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Concurrent training, is it safe?

concurrent training is performing both strength and cardio workouts during the same period of time, or in the same programming.

Today we are going to discuss a training style that combines both endurance and resistance training. A program that follows the simultaneous incorporation of endurance and resistance training is called concurrent training. Previous literature has found that concurrent training has negative effects on strength, power, and lean mass when compared to resistance training alone. Such effects are referred as “concurrent training effect” or “interference effect”. This effect is very specific in terms of what type of adaptions it attenuates. Only the physiological adaptations that come from resistance training are negatively impacted. It would be coherent to assume that since endurance training and resistance training, separately, induce positive adaptations, then the combination of the two would have even greater results, but since that it is not the case, then it makes sense to look further into the physiological responses induced by concurrent training. Considering that lean mass and strength seemed to be the most affected, it is worth to see how concurrent training affects one of our most important catabolic hormones. Thus, the purpose of this article is to explain the effect of both endurance and resistance training on cortisol levels in healthy active adults.

First, let’s talk about what cortisol is and what it does to our body. Cortisol is one of the hormones that has the most power in our skeletal muscle. It plays a key role in stress regulation. In terms of exercise and skeletal muscle, though, cortisol basically helps keep body homeostasis. Exercise induced cortisol plays a major role in muscle inflammation and damage as well. After intense exercise, muscles are extremely sensitive to glucocorticoids (cortisol being one of them) as a way to fight or counteract muscle inflammation and damage. Thus, cortisol is necessary during and after exercise. So, contrary to popular belief, cortisol itself is not harmful. What makes this hormone a threat to our health and performance has more to do with the when our levels are high and for how long. Naturally, cortisol levels are at their highest early in the morning and progressively decrease throughout the day. Towards nighttime, cortisol levels should be low in order for the body to have good quality sleep. Cortisol levels are associated with the “fight-or-flight” response or sympathetic state. Being in such state and having the body in a constant fight-or-flight state has proven to have detrimental effects to the overall health. Consequently, high cortisol levels only pose a threat to our health when they remain high for a long period of time and when they are high/low at the wrong time of the day.

Research has shown that exercise induced cortisol secretion in high levels decrease skeletal IGF-I (insulin-like growth factor 1) synthesis which is one of the most influential hormones in protein synthesis (building muscle.) It is then hard to refute the negative effects high levels cortisol have on lean mass. The extent of the neuroendocrine response, cortisol for our purposes, is determined by the volume of the exercise session. With regular exercise, being at a low-moderate intensity, the hormonal responses do not last long so the body only benefits from the positive aspects of cortisol. However, when the exercise is very high and frequent, the endocrine responses last way longer and that is what presents an issue.

So now let’s get into how exactly concurrent training affects cortisol. There has been research done in the topic, and most studies point to what is known as “concurrent training effect” or “interference effect” which suggests that concurrent training is detrimental to strength development or overall performance due to lack of recovery. Other literature suggests that the answer might be more complicated than that. A study done on both females and males found that gender might play a big role on the effects concurrent training might have. In this study, they witnessed lack of interference for men in terms of strength. All male participants still saw improvements on strength throughout the whole 16-week long program. Women, on the other hand, experimented decrements on strength. In terms of hormonal responses, all subjects (both strength training only group and concurrent training group) had elevated cortisol levels. Nevertheless, for the concurrent training group the levels went even higher during the last 4 weeks of training, while the strength only group progressively returned to baseline towards the end of the 16 weeks. Due to the inconsistency in findings, literature studying whether the order of training would make a difference or not, started to surface. Order meaning endurance-resistance and resistance-endurance. Multiple studies have looked into the effects of different orders of training in not only cortisol levels, but also testosterone. Older research has shown that the order of exercises on concurrent training does not present a significant difference in terms of cortisol levels. However, there was one study done in 2017 that proved the opposite. Said study found that order does indeed affect strength and cortisol levels. Their findings suggest that the best way to “protect” or keep strength from getting affected, is by doing strength training before endurance training when both adaptations are being trained the same day.

The exact reason why concurrent training negatively affects strength are not known. Some research has suggested that it has to do with the lack of, or reduced, hypertrophy. It is reasonable to think so, since high cortisol levels suppress IGF-1 which is a key hormone for building muscle. This reaction, along with some other hormonal responses, put the body in a catabolic state which has a direct effect on strength gains. However, there is a lot of inconsistency on the research. And so it is impossible to establish an exact reason why it has shown to be detrimental to strength, or why studies come to different conclusions and results. What can be taken from this, though is: a) main difference between concurrent training and regular exercise in terms of cortisol, is that concurrent training makes it harder to decrease cortisol levels. It keeps them high. Strength training only, does not. There is quicker clearance and return to baseline, b) there is research that shows risks of concurrent training in terms of maintaining strength, c) order and volume should be taken into consideration, d) general population could benefit more from this type of training, and e) for athletes, one adaptation should be prioritized over the other one while avoiding concurrent training during long blocks of periodized programs.

Author: Daniela Ruiz


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