Cardio during a bulk: how much and how

Among those gaining muscle mass, the fear of «burning muscle» with running or cycling is still alive. Because of this, many people completely give up cardio during a bulk and then wonder at their breathlessness on the stairs and worsening health indicators. Our editorial team looked into what is actually known about the effect of aerobic loads on hypertrophy and how to combine cardio with a bulk without losses.

The interference effect: where the fear came from

In 1980 Robert Hickson published a study in which participants combined intense strength training with a large volume of endurance training. The group that did both types of load stopped progressing in strength at a certain stage, while the strength-only group kept growing. This phenomenon was called the interference effect, or «concurrent training».

Later a molecular explanation appeared. Strength training activates the mTOR signaling pathway, associated with muscle protein synthesis, while prolonged aerobic work activates the enzyme AMPK, which is responsible for the cell’s energy balance and can inhibit mTOR. This «two switches» model became popular in sports media and cemented the idea that cardio «eats» muscle.

However, it is important to understand the conditions of the early studies. In Hickson’s work the volume of endurance training was very large — six sessions a week combined with five strength sessions. This is far more than an ordinary person in the gym performs when adding two or three cardio sessions to their program.

In addition, molecular interactions in vivo are more complex than the simple «AMPK switches off mTOR» scheme. Post-workout signals last for hours, and in real life there is usually time, meals and rest between cardio and strength work. So the direct transfer of the mechanism to practical recommendations turned out to be an oversimplification.

What modern meta-analyses show

A meta-analysis by Wilson and colleagues (2012) was one of the first to systematize the data on concurrent training. The authors found that interference depends on the type, frequency and duration of cardio. Running was associated with a greater negative effect on hypertrophy and strength than cycling, probably because of the greater eccentric load and micro-damage to the leg muscles.

A review by Murach and Bagley (2016) analyzed hypertrophy specifically and concluded that moderate volumes of aerobic load do not hinder muscle growth, and in some studies even promote it, especially in previously untrained people. Cardio improves muscle capillarization, which theoretically may help the delivery of nutrients and recovery.

An updated meta-analysis by Schumann and colleagues (2022) confirmed that combining aerobic and strength training does not reduce hypertrophy and maximal strength overall, but may slightly reduce gains in explosive strength, especially when both workouts are done in the same session. For an amateur whose goal is mass and strength, this means that moderate cardio is entirely compatible with a bulk.

As for interval training, a meta-analysis by Sabag and colleagues (2018) showed that combining HIIT with strength training may moderately reduce gains in lower-body strength, but not hypertrophy. So for the legs, the choice and placement of cardio matter most.

Why cardio during a bulk

The most important reason is the health of the cardiovascular system. A bulk is often accompanied by a calorie surplus, an increase in body weight and, in some people, a rise in blood pressure and a worsening lipid profile. Aerobic loads help soften these changes. The WHO guidelines (2020) recommend that adults get 150–300 minutes of moderate or 75–150 minutes of intense aerobic activity per week regardless of other goals.

The second reason is endurance in the gym itself. Better aerobic conditioning helps you recover faster between sets, especially in high-rep leg exercises. A person with poor endurance often finishes a set not because of muscle fatigue but because of breathlessness.

The third is controlling fat gain. Moderate cardio increases energy expenditure and allows a slightly more flexible attitude to the calorie content of the diet. This does not mean that cardio «compensates» for any surplus, but it helps keep the gain within reasonable limits.

Finally, cardio makes the subsequent transition to a weight-loss period easier. If a person did no aerobic work during the entire bulk, a sharp addition of a large volume of cardio during a «cut» is perceived by the body as strong stress. When an endurance base is preserved, the transition is smoother.

How much and what kind of cardio to choose

For most people during a bulk, a reasonable amount is 2–3 cardio sessions a week of 20–40 minutes at moderate intensity. This is enough to maintain cardiovascular health and endurance without a noticeable effect on muscle gain. The intensity guideline is that you can talk but singing is already hard.

If you love running, you do not have to give it up. Just keep an eye on leg recovery: if your strength numbers in squats and deadlifts drop, reduce the running volume or replace some sessions with cycling or walking.

Interval training is effective in terms of saving time but requires significant recovery. During a bulk it is better to limit yourself to one or two short interval sessions a week and do the rest of the aerobic work at a moderate pace.

The volume of cardio should be adjusted depending on how the bulk is going. If body weight is rising too fast and, along with muscle, fat tissue is noticeably increasing, you can add a session or lengthen the existing ones. But if mass is stalling while strength in the gym is dropping, first check calories and sleep, and only then whether there is too much aerobic work.

How to combine cardio with strength training

The simplest solution is to separate cardio and strength training into different days. If that is not possible, separate them by at least a few hours. Studies show that when performed in a single session, the most pronounced interference effect appears when cardio precedes strength work and fatigues the same muscles.

• Strength first, cardio after it — if the workouts are on the same day.
• Do not placeintense leg cardio the day before a squat or deadlift day.
• Light cardiocan be done on rest days — it does not hinder recovery.
• Account for calories:the additional energy expenditure must be compensated to keep the surplus.
• Watch your numbersin the compound exercises — they are the first to react to an excess of cardio.

Nutrition is the key variable. If cardio increases energy expenditure while the calorie content stays the same, the surplus may disappear and the bulk will stop. It is this, rather than «burning muscle», that most often explains the lack of progress in people who have added cardio.

Daily everyday activity — walks, stairs, moving actively around the city — is also an aerobic load. For many people 7–10 thousand steps a day plus two short cardio sessions a week cover the WHO recommendations without separate planning.

People with cardiovascular disease, high blood pressure or excess weight should get a check-up and agree the load with a doctor before starting intense aerobic training.

Editorial conclusions

The interference effect is real but appears mainly with large volumes of endurance training. Moderate cardio — 2–3 sessions of 20–40 minutes a week — has practically no effect on muscle-mass gain.

Cycling, walking and rowing affect leg recovery less than running and sprints. It is better to separate cardio from strength work or to do it afterward.

Giving up aerobic work during a bulk worsens heart health and endurance and complicates the later «cut», so our editorial team does not advise excluding cardio completely.

We also recommend reading our articles on HIIT for fat burning, on training for definition and on the three-day split.

References

1. Hickson RC. Interference of strength development by simultaneously training for strength and endurance. Eur J Appl Physiol Occup Physiol. 1980;45(2–3):255–263.
2. Wilson JM, Marin PJ, Rhea MR, et al. Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises. J Strength Cond Res. 2012;26(8):2293–2307.
3. Murach KA, Bagley JR. Skeletal muscle hypertrophy with concurrent exercise training: contrary evidence for an interference effect. Sports Med. 2016;46(8):1029–1039.
4. Schumann M, Feuerbacher JF, Sünkeler M, et al. Compatibility of concurrent aerobic and strength training for skeletal muscle size and function: an updated systematic review and meta-analysis. Sports Med. 2022;52(3):601–612.
5. Sabag A, Najafi A, Michael S, et al. 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. 2018;36(21):2472–2483.
6. Bull FC, Al-Ansari SS, Biddle S, et al. World Health Organization 2020 guidelines on physical activity and sedentary behaviour. Br J Sports Med. 2020;54(24):1451–1462.