Muscle injuries, from muscle fibre to system

New insights show that a muscle injury involves not only the muscle fibres themselves, but the entire connective tissue complex connected to them. This includes not only the layers within the muscle, such as the endomysium, perimysium and epimysium, as well as the larger structures that surround the muscle surrounded and connect with it. The aponeurosis, which lies partly ín and partly outside the muscle, is an important link in the force transmission between muscle fibres and tendon. In addition, the surrounding fascia also plays a role. The fascia is an external connective tissue layer that connects the muscle to adjacent structures and contributes to the lateral transmission of forces within the myofascial network. These structures function closely with the active structures or contractile elements(muscle fibres). Insights show that muscle injuries mainly occur within the transitions between muscle fibres and connective tissue. And thus often not isolated in the muscle fibres. Because these different connective tissue layers each have unique mechanical properties, the type of tissue damaged can have a major impact on recovery and rehabilitation time after a muscle injury.

Thus, a muscle injury does not always involve damage to the muscle fibres themselves. Often the problem lies in the connective tissue that surrounds the muscle in through the muscle, such as the fascia or aponeurosis. Therefore, it is important to properly name where the injury is contained and which fabric is affected. Because this affects recovery time and treatment.

Fascia, aponeurosis and tendon together form a continuous structure of connective tissue that transmits muscle power. The difference is mainly in the thickness and direction of the collagen fibres. 

• Fascia has a crosswise or random pattern of fibres and can therefore absorb forces from multiple directions. Is more elastic and flexible. 
• The fibres of the aponeurosis already run in a more parallel direction, making it stronger when pulling in a specific direction, but less flexible. Especially in muscles capable of producing a lot of explosive force.  
• A tendon has the most structured, straight fibre structure. Pretty much all in the same line as the tensile force, providing maximum strength but little flexibility. 

As you go from fascia, aponeurosis and tendon, the arrangement and strength of connective tissue increases, while elasticity decreases. 

So a muscle consists not only of muscle cells, but also of the connective tissue that supports and connects these cells. A muscle fibre (muscle cell) is elongated, cylindrical and has several cell nuclei. The main function of the muscle fibre is, of course, to provide force for movement. The length of a muscle is very different the ear or face has fibres that are only a few millimetres long and the longest muscle in the human body is the sartorius, also called the tailor muscle can be up to 50 cm long in adults. As with many other structures in our body, each muscle also has small blood vessels, nerve tissue and immune cells. Together, they allow a muscle to contract, transmit force and recover properly.

Each muscle fibre consists of thousands of myofibrils. These are made up of small parts called sarcomeres. The more sarcomeres there are in succession, the further a muscle can shorten.

• In a long muscle like the sartorius are estimated to be over 200,000 sarcomeres back to back.
• In the hamstrings approximately 100.000.
• In the calf muscle (gastrocnemius) around the 20.000.

Muscles with long fibres, such as the sartorius, can extend far but are less powerful. Muscles with short, oblique fibres, such as the gastrocnemius, can extend less far but are many times more powerful.

Within our muscles is a sophisticated network of connective tissue that has a much greater role than has always been thought. This intramuscular connective tissue in the literature often referred to as intramuscular connective tissue (IMCT) or the muscle's extracellular matrix (ECM) not only provides the structure and cohesion of muscle fibres, but is also essential for how force is transmitted during muscle contraction.

Although we often think of muscles as individual fibres that contract to cause movement, that picture is too simplistic. Together, the endomysium, perimysium and epimysium form a three-dimensional network that connects muscle fibres to each other and to surrounding structures. Instead of isolated units, the muscle forms a cooperative system, in which force is transmitted both longitudinally and circumferentially between the different fibres and muscle bundles.

The extra cellular matrix consists of several layers, each with a specific structure and function. The endomysium comprises each individual muscle fibre and consists mainly of collagen type I, III and V. The fibres within this layer are particularly thin and lie in an undulating pattern around the muscle fibres, providing a degree of elasticity and stretchability under load. Between the basal membranes of adjacent fibres, the endomysium forms a kind of honeycomb structure that allows communication between muscle fibres.

Around it lies the perimysium, which encloses these muscle bundles or fascicles. This layer contains thicker collagen fibres and a small amount of elastin. The perimysium plays an important role in the distribution and transmission of forces within the muscle and functions as an internal regulator of force that coordinates the direction and tension of muscle fibres.

The outer layer, the epimysium, surrounds the entire muscle. This layer contains mainly collagen type I and III and is often thicker and lamellar in pennate muscles. It forms a transitional structure towards the aponeurosis or tendon and thus contributes to efficient force transmission.

Importantly, these layers do not function in isolation. Together, they form a network that through the myofascial and myotendinous transition area (the myofascial junction and myotendinous junction) literally anchors the muscle to the surrounding connective tissue and tendon. This allows the strength of each individual muscle fibre to ultimately contribute to the movement of the entire system.

Besides this mechanical function, intramuscular connective tissue also has a biological role. It contains fibroblasts and other cells that send signals that affect the repair ability and growth of muscle tissue. It forms the structure through which new muscle fibres orientate and can develop after injury or damage.

In practice, the term fascia often used as an umbrella term or collective name for this system, but that does not do justice to the complexity of the different layers. The extra cellular matrix is not a passive envelope, but a dynamic network that combines structural, mechanical and biological functions.

The aponeurosis is an interesting structure within the musculoskeletal system. While muscles and tendons have been studied extensively, the aponeurosis has received remarkably little attention in the scientific literature by comparison. Yet this fibrous connective tissue forms an important link between muscle and tendon. The aponeurosis serves as a transitional area where muscle fibres transfer their force towards the tendon.

