In recent years, there has been a surge of interest in eccentric training, either in the form of supramaximal, eccentric-only training or in the form of eccentric overload training. There are now many technologies available that allow eccentric training to be performed for many common exercises. Yet, concentric-only training is also a valid method for producing strength gains and hypertrophy, and it has its own advantages and disadvantages, in much the same way as eccentric-only training.

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#1. Concentric-only and eccentric-only strength training produce similar hypertrophy

Although eccentric-only training is often believed to cause greater muscle growth than concentric-only training and conventional strength training, eccentric-only training causes only a very small (and uncertain) superior effect on muscle growth. In fact, when the literature is examined in totality by meta-analyses, hypertrophy is not significantly different after eccentric-only training and after concentric-only training.

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While this may appear counterintuitive, given that eccentric contractions allow muscle fibers to experience greater levels of mechanical tension, it can be easily explained by the fact that eccentrics typically allow lower levels of motor unit recruitment to be attained, especially in populations who do not regularly use eccentric contractions. Thus, eccentric-only does cause the muscle fibers that are activated to experience greater mechanical tension (and thus experience greater hypertrophy), fewer muscle fibers are activated.

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Indeed, when the problem of muscle activation is removed, by using electrically-stimulated contractions in rodent models, the amount of hypertrophy increases in tandem with increasing mechanical tension. This demonstrates that the reason that eccentric-only training does not produce much greater hypertrophy than concentric-only training in humans is the deficit in voluntary activation.

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#2. Concentric-only and eccentric-only strength training produce different muscle fiber hypertrophy

Although concentric-only and eccentric-only training produce similar overall hypertrophy, the nature of the hypertrophy differs. Eccentric-only training tends to cause greater muscle fiber growth through sarcomerogenesis, while concentric-only training tends to induce greater increases in muscle fiber diameter. This is relevant for athletes who wish to reduce their risk of muscle strain injury, which seems to be affected by muscle fiber lengths.

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#3. Concentric-only and eccentric-only strength training produce different regional hypertrophy

Although concentric-only and eccentric-only training produce similar overall hypertrophy, the location of the hypertrophy differs. Eccentric-only training tends to cause greater muscle growth in the distal part of the muscle, while concentric-only training tends to greater muscle growth in the middle region. While this effect was originally believed to occur due to eccentric-only training causing greater increases in fiber length due to sarcomerogenesis, recent research has shown that the effect is more wide-ranging. Although eccentric-only training does tend to cause greater increases in muscle fiber length (and smaller increases in muscle fiber diameter) compared to concentric-only training, there is also a genuine effect of muscle region.

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#4. Concentric-only strength training causes less muscle damage

One very large benefit of concentric-only training in comparison to eccentric-only training is that it produces much less muscle damage (and therefore less fatigue from one workout to the next). This may mean that concentric-only training can be performed more frequently than eccentric-only training, and this may translate to greater long-term gains in muscle size, if more workouts can be performed in the same period of time.

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Nevertheless, concentric-only training can still cause muscle damage. This might seem odd. However, it is easily explained. Muscle damage is not actually caused by muscle fibers being stretched until they are torn apart. In fact, muscle damage is caused by a series of biochemical processes that result from muscle fibers being activated. Consequently, any muscular contractions that involve a sustained period of muscle activation (including isometrics, concentrics, and eccentrics) can cause muscle damage. Greater damage occurs when a greater influx of calcium ions occurs in response to the muscle fiber being activated. The reason why eccentrics cause greater muscle damage is not because they involve greater mechanical tension, but because they open stretch-activated ion channels, which allow more calcium ions into the muscle fiber, while it is activated.

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#5. Concentric-only strength training removes the repeated bout effect

One interesting feature of concentric-only training is that it removes the repeated bout effect that is experienced after eccentric-only training workouts. In other words, when performing concentric-only training, a lifter will experience a large amount of muscle damage by incorporating a single eccentric-only training workout, or by performing exercise that involves eccentric contractions.

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This may be because concentric-only training could actually reduce the number of sarcomeres in series in a muscle fiber, as well as produce predominantly increases in muscle fiber diameter. Thus, when eccentric contractions are performed, this causes greater muscle fiber stretch, which causes greater opening of stretch-activated ion channels, a greater influx of calcium ions, and therefore greater muscle damage.

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Conclusions

Concentric-only training appears to cause only slightly less hypertrophy than eccentric-only training, and probably causes similar muscle growth to normal strength training. Yet, concentric-only training tends to cause greater increases in muscle fiber diameter and greater increases in the size of the middle region of a muscle, while eccentric-only training tends to cause greater increases in muscle fiber length and greater increases in the size of the distal region of a muscle. While both eccentric-only and concentric-only training cause muscle damage, damage is greater after eccentric-only training, likely due to the greater calcium ion influx that occurs due to the opening of stretch-activated ion channels. This may make concentric-only training an interesting option for bodybuilders, such that they can recover more quickly and train a muscle more frequently.