Do we need to think about connective tissues when strength training?
We tend to focus on the adaptations that occur inside muscle fibers after most types of exercise. However, adaptations also happen outside of muscle fibers, in the connective tissues, such as in the intramuscular connective tissue, tendons, ligaments, and fascia.
But how do different types of exercise affect these structures, and what are the implications of any adaptations that occur?
What are the connective tissues?
As far as the structures that contribute to movement are concerned, there are four main categories of connective tissue: (1) tendons, (2) intramuscular connective tissue, (3) ligaments, and (4) extramuscular connective tissue, which is often called “fascia,” although opinions on the terminology vary.
Tendons attach muscles to bones. They transmit longitudinal forces produced by the contracting muscles to the skeleton so that joints can be flexed and extended, thereby enabling movement.
Intramuscular connective tissue contains muscle fibers, fascicles, and the whole muscle, and transmits forces from the muscle fibers throughout the muscle itself and ultimately to the tendon. There are three main layers of intramuscular connective tissue: (1) the endomysium, which surrounds each muscle fiber, (2) the perimysium, which surrounds each muscle fascicle (group of muscle fibers), and (3) the epimysium, which surrounds the muscle. These structures (especially the epimysium) are sometimes also grouped under the wider term “fascia” alongside the extramuscular tissues, which are very similar in nature.
Ligaments connect bones to one another. Their main role is to provide stability to the joint that they surround.
Extramuscular connective tissue (fascia) contains individual muscles, connects them together, and transmits forces between muscles, causing movements to occur in adjacent segments.
What are connective tissues made of?
Like muscles, connective tissues contain a great deal of water. For example, tendons are approximately 55–70% water. After removing water, however, all connective tissues are largely comprised of collagen.
The collagen content of the various connective tissues varies only slightly, and most of the structures are made primarily out of types I, III, and V collagen, although there are many other types. These different types of collagen serve different purposes. For example, type I collagen forms into fibrils and is largely responsible for the mechanical properties of ligaments and tendons. Type III collagen is involved in collagen repair and development. Type V collagen regulates collagen fibril formation. Changes in the proportions of the various types of collagen will affect the mechanical properties of a tissue, due to the different roles played by each type of collagen.
In addition to collagen, the connective tissues contain a minority proportion of non-collagenous materials, which play distinct roles. Elastin lies adjacent to collagen fibrils and, as its name implies, contributes to the elastic behavior of a tissue. Fibrillin provides a structure for elastin fibers. Small, leucine-rich proteoglycans help regulate collagen fibrils. Large proteoglycans resist compressive loads. Changes in the content of these non-collagenous materials would logically have an influence on the mechanical properties of a tissue, but this has not been widely researched.
The structure of connective tissues is hierarchical.
Material changes in tendons, ligaments, and other structures can therefore occur both in relation to the content at each level and also in relation to the organization of the structures within each level, and such changes can affect the mechanical properties of the tissues. At the smallest level, tropocollagen molecules form collagen fibrils, and these fibrils are chemically crosslinked by enzymatic and non-enzymatic bonds. It has been suggested that increases in the density of the collagen fibrils or in the number of these bonds might increase tendon stiffness. As we ascend the hierarchy, the collagen fibrils are grouped into fibers and the fibers are grouped into fascicles. Both fibers and fascicles display a varying crimp (longitudinal wave) pattern, which may also affect the mechanical properties of the tissues.
How does connective tissue function during muscular contractions?
Each of the connective tissues (tendons, intramuscular connective tissue, ligaments, and extramuscular connective tissue) play important roles in muscular contractions. Since tendons work in series with the muscle, while the other tissues function in parallel, they behave more elastically and influence muscular function to a much more obvious degree.
Tendons connect the muscle to the skeleton. Originally, it was assumed that tendons were rigid links that acted merely to transmit tensile forces from the muscle to the bone. However, it has since become clear that tendons elongate a considerable amount when they are subjected to mechanical tension.
When a material elongates to a greater extent when it is exposed to a greater force, it is described as “elastic” (so long as it returns to its starting length once the force is removed). When a material elongates to a greater extent when a given force is applied more slowly, it is described as “viscous” (so long as it returns to its starting length once the force is removed). Materials that display both elastic and viscous properties are called “viscoelastic.” When a material is viscoelastic, it is not always clear which property will predominate during any given elongation. Whether a viscoelastic material displays mainly elastic behavior or mainly viscous behavior depends on both the force and the rate of force development (RFD) that it experiences.
