Does cardio kill muscle gains?

No, cardio doesn't kill muscle gains at normal amounts. Two to three sessions a week of 20 to 40 minutes won't get in the way of building muscle, as long as you're eating enough and still lifting properly. Very large amounts of cardio, especially running, can slow growth.

Where the idea comes from

Cardio and lifting send different signals to your muscles. Endurance work pushes them toward stamina, and lifting pushes them toward size and strength. Some research in the lab shows that doing a lot of both at the same time gives a smaller muscle gain than lifting alone.

But that shows up mostly at high volumes, like long runs several times a week on top of full training. In normal gym life, the effect is small.

What actually cuts into gains

Usually it's not the cardio itself. It's:

• Not eating enough. Cardio burns energy, and if you don't make up for it, you have less to build with. Protein of about 0.7 to 1 g per lb of bodyweight (1.6 to 2.2 g/kg) is a good target.
• Too tired to lift well. If you're wrecked after a long run, your squats suffer.
• Poor sleep and recovery.

How to fit them together

1. Lift first if building muscle is the priority, and do cardio after or on a separate day. Cardio before or after weights goes into the order.
2. Prefer cycling, rowing or incline walking over long runs if your legs are the thing you want to grow. Impact and long distances leave legs more sore.
3. Leave at least 6 hours between a hard cardio session and heavy leg work, if you can.
4. Keep hard intervals to one or two a week.

If you're mixing both, League of Fitness syncs lifting and cardio from Apple Health and scores each by intensity and heart rate zones. A moderate ride and a lifting session both count toward your daily match, so you aren't forced to pick one.

Why you'd want to keep it anyway

Cardio is good for your heart, your recovery between sets and your energy. A lifter with no cardio base gets winded between sets and tends to stall. Skipping it completely isn't a good trade.

My take is that for most people trying to build muscle, a few moderate sessions a week is the right amount, and fear of cardio costs more than the cardio itself. If you're training for a marathon, expect slower muscle gain while you do it, and that's a reasonable trade.