In Part 1, we discussed the properties and function of connective tissues. In this post, we’ll discuss the different variations of exercise and how connective tissues respond.
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 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.
- 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 involves 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 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.
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