How Much Cardio While Building Muscle?

Every lifter has heard that cardio kills gains. The claim comes from a real phenomenon, the interference effect, that was discovered in 1980 and then inflated by four decades of gym folklore into a rule that stops people doing any cardio at all. The actual research is reassuring, and specific about where the risk is.

Where the idea came from

In 1980, Robert Hickson published a study in which people trained strength only, endurance only, or both, for ten weeks. The combined group's strength gains kept pace for about seven weeks and then stalled and fell, while the strength-only group kept improving (Hickson 1980). That was the birth of the interference effect.

It was also an extreme protocol: six days a week of intense cycling and running, plus five days of heavy lifting, for people who had not been doing either. Nearly everything written since has been about finding where interference actually starts under conditions that resemble how people train.

What the meta-analyses say

The first major pooled analysis, in 2012, looked at 21 studies and found that combining aerobic and strength training reduced gains in hypertrophy, strength and power compared with strength alone, with power hit hardest. Crucially, the size of the interference rose with the volume and frequency of aerobic work, and running caused more interference than cycling (Wilson et al. 2012).

A larger 2022 update, pooling 43 studies, changed the headline. Concurrent training did not significantly reduce muscle hypertrophy or maximal strength compared with strength training alone. The only outcome that was consistently compromised was explosive strength (rate of force development), and that mainly when cardio and lifting were done in the same session (Schumann et al. 2022).

The current picture, then:

• Muscle size: no meaningful interference at moderate cardio volumes.
• Maximal strength: no meaningful interference at moderate volumes.
• Explosive power: a real but modest cost, worst when cardio is done in the same session as lifting.
• The dose matters: interference grows with cardio frequency, duration and intensity.
• The mode matters: running interferes more than cycling.

Why interference happens at all

Three mechanisms, all of which scale with how much and how hard you do cardio:

Recovery competition. Hard cardio creates fatigue and muscle damage, particularly eccentric-heavy running. If your legs are sore and depleted from a long run, the next squat session is worse, and worse sessions over months mean less growth. This is the mechanism that matters most in practice.

Molecular signalling. Endurance exercise activates the AMPK pathway, which in cell studies can dampen the mTOR pathway that drives muscle protein synthesis. This was the fashionable explanation for a decade, but in humans the effect is small and short-lived, and separating sessions by a few hours appears to eliminate it.

Energy balance. Cardio burns calories. If you do not eat them back, your surplus disappears and you are no longer bulking. This is not interference, it is arithmetic, but it explains a lot of "cardio killed my gains" stories.

How much is safe

Combining the meta-analytic data with the practical experience of coaches who manage both, the thresholds look like this:

Two to three sessions a week, 20 to 40 minutes, easy to moderate intensity. No measurable cost to muscle or strength in the research, meaningful benefits to heart health, work capacity, recovery between sets and appetite regulation. This is the recommendation for almost everyone in a building phase.

Four to five hours per week. Interference becomes detectable, mainly through recovery, and calorie needs rise substantially. Fine if your goal is a balance of fitness and muscle; suboptimal if muscle is the priority.

More than that, or frequent high intensity. This is where the Hickson-style trade-off lives. Appropriate for athletes whose sport demands it, not for a bulk.

Which cardio

Incline walking. Almost zero recovery cost, easy to do daily, no interference to speak of. If you are worried about gains, walk. See Walking for Fat Loss for why it is under-rated.

Cycling. The modality with the least interference in the research, probably because it is concentric-dominant (little muscle damage) and its movement pattern overlaps with squatting. Stationary bikes make intensity easy to control. This is the best choice for a proper Zone 2 session during a bulk. See Zone 2 Training.

Rowing, elliptical, swimming. Low impact, low damage, moderate interference. Rowing adds some upper-back work, which most lifters will not mind.

Running. Effective for fitness, but it is eccentric-heavy, damages leg muscle more than any of the above, and is the mode most associated with interference in the meta-analyses. If you love running, keep it easy and modest during a bulk.

HIIT and sprinting. High fatigue and the highest measured interference with power. Cap it at once a week, or drop it during a dedicated muscle-building block.

Scheduling

Order and spacing determine how much of the potential interference you actually experience:

1. Different days is best. Lift four days, cardio on two of the others.
2. Same day, separated. Six or more hours between sessions largely removes the molecular and fatigue overlap (Wilson et al. 2012). Lift in the morning, walk or cycle in the evening.
3. Same session. Lift first, then easy cardio. Never the reverse for anything heavy. Keep the cardio easy enough that it is a cooldown with benefits.
4. Protect leg days. Put hard cardio, if you do any, as far as possible from squat and deadlift sessions.

Eat the cardio back

A 40-minute easy session burns 250 to 400 calories. In a lean bulk with a surplus of 200 to 300 calories per day, that is enough to erase the surplus on cardio days. Add the calories, mostly as carbohydrate around training, or accept that the scale will stall. See The Lean Bulk Guide for setting the surplus.

The benefits you get in return

Lifters who keep some cardio in a bulk report the same things the research finds: better recovery between sets, more work capacity in high-volume sessions, less bloating and better appetite control, lower resting heart rate and blood pressure, and a fitter, healthier body at the end. A bulk with two or three easy cardio sessions ends with more muscle you can actually use, and a heart that can support the next block. See VO2 Max: Why It Predicts Longevity for why that matters beyond the gym.

The bottom line

Cardio does not kill gains at the doses most people should be doing. Two to three easy sessions a week of 20 to 40 minutes, preferably cycling or incline walking, separated from lifting by several hours, costs nothing measurable in muscle or strength and buys a lot of health. Interference is real only when cardio gets long, hard, or frequent, or when the calories are not replaced. Keep it easy, keep it modest, eat it back, and lift.

Sources

• Hickson RC. "Interference of strength development by simultaneously training for strength and endurance." European Journal of Applied Physiology and Occupational Physiology, 1980.
• Wilson JM, Marin PJ, Rhea MR, et al. "Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises." Journal of Strength and Conditioning Research, 2012.
• 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 Medicine, 2022.
• Petré H, Hemmingsson E, Rosdahl H, Psilander N. "Development of Maximal Dynamic Strength During Concurrent Resistance and Endurance Training in Untrained, Moderately Trained, and Trained Individuals: A Systematic Review and Meta-analysis." Sports Medicine, 2021.