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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.
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.
How do connective tissues respond to endurance training?
When exposed to the repetitive but low forces that are common to endurance training, connective tissues respond by displaying fatigue damage. Fatigue damage is a materials science term and does not mean “fatigue” in the wider context of exercise science. It describes the process by which a structure degrades after being exposed to cyclical or constant loading.
In living animals, this fatigue damage leads to a repair process. If this repair process can keep pace with the rate at which the damage accumulates (as is usually the case), then homeostasis is maintained. However, in the dead connective tissues that are used in some research studies, or when the repair process in living animals cannot keep pace with the rate at which damage accumulates, more severe damage occurs. This process has been proposed as an underlying mechanism that leads to tendon rupture or certain types of tendinopathy, and may also underpin the development of some fascial disorders, such as plantar fasciitis.
How do connective tissues respond to strength training?
Each of the connective tissues (tendons, intramuscular connective tissue, ligaments, and extramuscular connective tissue) adapt to long-term training. However, since the tendons work in series with the muscle, while the other tissues function in parallel, their adaptations have different implications.
When exposed to sufficiently high forces, tendons adapt initially by becoming stiffer (increasing in Young’s modulus) and later by increasing in size (albeit only in the peripheral regions, as the central region does not increase in size after adolescence). Exactly why there is a disconnect in time between the adaptations in tendon stiffness and size is unclear.
Some researchers have suggested that the process by which tendons increase in stiffness and size is a continuous one, and that increases in stiffness are early indicators of later increases in size. In this model, the mechanical loading produced by high forces stimulates both (1) an increase in collagen content, which is apparent initially as an increase in collagen density, and (2) an increase in water content, associated with the proteoglycans. Together, these adaptations lead to a very quick increase in tendon stiffness. Later, once the increase in collagen content reaches a certain threshold, there is an increase in tendon size.
Alternatively, it is possible that the early adaptations in stiffness could result from other changes inside the connective tissue, such as an increase in the number of cross-links between collagen molecules, or a change in the structural arrangement of the collagen molecules or fibrils. However, to date, research has been unable to detect any such changes.
Although there is a widespread belief that connective tissues in general, and tendons and ligaments in particular, can be strengthened by using a large number of repetitions of relatively light loads, this is not the case. In fact, tendons only increase increase in stiffness when the external loads are heavy (5RM+) or moderate (6–15RM). Light loads are unable to produce increases in tendon stiffness. Similarly, heavy or moderate loads are necessary for tendons to increase in size after long-term training.
Importantly, this means that the effects of load on muscles and tendons is different. Muscles increase in size to largely the same extent when training with light, moderate, and heavy loads, so long as sets are taken close to muscular failure. This is because it is mechanical loading that triggers muscle fibers to increase in size is applied by the individual muscle fibers, and the amount of mechanical loading is determined largely by the force-velocity relationship. Consequently, long-term strength training programs using heavy or moderate loads might be expected to produce concomitant increases in muscle size and tendon stiffness (and size), but long-term strength training programs using light loads might be expected to produce increases in muscle size without any simultaneous increases in tendon stiffness (and size).
In addition to the external load, tendons are affected by the duration of the rest period between contractions. Very short rest periods and very long rest periods both seem to be less effective than moderate (3-second) rest periods between each rep of a set. Very short periods seem to be especially ineffective, and promote greater fatigue damage but smaller stiffness and size adaptions, perhaps due to elevated shear stress caused by increased fluid flow.
#2. Other connective tissues
The way in which the other connective tissues respond to long-term strength training has not been well-studied. Even so, some research indicates that intramuscular collagen synthesis rates and expression are elevated after heavy strength training workouts, and intramuscular collagen content does seem to increase after long-term training programs. As for tendons, it is likely that the other connective tissues respond best to higher loads with sufficient rest periods between reps, and are more likely to display fatigue damage rather than beneficial adaptations when loads are lighter and loading cycles are more frequent.
How do connective tissues respond to plyometrics?
What are plyometrics?
Plyometrics are high-velocity movements that involve the SSC. Some coaches and researchers define plyometrics as any high-velocity movement that involves the SSC, while others include only a subset of such movements. One important subset of high-velocity movements that involve the SSC is the group of exercises that involve an impact that triggers the start of the eccentric phase. Examples of such exercises include the drop jump, bounding, hopping, and sprinting.
This definition is useful, because it fits well with the way in which plyometrics are used for preparing athletes for sport, and it also allows a very clear biological model to be developed to explain what is happening.
Defined in this way, plyometrics involve two phases: (1) forcible stretching of an already-activated muscle (the muscle is preactivated prior to landing, and the force of the landing then stretches the muscle while it produces a very high force), and (2) rapid force production of a shortening muscle. This means that plyometric training subjects the muscle-tendon unit to very high forces while muscle fibers are lengthening and also subjects the muscle-tendon unit to very high fascicle shortening velocities. As a result, we should expect muscle-tendon unit adaptations to comprise a mixture of those associated with eccentric training and with high-velocity training.
What are the adaptations to eccentric training?
Eccentric training causes large increases in eccentric strength and smaller (but still substantial) increases in maximum (concentric) strength. Eccentric training also differs from concentric and SSC strength training insofar as a large proportion of the muscle fiber growth that occurs is longitudinal rather than transverse.
