CrossFit aims to increase fitness in various ways and systems. A well rounded CrossFit program should include strength work, conditioning, speed work, mobility, bodyweight exercises, etc.
CrossFit can help individuals build muscle, especially when an individual has a background in endurance sports or never came in touch with any kind of resistance training. However, individuals that came from doing resistance training to doing CrossFit can have a loss in muscle mass. This depends on the genetics of the individual, the training age, the amounts of strength training they used to do, the way their CrossFit training is designed and even more.
There are loads of different programs to follow and of course, there are a lot of CrossFit boxes with their own programming. They all do CrossFit style training, but they don´t necessarily all build muscle. So the question ¨does CrossFit builds muscle¨ is always answered by the question ¨it depends¨.
For a muscle to be built, one needs to expose her/himself to mechanical tension. The muscle needs stress, in order to adapt. If an individual does not stress a muscle (enough), the body will not adapt because it doesn´t require any strength adaptations. The body wants to adapt if it notices that it was not able to complete a task, but if the task was easily managed then it won´t bother to become a better version and be able to get it right ¨the next time¨. I am not proposing that you always need to seek the limit in order to gain adaptations but you do need an amount of tension on the muscle to make it grow.
Time under tension is a much discussed topic in the CrossFit/fitness community, as much as it is important, it does have a flip side. A lot of muscles are constantly tensed, think about the coremuscles that help us keep upright through the entire day. The time under tension is really high but the resistance is not high enough for muscles to require any adaptations. If we look at marathon runners, their muscles are under tension for 3+ hours, but they usually are not the people that have a lot of muscle mass. This has to do with fast and slow twitch muscle fibers. During high bouts of energy fast twitch fibers are also used to complete a task instead of only the slow twitch fibers. The fast twitch fibers, which are prone to growth, are being worked more when the resistance is higher. So time under tension by itelf is not enough.
For this reason, just doing strength exercises is not enough, we need enough resistance to make the muscles grow. So saying that CrossFit builds muscle depends on the way you do CrossFit. CrossFit itself can have a variety of interpretations and we only spoke about the training end of the spectrum. We have not talked about hormonal effects on muscle building.
One example of how daily lifestyle affects muscle building is the impact cortisol has on testosterone which has a big impact on muscle building. So there are a lot of different factors that come in to play when one wants to increase muscle mass, whether that is with or without CrossFit.
CrossFit can build muscle if time under tension is well interpreted, lifestyle is on point as well as nutrition.