Does cardio interfere with strength and muscle gains?
It can — the phenomenon is called the interference effect — but the magnitude depends heavily on how much, what type, and how it is scheduled, and for most people training recreationally it is small enough to ignore.
What the research shows. Combining endurance and resistance training tends to blunt gains in strength and especially power compared with resistance training alone. Notably, hypertrophy is affected much less than strength, and endurance adaptations are barely affected at all — the interference runs mostly in one direction.
The proposed mechanisms:
Molecular signalling conflict, where endurance and resistance training activate partly opposing pathways. This was the original explanation and is now considered less important than initially thought, particularly at realistic training volumes.
Residual fatigue, which is probably the larger factor: tired legs produce worse resistance sessions, and the effect accumulates.
Energy availability, since doing both requires eating enough to recover from both, and under-fuelling looks exactly like interference.
What actually determines the size of the effect:
Frequency and volume of endurance work — the effect scales with how much you do, and becomes meaningful at high volumes.
Modality. Cycling interferes less than running, consistently, largely because running involves substantial eccentric loading and more muscle damage.
Proximity. Doing them in the same session, especially endurance first, hurts most.
How to minimise it if you want both:
Separate sessions by several hours, or put them on different days.
Do the priority first, when fresh.
Favour cycling or rowing for the endurance component if lower-body strength matters.
Keep most endurance work easy, since low-intensity volume interferes least.
Eat enough, which resolves a surprising proportion of apparent interference.
The important context: for general health, doing both is clearly better than doing either alone. The interference effect matters to competitive strength and power athletes, and rarely changes what a recreational trainee should do.