Does cardio make you lose muscle?

Short answer

No. As long as you keep lifting, adding cardio did not change muscle growth (only long, frequent running reduced it), and cardio alone grew leg muscle by 5-6%. What takes muscle away is a calorie shortfall, not cardio, and lifting is what protects muscle then.

Does cardio make you lose muscle?
'Cardio → burns muscle → muscle loss' — does this formula hold?
Among people building muscle, a common belief is that cardio melts muscle away. Some avoid the treadmill completely. We checked step by step whether cardio really takes muscle, and whether there are conditions where it does.
Does adding cardio to lifting shrink your gains?
Muscle size was no different
An analysis that pooled 43 studies and 1,090 people compared the same weight training with or without added cardio. In the 15 studies (389 people) that measured muscle size, muscle growth was no different between the two. Maximal strength was the same too.
The newest big-picture review agreed
Pooling 144 studies again, muscle was the same
In 2026, a review combined 17 analyses (144 studies, 1,492 people) and found the same thing. People who did both lifting and cardio gained strength and muscle about as well as those who only lifted, and their aerobic fitness improved more.
Long, frequent running is different
The type and amount of cardio mattered
An analysis of 21 studies found that adding running to lifting reduced muscle and strength gains, but cycling did not. The more often and the longer people did cardio, the smaller their muscle gains. Another analysis of 15 studies found that adding running led to less growth in endurance-type muscle fibers.
Flip side: cardio alone grew muscle
12 weeks of cycling grew thigh muscle 5-6%
13 people did nothing but indoor cycling for 12 weeks. The front thigh muscle grew 5-6%. The idea that cardio itself burns muscle breaks down right here.
The real cause of muscle loss is a calorie shortfall
Lifting is what protects muscle then
An analysis of 1,455 dieting people (34 studies) found that adding exercise cut the loss of lean mass (muscle and everything except fat) by 45.7%. Strength training saved 0.83 kg; cardio alone saved 0.51 kg, not a clear difference. In a trial of 160 people, the cardio-only group lost 5% of lean mass versus 2-3% in the lifting groups.
Bottom line: cardio does not take your muscle
Keep lifting and you can add cardio
If you keep lifting, adding cardio left muscle growth unchanged (only long, frequent running reduced it), and cardio alone even built muscle. Muscle goes when calories run short, and lifting is what holds on to it. If you worry about your legs, you can pick cycling over running and build up the amount of cardio gradually.