Lifting weights and aerobic training (often called "cardio)" lead to different physiological adaptations with some arguing that cardio will interfere with the adaptations induced by lifting weights and thus “killing your gains”. But is it really that simple or can you actually get away with doing cardio when looking to maximize muscle growth?
What did they test? The authors systematically reviewed and meta-analyzed the current literature to explore the effect of concurrent training on muscle fiber hypertrophy.
What did they find? Concurrent training led to small hypertrophy impairments in type I muscle fibers but did not seem to significantly impact whole muscle hypertrophy. Additionally, the negative effects of aerobic training were observed when running was the type of training selected, with cycling not leading to significant negative effects for hypertrophy.
What does it mean for you? Doing cardio when also training for muscle growth or strength is totally fine as long as you don’t overdo it with the duration and intensity of the type of cardio you decide to do. It’s also worth noting that cycling may be your best bet when trying to absolutely maximize hypertrophy.
What’s the Problem?
Combining lifting with aerobic training aka “cardio” may be one of the most discussed topics in the world of fitness, especially among those looking to maximize their muscle and strength potential. Training concurrently for strength and aerobic fitness is often referred to as “concurrent training” and has received quite a bit of attention in the literature, especially due to its practical implications for a plethora of populations, from elderly individuals looking to minimize the deleterious effects of aging and improve their health, to competitive athletes that need to train a multitude of athletic qualities, among those strength & aerobic fitness. It is currently recommended that people of all ages and fitness levels engage in both aerobic and resistance training to maximize health and longevity 1, although there is evidence to suggest that resistance training performed with sufficient intensity and at sufficient volumes of training may be able to induce some of the adaptations that you’d usually turn to aerobic training for 2.
In the world of fitness, and especially in the “old days”, the narrative that was pushed by strength enthusiasts was often that longer forms of endurance training (regardless of intensity) may kill your gains and you’re probably best off doing either short high intensity bouts of aerobic training (eg: HIIT) or swaying off cardio completely.
Such claims were often supported by pictures of competitive long distance runners next to pictures of competitive sprinters, highlighting the huge differences in their physiques. High level long distance runners are usually thin with minimal amounts of muscle while sprinters are jacked. Needless to say that comparing the average Jane/Joe who lifts 3-5 times per week and also wants to do some aerobic training (eg: running) to a competitive marathon runner who runs hundreds of miles per week, does not really count as sound scientific evidence to justify the idea that cardio will kill your gains.
A systematic review and meta-analysis from 2012 3 examined the interference effect when engaging in concurrent training and found that power seemed to be the variable most affected by concurrent training, whereas strength and hypertrophy seemed to be unaffected. They also found that different aerobic training modalities may have a different effect on strength and hypertrophy outcomes, with running having a more negative effect on strength and hypertrophy adaptations versus cycling. These findings were also consistent with a more recent systematic review and meta-analysis from 2022 4, by the same group of authors behind the study being reviewed, which found that although concurrent training did not seem to affect strength and hypertrophy, it had a negative effect on explosive strength, regardless of the intensity, type or frequency of aerobic training. Although the findings are encouraging for most fitness enthusiasts who want to be fit, muscular and strong, the authors noted that most of the studies that made it in their review did not use the most appropriate methods to assess muscle size when investigating the effects of concurrent training on hypertrophy (ie: they used magnetic resonance imaging or tomography) and with only one of the studies included looking at muscle fiber size. Before we get to why that may be an issue, here's a super short overview of muscle fiber types.
Overall there are two types of muscle fibers called type I and type II. Type I muscle fibers contract somewhat slowly and use aerobic respiration (oxygen and glucose) for adenosine triphosphate (ATP) production allowing for low power contractions to be produced over long periods of time. You may have heard people refer to them as “slow twitch” fibers and are developed through endurance training. Type II muscle fibers, or “fast twitch”, are split in two “subcategories”, type IIa and IIb (aka IIx). Type IIb muscle fibers primarily utilize anaerobic glycolysis to produce ATP and are able to produce quick contractions of high force used in high power movements. Type IIa muscle fibers are also known as the “in-between” muscle fibers as they have characteristics of both Type IIa and Type I fibers. Type IIa fibers fatigue slower than Type IIb fibers. Overall Type II fibers are developed through strength training.