the injury and is mediated by the capacity for muscle regeneration - fibrous scarring, so treatment is based on stimulating regeneration and competition with scarring (which is not elastic and predisposing to other injuries). In this phase, ultrasound plays an important role in monitoring the healing process, where the initial cavity occupied by a hematoma gradually begins to fill from its periphery with granulatory tissue (visualized as confluent echogenic foci that end up occupying it completely).^3,7,9,10 The management of muscle injuries is very simple but it must be taken into account for the proper performance of both its diagnosis and its therapeutic management. Lesions classified as mild can heal spontaneously and in a relatively short period of time, while those of a higher degree will require elements for their total restoration.^{3,7,9,10,15–17} Current therapy includes rest (especially for bruising, use of cold and compression therapy (use of a compression bandage for the purpose of pain control as well as the possible extension of the injury and to allow the diffusion of fluids), therapy physical - kinesiotherapy (therapeutic and targeted exercises), and in some cases drugs (NSAIDs, pain relievers and muscle relaxants. In some cases, the use of enzyme injections or the surgical technique of washing liquid extraction is accepted; but it should be avoided to how massages on the area give rise as well as the application of heat, since this favors the extension as well as the inflation of the affected area.^{3,7,9,10,15–17}

In addition to this therapy, there is the use of platelet-rich plasma (PRP), which aims to use growth factors and platelet-released cytokines to stimulate the repair process of a muscle injury, allowing multiple elite athletes being out of competition for a relatively short period of time. Caution in this type of therapy is the basis of these factors, which are fibrinogenic so they can be excessively harmful, due to the generation of more fibrocyte tissue and a risk of recurrence higher than natural; however, there is no consensus within the medical community regarding its use.^{3,7,9,10,15–17} The return of an athlete to their sport must be supported by various pillars such as strength-flexibility (pre-post-injury isokinetic tests, which are included in the athlete's file), imaging technique (RMI - US), tests functional on the affected muscle or muscles (mainly electromyography) and a strategic plan to reduce the risk of a recurrence or even avoid it entirely.^{3,7,9,10,15–17}

Muscle injuries tend to be more frequent acute - chronic symptoms due to the change in lifestyles with the implementation of physical activity and even the desire to perform a competitive sport, for which there is a higher risk of the appearance of the same. Knowing how they develop, risk factors that can influence both presentation and recovery allows a better approach to them. The healing model is divided into 3 phases: destruction, repair and remodeling, all 3 well defined. The destruction phase occurs in the first days, and presents degeneration and active inflammation, causing a breakdown of muscle tissue and necrosis of the myofibrils for the formation of hematoma between them, as well as an inflammatory cell reaction. It is followed by the repair phase, where the necrotic tissue is reabsorbed, with regeneration of the myofibrils and production of an area of ​​connective tissue and neovascularity; This phase occurs between 7-10 days post injury, with a peak after two weeks and a drop between 3-4 weeks. It ends with the remodeling phase, where there is a maturation of the neo myofibrils, a reorganization of the tissue that allows the muscular contractile capacity to be recovered. Once the process and its phases are known, the medical personnel and even the patient himself can know the evolution in the recovery as well as the lapses, means to use for its approach and resolution and even obtain better results by following the plan with understanding of it. Even if both parties are aware of imaging techniques, it is easier to determine the type of injury, its risks, possible complications and specific treatment (medical - pharmacological). Imaging techniques currently have various nomenclatures, which come from different points of view but upon reaching a consensus in the field, a better reading is allowed and therefore a better resolution of the picture of an athlete patient as well as an athlete of elite that allows you to reduce both risks and an increase in benefits for the return of the practice of sport. In other words, the use of general nomenclatures allows an approach aimed at a prompt resolution and restoration of the injury, as well as sports activity, so the use of ultrasound and magnetic resonance imaging, optimally and appropriately, is essential, as well as that anatomical knowledge and interpretation of the findings, for a prompt and effective evolution.

None.

The autor declares there is no conflict of interest.

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