The increases in eccentric and maximum (concentric) strength are achieved through increases in voluntary activation (motor unit recruitment), muscle fiber size, lateral force transmission, and tendon stiffness. The gains in eccentric strength are enhanced by increases in muscle titin and collagen content. Importantly, this does not necessarily lead to an increase in passive muscle stiffness, because the increase in muscle titin and collagen (which increase passive muscle stiffness) is counterbalanced by the increase in fascicle length (which decreases passive muscle stiffness).
What are the adaptations to high-velocity training?
High-velocity training causes large increases in high-velocity strength and smaller (but still substantial) increases in maximum (concentric) strength. When the loading used is conventional weight (and not isokinetic resistance) and there is no landing phase, there is little mechanical loading on the muscle fibers, and consequently minimal hypertrophy.
The increases in high-velocity and maximum (concentric) strength are achieved through increases in voluntary activation (motor unit recruitment) and reductions in antagonist muscle activation. The gains in high-velocity strength are further enhanced by increases in rate coding and as-yet unknown alterations in muscle fiber contractile properties that enhance maximum fiber shortening speed.
What are the known adaptations to plyometric training?
The adaptations that occur after plyometric training are not easy to predict and vary widely depending on the exact exercise used, since this affects the proportional amount of stimulus that arises from the eccentric and high-velocity phases of the movement.
- Voluntary activation — like both eccentric training and high-velocity strength training, plyometric training increases voluntary activation (motor unit recruitment) that transfers to increases in maximum strength.
- Tendon and passive muscle stiffness — like eccentric (but unlike high-velocity) training, plyometric training often increases tendon stiffness (albeit to a lesser extent than heavy dynamic strength training or isometric strength training) and sometimes also increases passive muscle stiffness in part by increasing muscle collagen content. Yet, some research has found no changes in tendon stiffness or passive muscle stiffness after plyometric training, most likely because of the relatively low time under tension that is experienced by the muscle-tendon unit. Indeed, higher volumes of plyometric training tend to cause proportionally larger increases in eccentric strength (vertical stiffness) while lower volumes tend to cause proportionally larger increases in high-velocity strength.
- Muscle fiber size — like high-velocity strength training, plyometric training usually does not cause any meaningful increases in muscle fiber size. However, when there is a meaningful amount of force exerted in the eccentric phase, muscle fiber growth can occur, and it tends to be longitudinal, as we would expect from eccentric training.
How do connective tissues adapt to plyometric training?
We know that connective tissues (including muscle collagen and tendons) do adapt after plyometric training, most likely due to the eccentric loading that is experienced. This loading allows high forces to be experienced by the tissues, leading them to increase first in stiffness and later in size.
Even so, exactly the same adaptations also happen after long-term heavy strength training. The interesting feature of plyometric training is that the proportional changes in active muscle stiffness and tendon stiffness differ from those that occur after heavy strength training.
Increases in active muscle stiffness are greater after plyometric training than after heavy dynamic strength training or isometric training, while increases in tendon stiffness are smaller. This is logical, given that the eccentric loading stimulus is much greater during plyometric training, but the total mechanical loading stimulus to the tendon is greater during heavy strength training. Consequently, the ratio of muscle-to-tendon stiffness probably increases after plyometric training but seems to remain the same or decrease slightly after heavy strength training or isometric training.
As a result, after heavy strength training or isometric training, the amount of tendon lengthening during a SSC contraction remains the same or decreases, because the tendon is now stiffer relative to the active stiffness of the muscle. In contrast, after plyometric training, the amount of tendon lengthening in a SSC contraction increases, because the tendon is now less stiff relative to the active stiffness of the muscle (absolute tendon stiffness increases, but active muscle stiffness increases by much more). This adaptation allows the muscle to remain at a shorter length (and change length at a slower speed) during the SSC contraction, thereby exerting a higher force.
What are the practical implications?
Athletes who perform rapid SSC movements (such as jumping, sprinting, and running while changing direction) will benefit greatly from using similar plyometrics in their training programs, to optimize the ratio of active muscle stiffness to tendon stiffness for SSC function in those movements. In contrast, overuse of heavy strength training or isometric training may have negative effects, because the ratio of active muscle stiffness to tendon stiffness will be altered such that the muscle must lengthen too far and too quickly during the SSC movements for it to produce force optimally.
Bodybuilders who use light loads exclusively may be at a greater risk of overuse injury to the connective tissues, owing to regular and sustained exposure to fatigue damage without the beneficial adaptations that occur after using moderate or heavy loads. Bodybuilders who prefer to use light loads may benefit from interspersing their customary training with regular training cycles of moderate or heavy loads to bring connective tissue stiffness and size to a level commensurate with their muscle size. When carrying out such cycles, using 3-second inter-rep rest periods may be optimal, as this will maximize tendon (and likely other connective tissue) adaptations.
What is the takeaway?
The connective tissues of the body (including the intramuscular and extramuscular connective tissues, tendons, and ligaments) contribute to muscular function and adapt after strength training and plyometrics. Understanding how and why these adaptations happen can help to reduce the risk of overuse injury and enhance performance.
If you enjoyed this article, you will like my first book (see on Amazon).