Ask the question does cardio kill gains to any old-school body builder and they'll most likely give you a strange look and wonder why you're even asking that question. The truth, however, is that it's not as simple as first thought. Of course if you only do cardio, then you'll struggle to retain any gains you may have made, but that doesn't mean you can't do any cardio at all. Let's have a look at the science behind it, to get a better idea of the truths and myths.
A study in the 80's(1) provided some proof that there's truth behind the claims, but if you look at the study, they actually compared six day a week of cardio verses 5 days a week of strength training. There's two things that make it difficult to draw any definite conclusions from this. First is that most people training for gains/muscle hypertrophy wouldn't intend on doing cardio six times a week anyway. The other factor when looking at the study is that it used strength training rather than hypertrophy training. This makes it impossible to apply exactly to the question we're discussing.
A couple of recent studies(2&3) have shown that combining resistance training and cardio training can potentially improve your muscle performance and hypertrophy development. Although both studies involved small amounts of cardiovascular training, the first was a 45 minute cycling session and the other involved two sessions of resistance training and two sessions of endurance training per week. This would suggest that moderate amounts of cardio training can be good for your muscular development. However the second study(3) also concluded that the endurance training interfered with explosive strength developments when compared to resistance training alone. So if you're training for strength gains, than maybe limit your cardio training or only perform low intensity exercise such as walking.
A 2014 study(4) which looked into the molecular biology of exercise recovery suggested that any high intensity cardio/endurance sessions should be performed in the morning with at least a 3 hour period of recovery before any resistance training. This is because the recovery and adaptation phases of resistance training and cardio training can compete for resources in the body. The study suggested a full refuel between the morning endurance session and the afternoon resistance session. It also concluded that sufficient protein before bedtime really helps in the synthesis of new muscle fibres and repair of muscle damage.
There's a fairly recent conclusion that cardio before weights will inhibit your ability to train maximally in your weights sessions. The study we mentioned above(4) actually concluded that performing a strength training session after low intensity training could actually help to improve strength gains and would also improve endurance adaptation when compared to endurance exercise alone. The idea that cardio before weights is bad, comes from performing high intensity cardio, which can tax your nervous system and also fatigue your muscles by using up large amounts of muscle glycogen. If your primary goal is muscle building and or strength training, then it's a good idea to avoid high intensity cardio exercise immediately before your weight training sessions.
Not all cardio training is the same. If you are looking to make gains in your muscle strength or size, then running isn't going to be ideal. Firstly the amount of impact from running is proportional to your weight, so if you're gaining mass, running will be more difficult and will lead to increased impact on your joints. Secondly, as a lot of the muscular work during running is eccentric, it can cause quite a bit of muscular stress and damage. This is fine if you want to be a runner, because your body will adapt accordingly, however if you're training for muscular strength or size then it's not ideal. Crossfit-style workouts that combine weightlifting or functional exercises to produce a cardio effect will be more specific to those wanting to keep their weight training gains. The other option is low impact exercise, such as swimming, cycling or calisthenics.
It's also a good idea to reduce the time you spend on cardio workouts and increase the intensity to maintain the effect. Fasted high intensity morning cardio two or three times a week will help with fat burning and give you a kick-start to your day. Follow this with a good afternoon or early evening weights session and you'll you'll be able to improve your cardiovascular fitness whilst maintaining your muscular gains!
So overall the good news is that you can do cardio and still keep training for muscle gains, the bad news is that you no longer have an excuse not to do cardio!
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1. Hickson, R. C. (1980). Interference of strength development by simultaneously training for strength and endurance. European Journal of Applied Physiology and Occupational Physiology, 45(2-3), 255-263.
2. Lundberg, T. R., Fernandez-Gonzalo, R., Gustafsson, T., & Tesch, P. A. (2013). Aerobic exercise does not compromise muscle hypertrophy response to short-term resistance training. Journal of Applied Physiology, 114(1), 81-89.
3. Mikkola, J., Rusko, H., Izquierdo, M., Gorostiaga, E. M., & Häkkinen, K. (2012). Neuromuscular and cardiovascular adaptations during concurrent strength and endurance training in untrained men. International Journal of Sports Medicine, 33(9), 702-710.
4. Baar, K. (2014). Using molecular biology to maximize concurrent training. Sports Medicine, 44(2), 117-125.