More than once we may have heard that cardio "burns muscle" or seen pictures that suggest that regular cardio training will lead us to the physique of an elite marathoner. By the way, an elite marathoner can run over 150 km in a week and every extra kilo is a significant burden for him.
But can cardio really cause muscle loss? When and what form of cardio to include so as not to interfere with the process of hypertrophy and muscle strength gain?
First of all - the main energy source during continuous sports activity is mainly glycogen and fat. In case of low glycogen levels, starvation or extreme exercise, even of a strength nature, some amino acids can also be used - e.g. leucine, isoleucine, valine or alanine. However, even in these cases, only about 5-10% of the total energy supply is involved. In addition, these amino acids are not necessarily taken up by our body from our own muscle proteins, but can be used from the diet, or from the so-called amino acid pool.
It even appears that aerobic exercise can cause muscle hypertrophy in individuals with sedentary lifestyles and prevent muscle atrophy in the elderly. Thus, cardio alone certainly does not cause muscle loss. However, if our main goal is muscle hypertrophy, its inappropriate inclusion may hinder training progress under certain conditions.
So, when is it ideal to include cardio?
Of course, the intensity and duration play an important role. If we go for five minutes on the treadmill before a warm-up or jump rope and then ride a bike for fifteen minutes after the workout, there can be no question of any disruption of muscle growth. If it's a moderate or high intensity load lasting longer than thirty minutes, it makes more sense to think about how and when we incorporate cardio.
According to the results of several studies, the best option seems to be the allocation of cardio as a separate training unit with strength training being ideally one day apart. One of the studies suggests that a six-hour interval is sufficient - for example, strength training in the morning and cardio in the evening.
If, e.g. for time reasons, the training units cannot be separated, cardio should definitely not precede strength training. There is no need to look for a big science in this. If we want to do our best in our training and maintain the principles of progressive overload, the last thing we want is to feel fatigue before we start working out.
It is much better to include cardio after the workout. Although one small study from 2017 suggests that interference or disruption of the hypertrophy process may occur even in this case, as cardio redistributes blood, which may slow recovery in certain body parts (Tomiya et al., 2017). However, this study has a number of limitations and so its results should be taken with some caution.
A 2012 meta-analysis pointed out that the type of aerobic exercise also plays a role (Wilson et al., 2012). In the case of running, there was a significantly greater reduction in hypertrophy and increase in muscle strength than cycling. This may be due to the greater muscle damage in the case of running or because lower limb locomotion during cycling is biomechanically closer to conventional strength training exercises. This suggests that the increasingly popular air bike or rowing or running machine might be similarly suitable. It remains to be seen whether future research will provide more information on this topic.
Cardio training can make a big difference in our energy balance. When our main goal is muscle hypertrophy, we should be in a slight caloric surplus. We must not forget not only to have sufficient energy intake, but also protein - it plays a key role in weight loss as well, as it protects us from losing muscle mass. Certainly, a poorly adjusted diet will be a more frequent and serious obstacle to growth than an extra hour of cardio per week.
Cardio alone can rarely be responsible for muscle loss or slowing down muscle growth, and it should not be completely abandoned even by those for whom the goal is maximum muscle hypertrophy and the development of strength. Although weightlifting, for example, is considered an anaerobic activity, a workout in which we repeatedly perform heavy sets of, say, ten repetitions will be largely covered aerobically. Insufficient aerobic capacity, simply put, a reduced maximum level of the body's ability to work in a steady state without significant lactate accumulation in muscle tissue, can negatively affect even pure strength performance.
In addition, cardio training also improves vascularisation or blood supply to muscle fibres and may also increase the number of mitochondria. This may in effect promote more pronounced hypertrophy. In fact, one theory holds that the reason we have a limit to building muscle tissue beyond a certain threshold is the body's inability to adequately supply this tissue with oxygen and nutrients due to the increased distance of transport.
In conclusion, let’s remember that properly set cardio training has a preventive effect against a number of diseases, improves regeneration, positively affects our cardiovascular system, musculoskeletal system and our psyche. The key is to schedule your training plan well, adapt your diet to it and get enough rest. Then we can go jogging, swimming, cycling or visit the cardio zone in the gym without worrying about our kilograms of muscle mass.
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