Why does eccentric overload training not cause more hypertrophy than normal strength training?

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Eccentric overload training tends to produce similar hypertrophy to normal strength training. This is strange, because we might expect the combination of eccentric training and concentric training within the same set to cause greater muscle growth compared to normal strength training. Why does this superior muscle growth not happen?

What is eccentric overload training?

In this article, when I refer to “eccentric overload,” I am talking about strength training that involves maximal efforts in the lowering (eccentric phase) of the exercise, in the same way that eccentric training involves maximal efforts in the lowering (eccentric phase) of the exercise. This is an important point, because the term “eccentric overload” is often incorrectly used to refer to slowing down the lowering (eccentric) phase during normal strength training. Yet, these types of training are fundamentally different from each other, at least from a physiological point of view.

Eccentric overload involves maximal efforts on every rep, and this means that it involves a very high level of motor unit recruitment on every rep. This in turn means that a lot of muscle fibers are activated on every rep, and are therefore stimulated to grow. In contrast, slow lowering (eccentric) phases during normal strength training involve submaximal efforts on each rep (and therefore a low level of motor unit recruitment). This means that only a relatively small proportion of the fibers in the muscle are activated and stimulated to grow.

How does fatigue develop during sets performed with different contraction types?

In my view, the reason why eccentric overload training does not produce greater hypertrophy than normal strength training is because of differences in the way that fatigue develops during [A] normal strength training, and [B] eccentric overload training. While it is often assumed that the fatigue that occurs in concentric and eccentric contractions is very similar (and therefore has similar effects), this is not actually the case.

During normal strength training, lifters perform stretch-shortening cycle actions comprising both lifting phases (concentric contractions) and lowering phases (eccentric contractions). At the start of the set, the level of motor unit recruitment (and therefore the number of activated muscle fibers) is necessarily much higher in the lifting phase than in the lowering phase, because of the extra passive tension that muscle fibers exert when they are actively stretched (more force per muscle fiber means fewer muscle fibers need to be activated to produce a given force).

Since the fast twitch muscle fibers of higher threshold motor units are more easily-fatigued than the slow twitch muscle fibers of lower threshold motor units, fatigue develops more quickly during the lifting (concentric) phase than in the lowering (eccentric) phase.

In concentric contractions, the formation of crossbridges is relatively inefficient, which leads to a high energy usage, and this in turn causes metabolite accumulation to occur quite quickly. The accumulation of metabolites primarily causes muscle fiber shortening speed to slow down, by means of acidosis (the effects of phosphate ions on force production are delayed by comparison). Importantly, this slowing down of the muscle fiber does not reduce the force that it produces, which means that the fiber continues to experience mechanical tension (which is the factor that stimulates muscle fibers to increase in size).

Obviously, acidosis has no fatiguing effect on eccentric contractions, because these involve muscle fiber lengthening, and not muscle fiber shortening. Reducing the speed at which the muscle fiber shortens cannot really influence the force that is produced while the same muscle fiber lengthens. Therefore, the primary type of fatigue that is generated by concentric contractions in the lifting phase of normal strength training does not affect the ability of muscle fibers to produce force in the lowering phase. Consequently, while effort (and therefore also motor unit recruitment) will increase due to increasing fatigue in the lifting phase as proximity to failure approaches, the same increases in effort (and motor unit recruitment) do not occur in the lowering phase.

In concentric-only training, fatigue progresses in very much the same way as in normal strength training. Metabolite accumulation occurs, which reduces muscle fiber shortening speed. When the strength training set is being done with a self-selected tempo, this then requires an increase in motor unit recruitment such that additional muscle fibers are activated to compensate, and the desired tempo can be maintained. Again, this slowing down of the muscle fiber does not reduce the force that it produces, which means that the fiber continues to experience mechanical tension (which is the factor that stimulates muscle fibers to increase in size).

In eccentric-only training, fatigue progresses differently compared to in normal strength training and in concentric-only training, for two important reasons. Firstly, eccentric contractions are very efficient, and do not require the same amount of ATP as concentric contractions in order to produce force, partly because of the large role played by the passive elements (which do not require energy), and partly because each crossbridge requires less energy to be pulled open than it does to form in the first place. For this reason, metabolite accumulation is fairly minimal during eccentric contractions. Secondly, the stretching of muscle fibers while they are activated opens stretch-activated ion channels, which cause a large number of calcium ions to enter the muscle fiber from the surrounding area. This causes calcium ion-related fatigue mechanisms to occur much more quickly than they do during isometric or concentric contractions.

