Myth: Cardio Kills Your Gains

Cardio won't erase your muscle. For most lifters, a sensible amount of cardio has no measurable effect on muscle growth or max strength. The real cost shows up in a narrow place: explosive power, and huge amounts of running.

Verdict: Cut. Cardio is a tool. You can use it to cut, to maintain or to bulk. There's a time and a place for it. Pick the right type, keep the dose sensible, and give it some space from your heavy lifting.

Start Here

The idea behind this myth is called the interference effect. The theory says your body can't adapt to two very different jobs at once. Train for endurance and strength at the same time, and one of them, usually strength and size, gets shortchanged.

That theory has a real study behind it. A well-known 1980 study by Hickson stacked endurance training six days a week on top of lifting, and the combined training held back strength gains (Murach and Bagley, 2016). That finding spread through gyms for decades. How extreme the setup was didn't spread with it.

The biggest recent review pulled together 43 studies. Adding aerobic training to strength training did not reduce muscle growth and did not reduce max strength. It did reduce explosive strength, like jumping and sprinting power, and that hit was bigger when both were done in the same session (Schumann et al., 2022).

Think of it like a phone with two apps running. Most of the time you never notice. Run a heavy app all day and the battery drains faster. The problem is how hard and how long you run it.

Your First Step: Look at your week. If your cardio sits right before your heavy leg day, move it to after your lifting or to a different day, and keep everything else the same.

The Breakdown

Where the Myth Comes From

Endurance athletes and bodybuilders look very different, and it's easy to blame the cardio. But a marathoner is built by how much they run, what they eat and their genetics, not by a few bike sessions. The early lab work used big doses. A later review of the topic pointed out that endurance volume like six days a week can simply wear people down, and that the human evidence for blunted muscle growth was weaker than many people assumed (Murach and Bagley, 2016).

The Type of Cardio Matters

An older review of 21 studies found that pairing lifting with running reduced muscle growth and strength, while pairing it with cycling did not. It also found that the more often and the longer people did their endurance work, the more their muscle, strength and power gains shrank (Wilson et al., 2012).

The newer review didn't find running versus cycling made a clear difference (Schumann et al., 2022). So I don't treat running as forbidden. I treat it as the option that costs more recovery, which matters when your legs already have a lot of work on the schedule.

Dose & Timing Matter More Than the Label

Across these reviews, the same practical points keep showing up:

• Keep the dose sensible. One review suggested 2 to 3 days a week of short sessions, around 30 to 40 minutes, as a reasonable place for people focused on muscle (Murach and Bagley, 2016).
• Give it some space. The explosive strength hit was bigger in the same session. When sessions were separated by at least 3 hours, the effect was no longer significant (Schumann et al., 2022).
• Pick low-impact when legs are the priority. Bikes, ellipticals and walking cost less recovery than long runs.

How I Use Cardio

I hate doing cardio. I do it anyway, and I make it useful. I'm up at 4:30, and I do 30 minutes to an hour of cardio first thing, fasted. While I'm moving, AI narrates my daily review: what I logged, what I need to get done that day. Then I brainstorm with Grok, my AI platform, and after that I go to the gym and lift. By the time I'm under a bar, my head is clear and my day is planned.

That morning session gets my heart rate up and my blood flowing, burns a little extra carbohydrate, and gets me insulin sensitive for my first meal. Some days that first meal is at 5 in the morning. Some days it's not until 1 in the afternoon. In the evening I do another 30 minutes or so.

Outside of those blocks and my client sessions, I'm fairly sedentary. I'm at a desk in front of a screen. That's why steps and staying active are paramount for me. My dog doesn't count as cardio anymore, either. When she was a younger pup, we did 2- to 3-mile walks a few times a day. Now she's older and would rather sniff and mosey around. It keeps me somewhat active, but it isn't cardio.

My go-to options are low-impact: the Arc Trainer, the elliptical, the recumbent bike and walking outside. I aim for 10,000 to 12,000 steps a day, and I make up any gap with cardio.

I also use 5 to 10 minutes of easy cardio before leg day as part of an active warm-up. That isn't the kind of cardio this myth is about, and it isn't counted as a working set.

What Actually Kills Gains

You're never going to out-train a bad diet. What really kills gains is how much fat you put on: not getting enough protein, eating too many carbs, and becoming insulin resistant instead of insulin sensitive. Cardio can help with insulin sensitivity, and that helps you build muscle, not lose it.

A lot of people also misunderstand cardio as all or nothing. My lifestyle clients tell me, "I worked out four times this week. I don't have to do cardio." You trained four times, fairly hard, and then sat on the couch the rest of the day. You can still walk around. You can take the dog out, mow the lawn and get things done around the house. That's activity, and it counts.

Compare that with a college student client I had who was giving campus tours, walking the campus for three sessions a day for two weeks. He didn't need extra cardio. He was getting his steps in. He earned skipping cardio by being active. The others treated the workout as the activity. If he wanted to do more, great, but he didn't need it. It's all relative. You have to do something in your life. Be honest with yourself.

Bro Science Check: "Every minute of cardio burns away muscle you worked for."

What the Research Says: A review of 43 studies found adding aerobic training to strength training did not reduce muscle growth or max strength. It did reduce explosive strength, especially when both were done in the same session (Schumann et al., 2022). Long, frequent endurance work, and running in particular, showed more cost in an older review (Wilson et al., 2012).

Verdict: Cut. Cardio doesn't kill gains. It's a tool. Too much of the wrong cardio, at the wrong time, can dent them.

The Asterisk for Athletes

One group should pay closer attention: athletes who rely on power. If your sport is about jumping, sprinting or exploding off the line, that explosive strength cost applies to you. That doesn't mean skip conditioning. Plan it so it doesn't sit right on top of your speed and power work, and match it to what your sport demands.

The Limits of the Research

Most of these studies are short, often 8 to 12 weeks, and many use untrained or recreationally active people. Very few look at highly trained lifters doing large amounts of both. For normal people doing normal amounts of cardio, the interference effect on muscle is small to none. At the extremes, we know less.

Common Mistakes

• Skipping cardio entirely out of fear. You give up the health and recovery benefits to avoid a cost most people will never see.
• Doing hard intervals right before heavy legs. Tired legs mean fewer quality reps where it counts.
• Using cardio to make up for eating. You can't out-train a bad diet. Our free Macros and Meal Planner quick start is a better place to begin.

Pain during cardio, especially in a joint, isn't something to push through. See a doctor or physical therapist. Not medical advice.

For recovery between sessions, read Sleep & Rest Days: The Recovery Levers Nobody Wants to Pull. Next: Myth: Lifting Makes Women Bulky.