02 Nov 2021

What is the repeated bout effect?

Dr. James L. Nuzzo
Head of Exercise Science Research at Vitruvian
Certified Strength & Conditioning Specialist

Let’s pretend 14 days ago was your first resistance exercise session in nearly a year. You were happy to be back exercising and decided to push yourself. You completed 3 sets of 6 lower-body exercises, with all 18 sets taken to failure – the point at which the resistance can no longer be lifted. 

In the days that followed your workout, you had some regrets about pushing yourself so hard. Your muscles became sore. Two days after your exercise session, you rated your level of muscle soreness as 7 out of 10 on a pain/soreness scale (0 = no pain, 7 = large amount of pain, 10 = maximal pain).

As a result of your soreness, and the joint stiffness that accompanied it, you decided to skip your second “legs days” that week. You rescheduled your second session for the following week, exactly 7 days after your first session. By that time, your pain/soreness score was 1.5 out of 10. Your muscles were nearly fully recovered.

For your second session, you completed the same lower-body workout you did the first time. You performed 3 sets to failure for the same 6 exercises. You also used the same resistances you used last time. 

In the days following your second workout, you noticed you only had a small amount of muscle soreness. The highest level of soreness you experienced after the second session was 3 out of 10, which occured two days after the session. You also did not have much joint stiffness.

Congratulations! You have just experienced the “repeated bout effect.”

The repeated bout effect has been known to exercise scientists for many years. It is studied by having a group of willing participants perform a session of intense resistance exercise, then quantifying how sore they get in the days after the exercise, and then asking them to repeat the same exercise a week later. Almost all participants experience less soreness and stiffness after the second session. 

The repeated bout effect can be thought of as your body building a defence system against future exercise. This defence system ensures that if your muscles are placed under the same stress as before, they will be ready to handle it.

The physiological changes that make up the defense system are not entirely clear. Researchers have proposed a number of specific mechanisms. These mechanisms have been summarised as follows: “adaptations to the nervous system, mechanical behaviour of musculotendinous structures, muscle extracellular matrix, and inflammatory responses.” In other words, a series of physiological events involving the brain, nerves, muscles, tendons, and immune system occur after exercise to ensure your muscles are stronger and protected against future high-intensity exercise.

So, if you have not exercised for a while, and you go hard your first day back, expect to get sore. Muscles become most sore approximately two days after exercise. This is why the term “delayed onset muscle soreness” is used to refer to the soreness experienced after an intense workout. Do not let this soreness scare you or prevent you from doing exercise in the future. It indicates your body is orchestrating a series of events that will make your muscles stronger. The soreness gradually fades, and your muscles will be ready to exercise again in a few days. The “repeated bout effect” has your back!


High levels of muscle soreness can be avoided when exercising with your V-Form or Vitruvian+ by carefully considering the resistance lifted, exercise volumes completed (sets x repetitions), and exercise modes selected. Heavy resistances, greater exercise volumes, and eccentric training will cause the highest levels of soreness. The best way to avoid high levels of muscle soreness during your first week of training is to use light or moderate resistances and to complete a low or moderate number of sets and repetitions. Then, in subsequent weeks, gradually increase the resistances and numbers of sets and repetitions. Also, because eccentric muscle contractions cause more muscle soreness than concentric muscle contractions, do not use maximal resistances or high training volumes if you plan to use “eccentric only” mode in your first week of training. Save intense eccentric training for subsequent weeks, after your muscles have become accustomed to exercise with lighter or moderate resistances.


Hyldahl RD, et al. Mechanisms and mediators of the skeletal muscle repeated bout effectExercise and Sports Sciences Reviews 45: 24-33, 2017. DOI: 10.1249/JES.0000000000000095.

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