I’ve you’ve been involved in the strength or hypertrophy training community for a while, you may have heard of the repeated bout effect.
This term is particularly common in sport science research.
In this article, we’ll define what the repeated bout effect is, link it to real-world training, and try to explore the potential mechanisms behind the repeated bout effect.
Table of Contents
What Is the Repeated Bout Effect?
When you first begin any training program that is at least moderately intense, high levels of muscle damage is a result.
At the level of the muscle, damage occurs to certain structures within muscle fibers (sarcomeres) and surrounding muscle fibers (extracellular matrix). Additionally, some proteins leak out of muscle fibers into the bloodstream (such as creatine kinase).
Muscle damage is believed to have symptoms that are easily recognized by an individual, such as a decrease in muscle force production, muscle stiffness, and potentially delayed onset muscle soreness.
However, the body seems to possess an adaptive response, whereby after training for a second time, the amount of muscle damage experienced is substantially less.
This is the repeated bout effect.
In other words, the repeated bout effect is the term given to the range of adaptations produced by your body that are protective against muscle damage.
Although the exact mechanisms behind muscle soreness have not been fully elucidated in the research, as mentioned, it seems likely that muscle soreness is at least partly due to muscle damage.
Indeed, if you have been training for some time, you would know that after your first training session, or a training session after a break, high levels of soreness is present.
But after repeating this session for a while, soreness virtually disappears. The repeated bout effect is potentially behind this.
Conditions for the Repeated Bout Effect
For the repeated bout effect to occur, the initial training session must induce muscle damage.
A range of factors would impact how much muscle damage a training session elicits.
All three contraction types concentric, eccentric, and isometric, have the potential to cause damage. However, eccentric training appears to cause the largest effect.
Exercises that involve contractions at long muscle lengths also likely induce more muscle damage than contractions at shorter lengths.
Interestingly, the arm muscles seem to be more susceptible to muscle damage than the leg muscles. This is probably because in everyday life, the leg muscles experience eccentric contractions (such as during walking), and so, some degree of protection is likely already there.
Furthermore, higher rep ranges appear to cause more muscle damage than lower rep ranges when reps are close to failure.
But regardless of these details, most people’s typical hypertrophy or strength program likely induces muscle damage if a person is unaccustomed to it.
Although the repeated bout effect is typically observed when performing the same exercise twice, there is some evidence of the repeated bout effect when performing a different type of exercise the second time.
For example, this study by Chen et al. found that performing maximal isometric contractions at 20 degrees of elbow flexion did elicit adaptations that protected the body for maximal eccentric exercise 3 weeks later.
Most people typically have a routine that includes the same exercises weekly, this of course ensures the repeated bout effect would occur.
However, as somewhat implied by the Chen et al. study, if you were to perform completely different exercises weekly, the repeated bout effect may still be present, though probably to a lesser extent.
For example, let’s say on one week you perform the bench press, the next week you perform chest flies, and the week after that you perform dips.
All three exercises involve the chest, and therefore the body will produce adaptations that protect the chest from being damaged.
However, at the same time, all three exercises do not work the chest identically, different angles, and perhaps different areas of the chest are targeted. Therefore, there very likely will be some degree of damage induced after each of those sessions.
Mechanisms Behind the Repeated Bout Effect
So, we’ve established what the repeated bout effect is and the conditions that allow the repeated bout effect to occur. But what actually causes the repeated bout effect?
In other words, what are the mechanisms that protect the body from experiencing muscle damage?
At the time of writing this, the exact mechanisms behind the repeated bout effect have not fully been identified. However, numerous papers give us an insight into the potential mechanisms behind the repeated bout effect.
Fortunately, there is a great review paper by Hyldahl et al. summarizing the potential mechanisms. Much of the following information is from this review paper.
Overall, there currently appear to be four main areas behind the repeated bout effect: neural adaptations, muscle-tendon adaptations, extracellular matrix adaptations, and the inflammation response.
When a muscle contracts and produces force, it does so because of the signals it receives from the nervous system (brain and spinal cord).
There is evidence that after damaging exercise, the nervous system distributes the load over a greater number of muscle fibers.
This should mean that less load is on one particular area of a muscle, reducing overall damage.
Moreover, there is evidence that the nervous system is able to recruit muscle fibers more easily through adaptations along the neurons that transmit the signals that cause muscle contraction.
During exercises that involve eccentric contractions (lowering of the weight), the muscles stretched. Fascicles are within muscles, and so these of course are stretched during eccentric contractions.
Contractions at long fascicle lengths appear to be quite muscle-damaging.
After damaging exercise, some evidence suggests that the extent to which fascicles stretch in subsequent contractions decreases. This is linked to an increase in the compliance of tendons (the tendons now stretch more), resulting on less loading to the fascicles.
Extracellular Matrix Adaptations
The extracellular matrix is a network of collagens, enzymes, and glycoproteins that surround and cover muscle fibers.
One role of the extracellular matrix is to generate passive tension (force production through stretching) during exercise (particular during eccentric contractions and contracting at long muscle lengths).
This passive tension from the extracellular matrix could potentially mean that muscle fibers would have to produce less force, therefore protecting muscle fibers from damage.
Therefore, adaptations (like collagen synthesis) to the extracellular matrix that allow it to produce greater passive tension could potentially be one factor in the repeated bout effect.
Secondly, evidence suggests that various matricellular proteins increase their activity after damaging exercise. Some of these proteins play a role in enabling various cells to help regenerate damaged muscles.
Lastly, integrins are structures that connect muscle fibers to the extracellular matrix. Adaptations appear to enable them to be stronger and less susceptible to damage.
After damaging exercise, the infiltration of immune cells to damaged areas play an important role in the repair process.
Interestingly, evidence suggests that this response may become enhanced and targeted with the repeated bout effect, thereby speeding up the recovery process.
So, as we can see, there likely is not a single mechanism behind the repeated bout effect, rather a variety of potential mechanisms are likely behind the repeated bout effect.