The names used for aponeurosis are not always clear. Several terms appear in different studies and books, such as central tendon, intramuscular tendon or intertendinous structure. This sometimes makes it confusing to know exactly what is meant by it. In fact, all these words describe the same thing. It refers to a firm connective tissue layer within the muscle that helps transfer force from the muscle fibres to eventually the free tendon. 

The aponeurosis is a special part of the musculoskeletal system. Although muscles and tendons have been widely studied, relatively little is known about the aponeurosis. Yet this connective tissue layer plays an important role in the transmission of force between muscle fibres and tendon. The aponeurosis lies partially inside the muscle and extends to the tendon. It thus forms the connection between the active muscle fibres and the firm tendon tissue. For a long time, it was thought that the aponeurosis was simply an extension of the tendon, but that image is not entirely accurate.

Research shows that aponeurosis and tendon do not simply function as two separate parts back to back, but neither do they work completely simultaneously. Together, they form a flexible system that distributes and transmits forces in a way that suits the specific muscle.

In muscles like the gastrocnemius or rectus femoris, the aponeurosis functions as an anchor where muscle fibres attach and can transfer force to eventually the tendon. In other muscles, the aponeurosis is actually thinner and more flexible. Here, the aponeurosis allows for more active force transfer between muscle fibres and smooth continuation of that force within the muscle itself.

Thus, the aponeurosis does not have a universal function, but rather adapts to the specific requirements of the muscle. Muscles that perform mainly explosive power tend to have a stiffer aponeurosis, while muscles that perform longer movements benefit from a more elastic structure.

Together with the muscle fibres and tendon, the aponeurosis forms the so-called muscle-tendon unit, also called muscle-tendon unit (MTU). The force generated in the muscle passes through the aponeurosis to the tendon and from there is transferred to the bone, allowing movement.

Although often referred to as the myotendinous transition, in many muscles the transition is not directly between muscle and tendon, but between muscle fibres and aponeurosis. In that case, it is better to speak of a myoaponeurotic transition. Microscopically, this area consists of folded connective tissue structures that penetrate deep into the muscle fibre wall and anchor in the perimysium and aponeurosis. These folds increase the contact area and provide a strong yet flexible connection that can absorb and transmit forces efficiently.

The muscle tendon unit has several properties. It can stretch, store energy and adapt to load. During movement, muscle, aponeurosis and tendon constantly switch roles. The muscle fibres actively generate force, while the aponeurosis and tendon store and release elastic energy. This creates an efficient system that enables strength, speed and control.

The aponeurosis and tendon can become stiffer or more flexible, depending on the load they are subjected to. These structures are trainable to a certain extent. This explains why a well-tuned training load is essential for both performance and injury prevention.

In practice, injuries are still often only referred to as muscle or tendon injuries, whereas many injuries actually occur in the aponeurotic or myotendinous transition. The injury is often located in the transition zone between muscle fibre, aponeurosis and extracellular matrix. The precise location and extent of the injury largely determine the recovery time and risk of relapse.

Anatomically, muscle and tendon merge seamlessly through a connective tissue continuum. The perimysium, which surrounds the muscle bundles, flows into the tendon structures, while the epimysium thickens at the end of the muscle and merges with the tendon. This continuum shows that muscles do not function as separate structures, but are part of a larger myofascial network. 

While much is known about muscle contraction and strength development, we still know relatively little about the adaptive mechanisms of the intramuscular connective tissue itself. So far, the trainability of this tissue remains largely underexplored, whereas this is often the key to injury prevention, recovery and optimal functioning of this entire system.

The tendon forms the end of the muscle-tendon unit and is responsible for transferring force from the muscle to the skeleton. It has a layered structure that provides both strength and flexibility during movement. This structure is very similar to the structure of muscle tissue. With an epitenon, endotenon and peritenon. 

On the outside, the tendon is surrounded by the epitenon. Around this, in some cases, there is an additional layer, the paratenon. This tissue is more elastic and well supplied with blood, allowing the tendon to move better in relation to the surrounding structures, such as muscle or skin. Together, the epitenon and paratenon form the peritenon, which acts as a gliding sheath that reduces friction and facilitates movement. In parts of our body that have a lot of range of motion, such as the ankle or wrist, this system often also contains a thin synovial membrane that provides additional lubrication.

Inside the tendon is the endotenon. This is a fine network of connective tissue that envelops each tendon fibre and joins multiple fibres together to form larger bundles, the tendon fascicles. 

The tendon is further surrounded by the deep fascia, which includes both muscles and tendons and contributes to the structural cohesion between the two. On ultrasound, these layers are sometimes difficult to distinguish; the paratenon and any tendon sheath can look very similar.

At the end of the tendon is the enthesis, the transition between tendon and bone. This transition is stepped: from tendon tissue to fibrocartilaginous tissue, then to calcified cartilage and finally to bone. This gradual transition allows forces to be fluidly distributed, reducing the risk of injury under high loads.

Muscle, aponeurosis, tendons and fascia together form a coherent system that provides strength, movement and stability. This connective tissue continuum makes it clear that muscle tissue cannot be separated from its supporting structures. What we often think of as a muscle injury in practice is, in reality, often damage to the connective tissue surrounding or intertwined with the muscle.

A good understanding of the anatomy, structure and adaptive mechanisms of this system is essential for correct diagnosis, effective treatment and optimal recovery. Not only muscle strength, but also other properties such as elasticity, stiffness and restorative capacity are important to consider.

For physiotherapists, trainers and researchers, there is an important challenge here. By looking beyond the muscle fibre alone, a better understanding of these injuries and more opportunities to properly treat and hopefully prevent them will emerge