Building muscle is hard enough as is - let alone having something possibly impair your potential. Many spend their whole lives trying to get 'too big,' only to find out it's very difficult to get too big! Endurance training (cardio) has an 'interference' effect on strength, power, and muscle hypertrophy gains (1). When added to a resistance training program, it has been shown to negatively impact the amount that can be gained (1, 2). This doesn't mean you can't make progress with some cardio training however - studies have shown you can still make good gains in strength, power, and muscle hypertrophy with the inclusion of cardio training in your resistance training program (1, 2) - it just means you have to be smart about how much and what type of cardio you decide to use. Different types and amounts of cardio have different effects on power, strength, and muscle hypertrophy gains.
Slow, Long Distance Cardio Sucks
If you had to pick a form of exercise that would likely have the worst impact on your gains, it would be slow, long-distance endurance training (1) (e.g. 10 km running). Long, slow distance endurance training has a different adaption effect compared to resistance training. It doesn't result in a significant increase in strength and power, and it doesn't cause muscle to hypertrophy like resistance training does (1). On its own, the gains you would get are absolutely minimal, if any. Added to a resistance training program, it can interfere with the gains you could make in strength, power, and muscle hypertrophy. Training programs that include both slow, long-distance endurance training and resistance training have been shown to produce fewer gains than programs that only included resistance training (1).
Slow, Long Distance Running Really Sucks
If you had to pick the worst type of slow, long-distance endurance training, it would be running. Both running and cycling have been shown to significantly impair lower body strength, power and muscle hypertrophy gains, however, running impaired gains the most (1). When running, the leg muscles experience a very high eccentric load when the lead foot strikes the ground. This high eccentric load repeated enough times (i.e. a long run) can result in a high amount of muscle damage (3). This high amount of muscle damage is believed to have an interference effect with strength, power, and muscle hypertrophy gains. If you have to do some slow, long-distance training, but don’t need to be good at running, then opting for other modalities such as cycling, rowing etc., could help to ensure you don't impair your strength, power, and hypertrophy gains as much.
High-Intensity Cardio is Better
High-intensity cardio, such as high-intensity interval training (HIIT), is a much smarter option if you need to work on your cardiovascular fitness but want to preserve as much of your strength, power, and hypertrophy gains. HIIT is as effective as slow, long-distance endurance training for improving cardiovascular performance (4), and included in a resistance training program has been shown to produce similar increases in muscle hypertrophy and upper body strength compared to resistance training only programs (2). However, lower body strength gains from resistance training will still likely be less with the inclusion of HIIT compared to a resistance training only program. Using running as your modality for HIIT has been shown to be better than using cycling in mitigating the negative impacts of HIIT on gains (2).
The More You Do, The Worse It Is
It's pretty clear by now that cardio training isn't great for improving strength and power or building muscle. However, this doesn't mean that any amount of cardio is going to have a devastating effect. The total amount (frequency and duration) of cardio you complete will have the greatest influence on how much it impacts your gains (1). If you can avoid it entirely, then you'll get the best lower body strength, power, and hypertrophy gains - I'm looking at you, cardio bunnies, who feel compelled to do cardio after every workout to 'help burn fat.' (FYI - you don't!). If you need to improve/maintain your cardiovascular fitness for your sport, then do the least amount you can get away with.
Your Upper Body is Fine, It's Just Your Legs!
If you haven't noticed already, all the negative effects of cardio training we've been talking about have been to the lower body - the upper body doesn't get affected at all by cardio that uses the lower body. The effects of cardio are specific to the body area that is involved in the exercise modality. Cycling and running have been shown to only affect strength, power, and muscle hypertrophy gains in the lower body; the upper body's gains are unaffected (1, 2). There hasn't been sufficient research to conclusively determine if cardio training modalities that include the upper body (e.g. arm ergometers, swimming, rowing etc.) negatively impact upper body strength, power, and muscle hypertrophy (1), however, given that lower body endurance training modalities can, it's possible upper body modalities will negatively impact gains as well if you do enough of it.
If your main goal is to purely develop as much strength, power, and muscle hypertrophy as possible, it's best to avoid cardio as much as possible. Slow, long-distance endurance training will have the worst impact on your gains - especially running. Cycling is a better option. Utilise HIIT as much as possible and keep the total amount of cardio completed to the absolute least needed. If you do decide to use HIIT to work on your cardiovascular fitness, then running is actually a better option than cycling.
- Wilson, J. M., Marin, P. J., Rhea, M. R., Wilson, S. M.C., Loenneke, Jeremy P., & Anderson, Judy C. (2012), Concurrent training: A meta-analysis examining interference of aerobic and resistance exercises, J Strength Cond Res., 26(8), 2293-2307
- Sabag, A., Najafi, A., Michael, S., Esgin, T., Halaki, M., Hackett, D. (2018), The compatibility of concurrent high intensity interval training and resistance training for muscular strength and hypertrophy: a systematic review and meta-analysis, J Sports Sci., 36(21), 2472-2483
- Koller, A., Mair, J., Schobersberger, W., Wohlfarter, T., Haid, C., Mayr, M., Villiger, B., Frey, W., Puschendorf, B. (1998), Effects of prolonged endurance exercise on plasma myosin heavy chain fragments and other muscular proteins. Cycling vs running., J Sports Med Phys Fitness, 38(1), 10-17
- Milanović, Z., Sporiš, G. & Weston, M., 2015. Effectiveness of High-Intensity Interval Training (HIT) and Continuous Endurance Training for VO2max Improvements: A Systematic Review and Meta-Analysis of Controlled Trials. Sports Medicine, 45(10), pp. 1469-81