Since tendons are viscoelastic, they sometimes display viscous properties in a muscular contraction, and sometimes display elastic properties. Generally, when external resistance is small (such as during high-velocity or plyometric exercises), tendons display very elastic properties, and elongate a long way. In contrast, when external resistance is high (such as in heavy strength training), tendons display more viscous properties and elongate only slightly.
Tendons lie in series with muscles, and therefore their behavior has a large impact on muscle force production. Whenever the tendon displays mainly viscous properties, the muscle changes length to the same extent (and therefore at the same speed) as the whole muscle-tendon unit. In contrast, when the tendon displays elastic properties, the muscle changes length to a greater or lesser extent (and therefore at a faster or slower speed) as the whole muscle-tendon unit.
In a concentric (shortening) muscular contraction, muscles exert a higher force and RFD when the tendon is stiff (whether because of its inherent properties or because it is being viscous). When a tendon is stiff, the tendon does not change length once the muscle starts producing force. It simply transmits the muscle force to the skeleton. Therefore, the muscle and the muscle-tendon unit shorten at the same speed. In contrast, when a tendon is compliant, the tendon elongates once the muscle starts producing force, and this causes the muscle to shorten more quickly than the muscle-tendon unit. Since the main factor that determines muscle fiber force production is the force-velocity relationship, this means that the force that is produced is smaller when a tendon is compliant compared to when a tendon is stiff.
During coupled eccentric-concentric or stretch-shortening cycle (SSC) contractions, the same phenomenon occurs but with a different end result. When the tendon is compliant, it again elongates to a greater degree as soon as its muscle starts producing force. Yet, since this occurs during an eccentric phase, it means that the muscle lengthens less than the muscle-tendon unit. Consequently, when the muscle produces force in the subsequent concentric phase, it begins from a shorter length and shortens less, which allows it to exert a greater force. In addition, as the contraction reaches the end of the concentric phase and muscle force starts to decrease, the tendon recoils and releases elastic energy as it shortens, which increases the overall force being exerted at the muscle-tendon junction.
To the extent that this phenomenon contributes to the greater force that we exert in SSC contractions than in concentric-only contractions, it should vary with the external load being lifted. In other words, if tendon compliance can help explain why we exert more force in a SSC contraction, then we should be able to exert proportionally much more force in a SSC contraction relative to a similar concentric-only contraction when the external load is light, but only a little more force when the external load is heavy. This is exactly what has been reported for the bench press and the back squat exercises.
#2. Other connective tissues
The way in which the other connective tissues function during muscular contractions has been less well-studied. However, the research shows that, just like tendons, many of these structures (1) transmit forces between contracting muscle fibers and the skeleton, and (2) store elastic energy in muscular contractions.
The intramuscular connective tissues (particularly the endomysium) are important structures for force transmission between the contracting muscle fibers and the skeleton. Some research has even suggested that a large proportion of the force produced by contracting muscle fibers is transmitted to the tendon laterally through intramuscular connective tissues and not longitudinally along the muscle fibers themselves. Force is transmitted from contracting muscle fibers to the surrounding endomysium by structures called costameres. In contrast, it seems unlikely that the endomysium stores a great deal of elastic energy in muscular contractions, since it does not contribute substantially to force production when the muscle fibers are stretched, in contrast to the titin molecules inside the fibers themselves.
The extramuscular connective tissues also transmits forces between muscles, which causes movements to occur in adjacent segments. Yet, the amount of force that is transmitted varies between tissues around the body. In some cases, a meaningful proportion of the force exerted by muscles in contractions is transmitted transversely to the surrounding fascia. However, in other cases, only small forces are transmitted. Also, the extramuscular connective (fascial) tissues such as the plantar fascia, the iliotibial band (ITB), and the fascia lata store elastic energy during muscular contractions, which enables cyclical movements like walking or running to be more efficient.
In Part 2 we’ll discuss how connective tissue responds to endurance, strength, plyometric, eccentric and high-velocity training, as well as practical applications for athletes and clients.
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