The main types of calcium ion-related fatigue are [A] excitation-contraction coupling failure, and [B] reductions in sarcolemmal excitability. These types of fatigue do not cause a reduction in muscle fiber shortening speed. Rather, they both cause a dramatic reduction in muscle fiber force by preventing the crossbridges inside the muscle fiber from forming in the first place. This is why eccentric-only training causes much larger reductions in strength after a set than concentric-only training (but causes smaller changes in maximum muscle shortening speed). It is also why continuing to perform more and more reps of eccentric contractions in a set doesn’t continue to produce more and more hypertrophy. The calcium ion-related fatigue suppresses the ability of muscle fibers to produce force (and therefore experience mechanical tension) in a way that metabolite-related fatigue does not.

During eccentric overload training, the level of effort (and therefore the level of motor unit recruitment) is high in both the lifting and lowering phases of an exercise. Hence, the easily-fatigued, fast twitch muscle fibers experience fatigue from both sets of mechanisms (metabolite accumulation in the lifting phase and calcium ion-related mechanisms in the lowering phase).

This has a very interesting effect.

Although the primary fatigue mechanism that occurs during concentric contractions (metabolite accumulation) does not affect force production in eccentric contractions, the reverse is not true. Calcium ion-related fatigue mechanisms that are provoked during eccentric contractions absolutely do reduce force production during concentric contractions.

Consequently, during eccentric overload training, the muscle fibers of the high-threshold motor units start to experience excitation-contraction coupling failure from the very first rep of the set. This is not a major problem for any adaptations that are stimulated by the lowering phase, since that happens during eccentric-only training anyway. However, it is a big problem for any adaptations that might be stimulated by the lifting phase, because it causes reductions in muscle fiber force that would not occur in normal strength training or in concentric–only training (where metabolite accumulation is the primary mechanism of fatigue).

How does hypertrophy occur after training with different contraction types?

One interesting feature of strength training is that it causes different types of hypertrophy depending on the extent to which passive tension is experienced by the activated (and responsive) muscle fibers. When muscle fibers experience a lot of passive tension due to titin being stretched, they tend to grow by increasing in length. When they experience mechanical tension without passive tension, they tend to grow by increasing in diameter. We can use this observation to draw inferences about how hypertrophy is stimulated after different types of strength training.

After normal strength training and concentric-only training, most (if not all) muscle fiber growth occurs in the form of increases in muscle cross-sectional area. Unless the concentric phase starts from a stretched position (which causes passive tension), there is no substantial increase in fascicle length. This tells us that the muscle fibers that belong to the high-threshold motor units (which are the ones that respond to strength training) do not experience very much passive tension during these types of training.

This is logical if we take into account the fact that concentric-only training does not involve a lowering phase, and therefore cannot experience passive tension (unless each concentric phase starts from a stretched position). It is also logical if we take into account the fact that normal strength training involves a relatively low level of muscle activation in each lowering phase, such that only a relatively small proportion of the fibers in the muscle are activated and stimulated to grow longitudinally.

After eccentric-only training and eccentric overload training, most muscle fiber growth occurs in the form of increases in muscle fiber length. In contrast, there are comparatively small increases in muscle cross-sectional area (and probably even smaller increases in muscle fiber diameter, since some increases in muscle cross-sectional area can arise due to increases in muscle fiber length).

This tells us that both eccentric-only training and eccentric overload training mainly cause muscle fiber growth due to experiencing passive tension in the eccentric phase. While this may be obvious for eccentric-only training, it is perhaps surprising for eccentric overload training, because it implies that the concentric phase doesn’t really do very much for hypertrophy.

Why might this happen? There is an obvious explanation.

Unlike in normal strength training, considerable fatigue is produced in the eccentric phase of eccentric overload strength training. While the metabolite-related fatigue produced in concentric phases does not affect eccentric muscle fiber force production, the calcium ion-related fatigue produced in eccentric phases does affect concentric muscle fiber force production.

In fact, by the time a set of eccentric overload training reaches the final few reps of the set (in which concentric phases are able to stimulate hypertrophy), the level of calcium ion-related fatigue is has reduced muscle fiber force to the point where the mechanical tension cannot stimulate much muscle growth. Thus, only the eccentric phase is able to trigger hypertrophy in eccentric overload training, which is why it mainly increases muscle fiber length, in the same way as eccentric-only training.

What is the takeaway?

Eccentric overload training does not cause more hypertrophy than normal strength training, and this probably happens because of the different type of fatigue that develops in the eccentric phase of each rep, which impairs the level of mechanical tension that is produced (and therefore experienced) by muscle fibers in each concentric phase to a greater degree than occurs during normal strength training. This reduces the stimulus that is experienced in the concentric phase, and thus most of the hypertrophy is actually caused by the eccentric phase (which we can see must be the case, because the hypertrophy that occurs after eccentric overload training is mainly in the form of increases in fascicle length, the same as after eccentric-only training).

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